Read a specific line from a file
You are encouraged to solve this task according to the task description, using any language you may know.
Some languages have special semantics for obtaining a known line number from a file.
- Task
Demonstrate how to obtain the contents of a specific line within a file.
For the purpose of this task demonstrate how the contents of the seventh line of a file can be obtained, and store it in a variable or in memory (for potential future use within the program if the code were to become embedded).
If the file does not contain seven lines, or the seventh line is empty, or too big to be retrieved, output an appropriate message.
If no special semantics are available for obtaining the required line, it is permissible to read line by line.
Note that empty lines are considered and should still be counted.
Also note that for functional languages or languages without variables or storage, it is permissible to output the extracted data to standard output.
11l
<lang 11l>V f = File(‘input.txt’) V line = 3
L 0 .< line - 1
f.read_line()
print(f.read_line())</lang>
Ada
<lang ada>with Ada.Text_IO; use Ada.Text_IO;
procedure Rosetta_Read is
File : File_Type;
begin
Open (File => File,
Mode => In_File,
Name => "rosetta_read.adb");
Set_Line (File, To => 7);
declare
Line_7 : constant String := Get_Line (File);
begin
if Line_7'Length = 0 then
Put_Line ("Line 7 is empty.");
else
Put_Line (Line_7);
end if;
end;
Close (File);
exception
when End_Error =>
Put_Line ("The file contains fewer than 7 lines.");
Close (File);
when Storage_Error =>
Put_Line ("Line 7 is too long to load.");
Close (File);
end Rosetta_Read;</lang>
Aime
<lang aime>void read_line(text &line, text path, integer n) {
file f;
f.affix(path);
call_n(n, f_slip, f);
f.line(line);
}
integer
main(void)
{
if (1 < argc()) {
text line;
read_line(line, argv(1), 6);
o_("7th line is:\n", line, "\n");
}
0;
}</lang>
ALGOL 68
<lang algol68># reads the line with number "number" (counting from 1) #
- from the file named "file name" and returns the text of the #
- in "line". If an error occurs, the result is FALSE and a #
- message is returned in "err". If no error occurs, TRUE is #
- returned #
PROC read specific line = ( STRING file name
, INT number # line 7 #
, REF STRING line
, REF STRING err
)BOOL:
IF FILE input file;
line := "";
open( input file, file name, stand in channel ) /= 0
THEN
# failed to open the file #
err := "Unable to open """ + file name + """";
FALSE
ELSE
# file opened OK #
err := "";
BOOL at eof := FALSE;
# set the EOF handler for the file #
on logical file end( input file
, ( REF FILE f )BOOL:
BEGIN
# note that we reached EOF on the #
# latest read #
at eof := TRUE;
# return TRUE so processing can continue #
TRUE
END
);
INT line number := 0;
STRING text;
WHILE line number < number
AND NOT at eof
DO
get( input file, ( text, newline ) );
line number +:= 1
OD;
# close the file #
close( input file );
# return the line or an error message depending on whether #
# we got a line with the required number or not #
IF line number = number
THEN
# got the required line #
line := text;
TRUE
ELSE
# not enough lines in the file #
err := """" + file name + """ is too short";
FALSE
FI
FI; # read specific line #
main:(
# read the seventh line of this source and print it #
# (or an error message if we can't) #
STRING line;
STRING err;
IF read specific line( "read-specific-line.a68", 7, line, err )
THEN
# got the line #
print( ( "line seven is: """ + line + """", newline ) )
ELSE
# got an error #
print( ( "unable to read line: """ + err + """" ) )
FI
)</lang>
AutoHotkey
<lang AutoHotkey>FileReadLine, OutputVar, filename.txt, 7 if ErrorLevel
MsgBox, There was an error reading the 7th line of the file</lang>
AWK
<lang awk>#!/usr/bin/awk -f
- usage: readnthline.awk -v lineno=6 filename
FNR==lineno { storedline=$0; found++ }
END {
if (found < 1) {
print "ERROR: Line", lineno, "not found"
exit 1
}
print storedline
} </lang>
Batch File
<lang dos> @echo off
for /f "skip=6 tokens=*" %%i in (file.txt) do (
set line7=%%i goto break
)
- break
echo Line 7 is: %line7% pause>nul </lang>
- Input:
This is line 1.
This is line 2.
This is line 3. This line has special characters !@#$%^&*()
This is line 4. The next line is blank
This is line 6. This line has trailing spaces.
This is line 7
This is line 8
etc...
- Output:
Line 7 is: This is line 7
BBC BASIC
<lang bbcbasic> filepath$ = @lib$ + "..\licence.txt"
requiredline% = 7
file% = OPENIN(filepath$)
IF file%=0 ERROR 100, "File could not be opened"
FOR i% = 1 TO requiredline%
IF EOF#file% ERROR 100, "File contains too few lines"
INPUT #file%, text$
NEXT
CLOSE #file%
IF ASCtext$=10 text$ = MID$(text$,2)
PRINT text$</lang>
C
Mmap file and search for offsets to certain line number. Since mapped file really is memory, there's no extra storage procedure once offsets are found. <lang c>#include <unistd.h>
- include <sys/types.h>
- include <sys/mman.h>
- include <sys/stat.h>
- include <fcntl.h>
- include <err.h>
/* following code assumes all file operations succeed. In practice,
* return codes from open, close, fstat, mmap, munmap all need to be * checked for error.
- /
int read_file_line(const char *path, int line_no) { struct stat s; char *buf; off_t start = -1, end = -1; size_t i; int ln, fd, ret = 1;
if (line_no == 1) start = 0; else if (line_no < 1){ warn("line_no too small"); return 0; /* line_no starts at 1; less is error */ }
line_no--; /* back to zero based, easier */
fd = open(path, O_RDONLY); fstat(fd, &s);
/* Map the whole file. If the file is huge (up to GBs), OS will swap * pages in and out, and because search for lines goes sequentially * and never accesses more than one page at a time, penalty is low. * If the file is HUGE, such that OS can't find an address space to map * it, we got a real problem. In practice one would repeatedly map small * chunks, say 1MB at a time, and find the offsets of the line along the * way. Although, if file is really so huge, the line itself can't be * guaranteed small enough to be "stored in memory", so there. */ buf = mmap(0, s.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
/* optional; if the file is large, tell OS to read ahead */ madvise(buf, s.st_size, MADV_SEQUENTIAL);
for (i = ln = 0; i < s.st_size && ln <= line_no; i++) { if (buf[i] != '\n') continue;
if (++ln == line_no) start = i + 1; else if (ln == line_no + 1) end = i + 1; }
if (start >= s.st_size || start < 0) { warn("file does not have line %d", line_no + 1); ret = 0; } else { /* do something with the line here, like write(STDOUT_FILENO, buf + start, end - start); or copy it out, or something */ }
munmap(buf, s.st_size); close(fd);
return ret; }</lang>
Alternate Version
This version does not rely on POSIX APIs such as mmap, but rather sticks to ANSI C functionality. This version also works with non-seekable files, so it can be fed by a pipe. It performs limited but adequate error checking. That is, get_nth_line returns NULL on all failures, and the caller can distinguish EOF, file read error and out of memory by calling feof() and ferror() on the input file.
<lang c>#include <stdio.h>
- include <stdlib.h>
- define BUF_SIZE ( 256 )
char *get_nth_line( FILE *f, int line_no ) {
char buf[ BUF_SIZE ]; size_t curr_alloc = BUF_SIZE, curr_ofs = 0; char *line = malloc( BUF_SIZE ); int in_line = line_no == 1; size_t bytes_read;
/* Illegal to ask for a line before the first one. */
if ( line_no < 1 )
return NULL;
/* Handle out-of-memory by returning NULL */
if ( !line )
return NULL;
/* Scan the file looking for newlines */
while ( line_no &&
( bytes_read = fread( buf, 1, BUF_SIZE, f ) ) > 0 )
{
int i;
for ( i = 0 ; i < bytes_read ; i++ )
{
if ( in_line )
{
if ( curr_ofs >= curr_alloc )
{
curr_alloc <<= 1;
line = realloc( line, curr_alloc );
if ( !line ) /* out of memory? */
return NULL;
}
line[ curr_ofs++ ] = buf[i];
}
if ( buf[i] == '\n' )
{
line_no--;
if ( line_no == 1 )
in_line = 1;
if ( line_no == 0 )
break;
}
}
}
/* Didn't find the line? */
if ( line_no != 0 )
{
free( line );
return NULL;
}
/* Resize allocated buffer to what's exactly needed by the string
and the terminating NUL character. Note that this code *keeps*
the terminating newline as part of the string.
*/
line = realloc( line, curr_ofs + 1 );
if ( !line ) /* out of memory? */
return NULL;
/* Add the terminating NUL. */ line[ curr_ofs ] = '\0';
/* Return the line. Caller is responsible for freeing it. */ return line;
}
/* Test program. Prints out the 7th line of input from stdin, if any */
int main( int argc, char *argv[] )
{
char *line7 = get_nth_line( stdin, 7 );
if ( line7 )
{
printf("The 7th line of input was:\n%s\n", line7 );
free( line7 );
} else
{
printf("Did not find the 7th line of input. Reason: ");
if ( feof( stdin ) )
puts("End of file reached.");
else if ( ferror( stdin ) )
puts("Error reading input.");
else
puts("Out of memory.");
}
return 0;
} </lang>
C#
<lang C sharp>using System; using System.IO;
namespace GetLine {
internal class Program
{
private static void Main(string[] args)
{
Console.WriteLine(GetLine(args[0], uint.Parse(args[1])));
}
private static string GetLine(string path, uint line)
{
using (var reader = new StreamReader(path))
{
try
{
for (uint i = 0; i <= line; i++)
{
if (reader.EndOfStream)
return string.Format("There {1} less than {0} line{2} in the file.", line,
((line == 1) ? "is" : "are"), ((line == 1) ? "" : "s"));
if (i == line)
return reader.ReadLine();
reader.ReadLine();
}
}
catch (IOException ex)
{
return ex.Message;
}
catch (OutOfMemoryException ex)
{
return ex.Message;
}
}
throw new Exception("Something bad happened.");
}
}
}</lang>
C++
<lang cpp>#include <string>
- include <fstream>
- include <iostream>
int main( ) {
std::cout << "Which file do you want to look at ?\n" ;
std::string input ;
std::getline( std::cin , input ) ;
std::ifstream infile( input.c_str( ) , std::ios::in ) ;
std::string file( input ) ;
std::cout << "Which file line do you want to see ? ( Give a number > 0 ) ?\n" ;
std::getline( std::cin , input ) ;
int linenumber = std::stoi( input ) ;
int lines_read = 0 ;
std::string line ;
if ( infile.is_open( ) ) {
while ( infile ) {
getline( infile , line ) ; lines_read++ ; if ( lines_read == linenumber ) { std::cout << line << std::endl ; break ; }
}
infile.close( ) ;
if ( lines_read < linenumber )
std::cout << "No " << linenumber << " lines in " << file << " !\n" ;
return 0 ;
}
else {
std::cerr << "Could not find file " << file << " !\n" ;
return 1 ;
}
}</lang>
Clojure
<lang clojure>(defn read-nth-line
"Read line-number from the given text file. The first line has the number 1." [file line-number] (with-open [rdr (clojure.java.io/reader file)] (nth (line-seq rdr) (dec line-number))))</lang>
- Output:
user=> (read-nth-line "/tmp/test.txt" 7) "foo"
Common Lisp
<lang lisp>(defun read-nth-line (file n &aux (line-number 0))
"Read the nth line from a text file. The first line has the number 1"
(assert (> n 0) (n))
(with-open-file (stream file)
(loop for line = (read-line stream nil nil)
if (and (null line) (< line-number n))
do (error "file ~a is too short, just ~a, not ~a lines long"
file line-number n)
do (incf line-number)
if (and line (= line-number n))
do (return line))))
</lang>
Example call:
CL-USER> (read-nth-line "/tmp/test1.text" 7) "foo"
D
simply <lang d> void main() {
import std.stdio, std.file, std.string;
auto file_lines = readText("input.txt").splitLines();
//file_lines becomes an array of strings, each line is one element
writeln((file_lines.length > 6) ? file_lines[6] : "line not found");
} </lang>
or, line by line <lang d>import std.stdio;
void main() {
int countLines;
char[] ln;
auto f = File("linenumber.d", "r");
foreach (char[] line; f.byLine()) {
countLines++;
if (countLines == 7) {
ln = line;
break;
}
}
switch(countLines) {
case 0 : writeln("the file has zero length");
break;
case 7 : writeln("line 7: ", (ln.length ? ln : "empty"));
break;
default :
writefln("the file only contains %d lines", countLines);
}
}</lang>
line 7: foreach (char[] line; f.byLine()) {
Delphi
<lang Delphi> program Read_a_specific_line_from_a_file;
{$APPTYPE CONSOLE}
uses
System.SysUtils;
function ReadLine(position: Cardinal; FileName: TFileName): string; begin
Result := ;
if not FileExists(FileName) then
raise Exception.Create('Error: File does not exist.');
var F: TextFile;
var line: string;
AssignFile(F, FileName);
Reset(F);
for var _ := 1 to position do
begin
if Eof(F) then
begin
CloseFile(F);
raise Exception.Create(Format('Error: The file "%s" is too short. Cannot read line %d',
[FileName, position]));
end;
Readln(F, line); end; CloseFile(F); Result := line;
end;
begin
Writeln(ReadLine(7, 'test')); Readln;
end.</lang>
Elixir
The idea is to stream the file, filter the elements and get the remaining element. If does not exist, nil will be throw and we will pattern match with print_line(_). If the value exists, it will match print_line({value, _line_number}) and the value of the line will be printed. <lang Elixir> defmodule LineReader do
def get_line(filename, line) do
File.stream!(filename)
|> Stream.with_index
|> Stream.filter(fn {_value, index} -> index == line-1 end)
|> Enum.at(0)
|> print_line
end
defp print_line({value, _line_number}), do: String.trim(value)
defp print_line(_), do: {:error, "Invalid Line"}
end </lang>
Erlang
Using function into_list/1 from Read_a_file_line_by_line. There is no behaviour specified after printing an error message, so I throw an exception. An alternative would be to continue with a default value? <lang Erlang> -module( read_a_specific_line ).
-export( [from_file/2, task/0] ).
from_file( File, N ) -> line_nr( N, read_a_file_line_by_line:into_list(File) ).
task() ->
Lines = read_a_file_line_by_line:into_list( "read_a_specific_line.erl" ),
Line_7 = line_nr( 7, Lines ),
Line_7.
line_nr( N, Lines ) ->
try
case lists:nth( N, Lines )
of "\n" -> erlang:exit( empty_line )
; Line -> Line
end
catch
_Type:Error0 ->
Error = line_nr_error( Error0 ),
io:fwrite( "Error: ~p~n", [Error] ),
erlang:exit( Error )
end.
line_nr_error( function_clause ) -> too_few_lines_in_file; line_nr_error( Error ) -> Error. </lang>
- Output:
27> read_a_specific_line:task().
"task() ->\n"
28> read_a_specific_line:from_file("read_a_specific_line.erl", 6).
Error: empty_line
** exception exit: empty_line
in function read_a_specific_line:line_nr/2 (read_a_specific_line.erl, line 25)
29> read_a_specific_line:from_file("read_a_specific_line.erl", 66).
Error: too_few_lines_in_file
** exception exit: too_few_lines_in_file
in function read_a_specific_line:line_nr/2 (read_a_specific_line.erl, line 25)
F#
<lang fsharp>open System open System.IO
[<EntryPoint>] let main args =
let n = Int32.Parse(args.[1]) - 1
use r = new StreamReader(args.[0])
let lines = Seq.unfold (
fun (reader : StreamReader) ->
if (reader.EndOfStream) then None
else Some(reader.ReadLine(), reader)) r
let line = Seq.nth n lines // Seq.nth throws an ArgumentException,
// if not not enough lines available
Console.WriteLine(line)
0</lang>
Factor
<lang factor>USING: continuations fry io io.encodings.utf8 io.files kernel math ; IN: rosetta-code.nth-line
- nth-line ( path encoding n -- str/f )
[ f ] 3dip '[
[ _ [ drop readln [ return ] unless* ] times ]
with-return
] with-file-reader ;
- nth-line-demo ( -- )
"input.txt" utf8 7 nth-line [ "line not found" ] unless* print ;
MAIN: nth-line-demo</lang>
Fortran
A lot of petty annoyances can arise in the attempt to complete the desired action, and so the function does not simply return true or false, nor does it return some drab integer code that would require an auxiliary array of explanatory texts somewhere... Instead, it returns a message reporting on its opinion, with an ad-hoc scheme. If the first character is a space, all is well, otherwise a ! indicates some severe problem while a + indicates a partial difficulty. The text of the desired record is returned via a parameter, thus the caller can be the one responsible for deciding how much space to provide for it. F2000 has provision for allocating character strings of the needed length, but there is no attempt to use that here as the key requirement is for the length to be decided during the process of the READ statement.
The example uses F90 only because the MODULE protocol enables usage of a function without having to re-declare its type in every calling routine. Otherwise this is F77 style. Some compilers become confused or raise an error over the manipulation of a function's name as if it were an ordinary variable. In such a case an auxiliary variable can be used with its value assigned to the function name on exit.<lang Fortran> MODULE SAMPLER !To sample a record from a file. SAM00100
CONTAINS SAM00200
CHARACTER*20 FUNCTION GETREC(N,F,IS) !Returns a status. SAM00300
Careful. Some compilers get confused over the function name's usage. SAM00400
INTEGER N !The desired record number. SAM00500
INTEGER F !Of this file. SAM00600
CHARACTER*(*) IS !Stashed here. SAM00700
INTEGER I,L !Assistants. SAM00800
IS = "" !Clear previous content, even if null...SAM00900
IF (N.LE.0) THEN !Start on errors. SAM01000
WRITE (GETREC,1) "!No record",N !Could never be found. SAM01100
1 FORMAT (A,1X,I0) !Message, number. SAM01200
ELSE IF (F.LE.0) THEN !Obviously wrong? SAM01300
WRITE (GETREC,1) "!No unit number",F!Positive is valid. SAM01400
ELSE IF (LEN(IS).LE.0) THEN !Space awaits? SAM01500
WRITE (GETREC,1) "!String size",LEN(IS) !Nope. SAM01600
ELSE !Otherwise, there is hope. SAM01700
REWIND (F) !Clarify the file position. SAM01800
DO I = 1,N - 1 !Grind up to the desired record. SAM01900
READ (F,2,END=3) !Ignoring any content. SAM02000
END DO !Are we there yet? SAM02100
READ (F,2,END = 3) L,IS(1:MIN(L,LEN(IS))) !At last. SAM02200
2 FORMAT (Q,A) !Q = characters yet unread. SAM02300
IF (L.LT.LEN(IS)) IS(L + 1:) = "" !Clear the tail. SAM02400
IF (L.GT.LEN(IS)) THEN !Now for more silliness.SAM02500
WRITE (GETREC,1) "+Length",L !Too long to fit in IS. SAM02600
ELSE IF (L.LE.0) THEN !A zero-length record SAM02700
WRITE (GETREC,1) "+Null" !Is not the same SAM02800
ELSE IF (IS.EQ."") THEN !As a record SAM02900
WRITE (GETREC,1) "+Blank",L !Containing spaces. SAM03000
ELSE !But otherwise, SAM03100
WRITE (GETREC,1) " Length",L !Note the leading space.SAM03200
END IF !Righto, we've decided. SAM03300
END IF !And, no more options. SAM03400
RETURN !So, done. SAM03500
3 WRITE (GETREC,1) "!End on read",I !An alternative ending. SAM03600
END FUNCTION GETREC !That was interesting. SAM03700
END MODULE SAMPLER !Just a sample of possibility. SAM03800
SAM03900
PROGRAM POKE POK00100
USE SAMPLER POK00200
INTEGER ENUFF !Some sizes. POK00300
PARAMETER (ENUFF = 666) !Sufficient? POK00400
CHARACTER*(ENUFF) STUFF !Lots of memory these days. POK00500
CHARACTER*20 RESULT POK00600
INTEGER MSG,F !I/O unit numbers. POK00700
MSG = 6 !Standard output. POK00800
F = 10 !Chooose a unit number. POK00900
WRITE (MSG,*) " To select record 7 from a disc file." POK01000
POK01100
WRITE (MSG,*) "As a FORMATTED file." POK01200
OPEN (F,FILE="FileSlurpN.for",STATUS="OLD",ACTION="READ") POK01300
RESULT = GETREC(7,F,STUFF) POK01400
WRITE (MSG,1) "Result",RESULT POK01500
WRITE (MSG,1) "Record",STUFF POK01600
1 FORMAT (A,":",A) POK01700
POK01800
CLOSE (F) POK01900
WRITE (MSG,*) "As a random-access unformatted file." POK02000
OPEN (F,FILE="FileSlurpN.for",STATUS="OLD",ACTION="READ", POK02100
1 ACCESS="DIRECT",FORM="UNFORMATTED",RECL=82) !Not 80! POK02200
STUFF = "Cleared." POK02300
READ (F,REC = 7,ERR = 666) STUFF(1:80) POK02400
WRITE (MSG,1) "Record",STUFF(1:80) POK02500
STOP POK02600
666 WRITE (MSG,*) "Can't get the record!" POK02700
END !That was easy. POK02800
</lang> Output:
To select record 7 from a disc file. As a FORMATTED file. Result: Length 80 Record: CHARACTER*(*) IS !Stashed here. SAM00700 As a random-access unformatted file. Record: CHARACTER*(*) IS !Stashed here. SAM00700
Fortran's file reading counts a null line as a valid record so fortunately, there is no difficulty there. Trailing spaces appear in IS because of fixed-size CHARACTER variables. A length parameter for the length of the record as read (with possible trailing spaces counted) could easily enough be passed back.
Random access
An entirely different approach is possible if the file is opened for random access, and has fixed-size records. In such a case, READ (F,REC=7,ERR=666) STUFF would suffice (where STUFF was the right size for the record) and if the record did not exist (being beyond the last record of a short file) then label 666 would be jumped to - without that, a crash results. In the ASCII world, text files have varying-length records so the example file reactivates source line sequence numbers of the Fortran fixed-format style and yet again, the source file highlighter doesn't recognise another foible of Fortran layout. Although there are many conventions (the simplest being digits only), here a text name is crammed in to the sequence field of columns 73-80.
This results in every line being of the same length with an obvious route towards calculating the location of a random record. But, the actual record length is not 80, because in the ASCII world, plain text files have their records separated by CR, or CRLF, or LFCR, or CR - depending on the system. Experiment shows that this system (Windows XP) uses CRLF, and so the record length is 82. But that's not the end of it. The RECL parameter by default is in terms of the default integer size, which is four bytes, and so the record length cannot be specified correctly! Fortunately, the Compaq Visual Fortran compiler has an option to specify that the RECL value is to be in bytes, and, invisibly so here, this has been done...
Obviously, this only works because of the special nature of the file being read. Other systems offer a filesystem that does not regard record sizes or separators as being a part of the record that is read or written, and in such a case, a CR (or CRLF, or whatever) does not appear amongst the data. Some systems escalate to enabling such random access for varying-length records, or access by a key text rather than a record number (so that a key of "SAM00700" might be specified), and acronyms such as ISAM start appearing.
In Fortran 77 there was no verbose REC=n facility, instead one used <lang Fortran>READ (F'7) STUFF(1:80)</lang> - that is, an apostrophe even though an at-sign was available - and again the source file highlighting is confused. An interesting alternative was to use the FIND(F'7) statement instead, followed by an ordinary READ (or WRITE) not necessarily specifying the desired record number. The point of this is that the FIND statement would initiate the pre-positioning for the next I/O asynchronously so that other processing could intervene between it and the READ or WRITE, and in situations more complex than this example, there could be startling changes in performance. If not always positive ones when many files were being accessed on one physical disc drive. Unfortunately, later Fortran extensions have abandoned this statement, while multiprocessing has proliferated.
The GO TO style of handling mishaps in an I/O statement makes a simple structure difficult - note that the reception area ought not be fallen into by normal execution, thus the STOP. Unfortunately, READ and WRITE statements do not return a result that could be tested in an IF-statement or WHILE-loop, but this can be approached with only a little deformation: <lang Fortran> READ (F,REC = 7,ERR = 666, IOSTAT = IOSTAT) STUFF(1:80)
666 IF (IOSTAT.NE.0) THEN
WRITE (MSG,*) "Can't get the record: code",IOSTAT
ELSE
WRITE (MSG,1) "Record",STUFF(1:80)
END IF</lang>
Where IOSTAT is an integer variable (and also a key word) and this formulation means not having to remember which way the assignment goes; it is left to right. The error code numbers are unlikely to be the same across different systems, so experimentation is in order. It would be much nicer to be able to write something like IF (READ (F,REC = 7, IOSTAT = IOSTAT) STUFF(1:80)) THEN etc. or DO WHILE(etc.)
FreeBASIC
<lang freebasic>' FB 1.05.0 Win64
Open "input.txt" For Input As #1 Dim line_ As String Dim count As Integer = 0 While Not Eof(1)
Line Input #1, line_ read each line
count += 1
If count = 7 Then
line_ = Trim(line_, Any !" \t") remove any leading or trailing spaces or tabs
If line_ = "" Then
Print "The 7th line is empty"
Else
Print "The 7th line is : "; line_
End If
Exit While
End If
Wend If count < 7 Then
Print "There are only"; count; " lines in the file"
End If Close #1 Print Print "Press any key to quit" Sleep </lang>
FutureBasic
Uses FB's native file$ command that opens a dialog window and allows the user to select the file to read. <lang futurebasic> include "ConsoleWindow"
dim as long i : i = 1 dim as Str255 s, lineSeven dim as CFURLRef url
if ( files$( _CFURLRefOpen, "TEXT", "Select text file", @url ) )
open "I", 2, @url
while ( not eof(2) )
line input #2, s
if ( i == 7 )
lineSeven = s
end if
i++
wend
close 2
end if
if ( lineSeven[0] )
print lineSeven
else
print "File did not contain seven lines, or line was empty."
end if </lang>
Input text file:
Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8 Line 9 Line 10
Output:
Line 7
Go
<lang go>package main
import ( "bufio" "errors" "fmt" "io" "os" )
func main() { if line, err := rsl("input.txt", 7); err == nil { fmt.Println("7th line:") fmt.Println(line) } else { fmt.Println("rsl:", err) } }
func rsl(fn string, n int) (string, error) { if n < 1 { return "", fmt.Errorf("invalid request: line %d", n) } f, err := os.Open(fn) if err != nil { return "", err } defer f.Close() bf := bufio.NewReader(f) var line string for lnum := 0; lnum < n; lnum++ { line, err = bf.ReadString('\n') if err == io.EOF { switch lnum { case 0: return "", errors.New("no lines in file") case 1: return "", errors.New("only 1 line") default: return "", fmt.Errorf("only %d lines", lnum) } } if err != nil { return "", err } } if line == "" { return "", fmt.Errorf("line %d empty", n) } return line, nil }</lang>
Groovy
<lang groovy>def line = null new File("lines.txt").eachLine { currentLine, lineNumber ->
if (lineNumber == 7) {
line = currentLine
}
} println "Line 7 = $line"</lang>
Haskell
<lang Haskell>main :: IO () main = do contents <- readFile filename
case drop 6 $ lines contents of
[] -> error "File has less than seven lines"
l:_ -> putStrLn l
where filename = "testfile"</lang>
Icon and Unicon
The procedure readline uses repeated alternation (i.e. |read()) to generate the lines of the file one at a time and limitation (i.e. \ n) to limit the generation to n results. If the file is not large enough readline will fail.
While it is certainly possible to read at file at specific offsets without reading each line via seek, with files using line feed terminated variable length records something has to read the data to determine the 7th record. This solution uses a combination of repeated alternation and generation limiting to achieve this. The counter is simply to discover if there are enough records.
<lang Icon>procedure main() write(readline("foo.bar.txt",7)|"failed") end
procedure readline(f,n) # return n'th line of file f f := open(\f,"r") | fail # open file every i := n & line := |read(f) \ n do i -:= 1 # <== here close(f) if i = 0 then return line end</lang>
J
<lang j>readLine=: 4 :0
(x-1) {:: <;.2 ] 1!:1 boxxopen y
)</lang>
Thus: <lang bash>$ cal 2011 > cal.txt</lang>
<lang j> 7 readLine 'cal.txt'
9 10 11 12 13 14 15 13 14 15 16 17 18 19 13 14 15 16 17 18 19</lang>
Note that this code assumes that the last character in the file is the line end character, and that the line end character is a part of the line to be retrieved.
Tacit alternative <lang j>require 'files' NB. required for versions before J701 readLineT=: <:@[ {:: 'b'&freads@]</lang> This is not quite equivalent to the code above as it handles cross-platform line-endings and those line end character(s) are removed from the result.
Java
example: java -cp . LineNbr7 LineNbr7.java
output : line 7: public static void main(String[] args) throws Exception {;
<lang java>package linenbr7;
import java.io.*;
public class LineNbr7 {
public static void main(String[] args) throws Exception {
File f = new File(args[0]);
if (!f.isFile() || !f.canRead())
throw new IOException("can't read " + args[0]);
BufferedReader br = new BufferedReader(new FileReader(f));
try (LineNumberReader lnr = new LineNumberReader(br)) {
String line = null;
int lnum = 0;
while ((line = lnr.readLine()) != null
&& (lnum = lnr.getLineNumber()) < 7) {
}
switch (lnum) {
case 0:
System.out.println("the file has zero length");
break;
case 7:
boolean empty = "".equals(line);
System.out.println("line 7: " + (empty ? "empty" : line));
break;
default:
System.out.println("the file has only " + lnum + " line(s)");
}
}
}
}</lang>
jq
Using jq 1.4, one would have to read the entire file in order to extract a particular line. Since April 24, 2015, however, the task can be accomplished by only reading the lines up to the desired line number. We accordingly showcase here these recently added features of jq:
- "inputs" - a builtin which produces a stream
- "foreach" - a control structure for iterating over a stream
- "break" - for breaking out of a loop
<lang jq># Input - a line number to read, counting from 1
- Output - a stream with 0 or 1 items
def read_line:
. as $in
| label $top
| foreach inputs as $line
(0; .+1; if . == $in then $line, break $top else empty end) ;</lang>
Example: Read line number $line (to be provided on the command line), counting from 1 <lang jq>$line | tonumber | if . > 0 then read_line
else "$line (\(.)) should be a non-negative integer" end</lang>
- Output:
<lang sh>$ jq -n -r 'range(0;20) | tostring' | jq --arg line 10 -n -R -r -f Read_a_specific_line_from_a_file.jq 9</lang>
Julia
The short following snippet of code actually stores all the lines from the file in an array and displays the seventh element of the array, returning an error if there is no such element. Since the array is not referenced, it will be garbage collected when needed. The filehandle is closed upon completion of the task, be it successful or not. <lang Julia>open(readlines, "path/to/file")[7]</lang> The next function reads n lines in the file and displays the last read if possible, or returns a short message. Here again, the filehandle is automatically closed after the task. Note that the first line is returned if a negative number is given as the line number. <lang Julia>function read_nth_lines(stream, num)
for i = 1:num-1 readline(stream) end result = readline(stream) print(result != "" ? result : "No such line.")
end</lang>
- Output:
julia> open(line -> read_nth_lines(line, 7), "path/to/file") "Hi, I am the content of the seventh line\n"
Kotlin
<lang scala>// version 1.1.2
import java.io.File
fun main(args: Array<String>) {
/* The following code reads the whole file into memory
and so should not be used for large files
which should instead be read line by line until the
desired line is reached */
val lines = File("input.txt").readLines()
if (lines.size < 7)
println("There are only ${lines.size} lines in the file")
else {
val line7 = lines[6].trim()
if (line7.isEmpty())
println("The seventh line is empty")
else
println("The seventh line is : $line7")
}
}
/* Note that 'input.txt' contains the eight lines: Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8
- /</lang>
- Output:
The seventh line is : Line 7
Lasso
<lang Lasso>local(f) = file('unixdict.txt') handle => { #f->close } local(this_line = string,line = 0)
- f->forEachLine => {
#line++ #line == 7 ? #this_line = #1 #line == 7 ? loop_abort }
- this_line // 6th, which is the 7th line in the file
</lang>
Liberty BASIC
We read the whole file into memory, and use 'word$( string, number, delimiter)'. Line delimiter is assumed to be CRLF, and the file is assumed to exist at the path given. <lang lb>fileName$ ="F:\sample.txt" requiredLine =7
open fileName$ for input as #i
f$ =input$( #i, lof( #i))
close #i
line7$ =word$( f$, 7, chr$( 13)) if line7$ =chr$( 13) +chr$( 10) or line7$ ="" then notice "Empty line! ( or file has fewer lines)."
print line7$</lang>
Lua
<lang lua>function fileLine (lineNum, fileName)
local count = 0 for line in io.lines(fileName) do count = count + 1 if count == lineNum then return line end end error(fileName .. " has fewer than " .. lineNum .. " lines.")
end
print(fileLine(7, "test.txt"))</lang>
Maple
<lang Maple>path := "file.txt": specificLine := proc(path, num) local i, input: for i to num do input := readline(path): if input = 0 then break; end if: end do: if i = num+1 then printf("Line %d, %s", num, input): elif i <= num then printf ("Line number %d is not reached",num): end if: end proc:</lang>
Mathematica/Wolfram Language
<lang Mathematica> If[# != EndOfFile , Print[#]]& @ ReadList["file", String, 7]</lang>
MATLAB / Octave
<lang Matlab>
eln = 7; % extract line number 7
line = ;
fid = fopen('foobar.txt','r');
if (fid < 0)
printf('Error:could not open file\n')
else
n = 0;
while ~feof(fid),
n = n + 1;
if (n ~= eln),
fgetl(fid);
else
line = fgetl(fid);
end
end;
fclose(fid);
end;
printf('line %i: %s\n',eln,line);
</lang>
Insert non-formatted text here
MoonScript
<lang MoonScript>iter = io.lines 'test.txt' for i=0, 5
error 'Not 7 lines in file' if not iter!
print iter!</lang>
Nanoquery
<lang Nanoquery>def getline(fname, linenum) contents = null try contents = new(Nanoquery.IO.File).read() return contents[linenum] catch if contents = null throw new(Exception, "unable to read from file '" + fname + "'") else throw new(Exception, "unable to retrieve line " + linenum + " from file: not enough lines") end end end</lang>
NetRexx
<lang NetRexx>/* NetRexx */ options replace format comments java crossref symbols nobinary
parse arg inFileName lineNr .
if inFileName = | inFileName = '.' then inFileName = './data/input.txt' if lineNr = | lineNr = '.' then lineNr = 7
do
lineTxt = readLine(inFileName, lineNr) say '<textline number="'lineNr.right(5, 0)'">'lineTxt'</textline>'
catch ex = Exception
ex.printStackTrace()
end
return
-- ============================================================================= -- NetRexx/Java programs don't have a special mechanism to seek to a specified line number -- the simple solution is to iterate through file. (Costly for very large files) method readLine(inFileName, lineNr) public static signals IOException, FileNotFoundException
lineReader = LineNumberReader(FileReader(File(inFileName)))
notFound = isTrue
lineTxt =
loop label reading forever
line = lineReader.readLine()
select
when lineReader.getLineNumber() = lineNr then do
lineTxt = line
notFound = isFalse
leave reading -- terminate I/O loop
end
when line = null then do
leave reading -- terminate I/O loop
end
otherwise nop
end
finally
lineReader.close()
end reading
if notFound then signal RuntimeException('File' inFileName 'does not contain line' lineNr.right(5))
return lineTxt
-- ============================================================================= method isTrue() public static returns boolean
return 1 == 1
-- ============================================================================= method isFalse() public static returns boolean
return \(1 == 1)
</lang>
Nim
<lang nim>import strformat
proc readLine(f: File; num: Positive): string =
for n in 1..num:
try:
result = f.readLine()
except EOFError:
raise newException(IOError, &"Not enough lines in file; expected {num}, found {n - 1}.")
let f = open("test.txt", fmRead) echo f.readLine(7) f.close() </lang>
OCaml
OCaml does not provide built-in facilities to obtain a particular line from a file. It only provides a function to read one line from a file from the current position in the input channel input_line. We can use this function to get the seventh line from a file, for example as follows:
<lang ocaml>let input_line_opt ic =
try Some (input_line ic) with End_of_file -> None
let nth_line n filename =
let ic = open_in filename in
let rec aux i =
match input_line_opt ic with
| Some line ->
if i = n then begin
close_in ic;
(line)
end else aux (succ i)
| None ->
close_in ic;
failwith "end of file reached"
in
aux 1
let () =
print_endline (nth_line 7 Sys.argv.(1))</lang>
PARI/GP
GP is not able to read specific lines, only whole files. For this capability one can use the extern, externstr, or system commands together with, e.g., the AWK solution, or else use the C solution from within PARI itself.
Pascal
<lang pascal>Program FileTruncate;
uses
SysUtils;
const
filename = 'test'; position = 7;
var
myfile: text; line: string; counter: integer;
begin
if not FileExists(filename) then
begin
writeln('Error: File does not exist.');
exit;
end;
Assign(myfile, filename);
Reset(myfile);
counter := 0;
Repeat
if eof(myfile) then
begin
writeln('Error: The file "', filename, '" is too short. Cannot read line ', position);
Close(myfile);
exit;
end;
inc(counter);
readln(myfile);
until counter = position - 1;
readln(myfile, line);
Close(myfile);
writeln(line);
end.</lang> Output:
line 7 from file test
Perl
<lang perl>#!/usr/bin/perl -s
- invoke as <scriptname> -n=7 [input]
while (<>) { $. == $n and print, exit } die "file too short\n";</lang>
Phix
No specific mechanism, but simple enough. If the file is suitably small:
object lines = get_text("TEST.TXT",GT_LF_STRIPPED) if sequence(lines) and length(lines)>=7 then ?lines[7] else ?"no line 7" end if
For bigger files:
integer fn = open("TEST.TXT","r") for i=1 to 6 do {} = gets(fn) end for ?gets(fn) -- (shows -1 if past eof) close(fn)
Phixmonti
<lang Phixmonti>include ..\Utilitys.pmt
argument 1 get "r" fopen var f drop 0 7 for
f fgets number? if f fclose exitfor else nip nip endif
endfor print /# show -1 if past eof #/</lang>
PHP
<lang php><?php
$DOCROOT = $_SERVER['DOCUMENT_ROOT'];
function fileLine ($lineNum, $file) {
$count = 0;
while (!feof($file)) {
$count++;
$line = fgets($file);
if ($count == $lineNum) return $line;
}
die("Requested file has fewer than ".$lineNum." lines!");
}
@ $fp = fopen("$DOCROOT/exercises/words.txt", 'r');
if (!$fp) die("Input file not found!");
echo fileLine(7, $fp);
?></lang>
PicoLisp
<lang PicoLisp>(in "file.txt"
(do 6 (line)) (or (line) (quit "No 7 lines")) )</lang>
PL/I
<lang pli> declare text character (1000) varying, line_no fixed;
get (line_no); on endfile (f) begin;
put skip list ('the specified line does not exist');
go to next;
end;
get file (f) edit ((text do i = 1 to line_no)) (L);
put skip list (text); next: ; </lang>
PL/M
This is written for the original 8080 PL/M compiler and can be run under CP/M or an emulator or clone.
The name of the file to read and the line number to read should be specified on the command line. E.g., if the source is in a file called READLINE.PLM and has been compiled to READLINE.COM, then the command READLINE READLINE.PLM 7 would display the 7th line of the source.
<lang pli>100H: /* READ A SPECIFIC LINE FROM A FILE */
DECLARE FALSE LITERALLY '0', TRUE LITERALLY '0FFH';
DECLARE NL$CHAR LITERALLY '0AH'; /* NEWLINE: CHAR 10 */
DECLARE EOF$CHAR LITERALLY '26'; /* EOF: CTRL-Z */
/* CP/M BDOS SYSTEM CALL, RETURNS A VALUE */
BDOS: PROCEDURE( FN, ARG )BYTE; DECLARE FN BYTE, ARG ADDRESS; GOTO 5; END;
/* CP/M BDOS SYSTEM CALL, NO RETURN VALUE */
BDOS$P: PROCEDURE( FN, ARG ); DECLARE FN BYTE, ARG ADDRESS; GOTO 5; END;
EXIT: PROCEDURE; CALL BDOS$P( 0, 0 ); END; /* CP/M SYSTEM RESET */
PR$CHAR: PROCEDURE( C ); DECLARE C BYTE; CALL BDOS$P( 2, C ); END;
PR$STRING: PROCEDURE( S ); DECLARE S ADDRESS; CALL BDOS$P( 9, S ); END;
PR$NL: PROCEDURE; CALL PR$STRING( .( 0DH, NL$CHAR, '$' ) ); END;
PR$NUMBER: PROCEDURE( N ); /* PRINTS A NUMBER IN THE MINIMUN FIELD WIDTH */
DECLARE N ADDRESS;
DECLARE V ADDRESS, N$STR ( 6 )BYTE, W BYTE;
V = N;
W = LAST( N$STR );
N$STR( W ) = '$';
N$STR( W := W - 1 ) = '0' + ( V MOD 10 );
DO WHILE( ( V := V / 10 ) > 0 );
N$STR( W := W - 1 ) = '0' + ( V MOD 10 );
END;
CALL PR$STRING( .N$STR( W ) );
END PR$NUMBER;
FL$EXISTS: PROCEDURE( FCB )BYTE; /* RETURNS TRUE IF THE FILE NAMED IN THE */
DECLARE FCB ADDRESS; /* FCB EXISTS */
RETURN ( BDOS( 17, FCB ) < 4 );
END FL$EXISTS ;
FL$OPEN: PROCEDURE( FCB )BYTE; /* OPEN THE FILE WITH THE SPECIFIED FCB */
DECLARE FCB ADDRESS;
RETURN ( BDOS( 15, FCB ) < 4 );
END FL$OPEN;
FL$READ: PROCEDURE( FCB )BYTE; /* READ THE NEXT RECORD FROM FCB */
DECLARE FCB ADDRESS;
RETURN ( BDOS( 20, FCB ) = 0 );
END FL$READ;
FL$CLOSE: PROCEDURE( FCB )BYTE; /* CLOSE THE FILE WITH THE SPECIFIED FCB */
DECLARE FCB ADDRESS;
RETURN ( BDOS( 16, FCB ) < 4 );
END FL$CLOSE;
PR$BYTE: PROCEDURE( B ); /* PRINT B EITHER AS A CHAR OR IN HEX */
DECLARE B BYTE;
PR$HEX: PROCEDURE( H ); /* PRINT A HEX DIGIT */
DECLARE H BYTE;
IF H < 10 THEN CALL PR$CHAR( H + '0' );
ELSE CALL PR$CHAR( ( H - 10 ) + 'A' );
END PR$HEX;
IF B >= ' ' AND B < 7FH AND B <> '$' THEN DO;
/* PRINTABLE CHAR AND NOT A $, SHOW AS A CHARACTER */
CALL PR$CHAR( B );
END;
ELSE DO;
/* NON-PRINTING CHAR OR $ - SHOW IN HEX */
CALL PR$CHAR( '$' );
CALL PR$HEX( SHR( B, 4 ) );
CALL PR$HEX( B AND 0FH );
END;
END PR$BYTE;
/* I/O USES FILE CONTROL BLOCKS CONTAINING THE FILE-NAME, POSITION, ETC. */ /* WHEN THE PROGRAM IS RUN, THE CCP WILL FIRST PARSE THE COMMAND LINE AND */ /* PUT THE FIRST PARAMETER IN FCB1, THE SECOND PARAMETER IN FCB2 */ /* BUT FCB2 OVERLAYS THE END OF FCB1 AND THE DMA BUFFER OVERLAYS THE END */ /* OF FCB2, SO WE NEED TO GET THE LINE NUMBER FROM FCB2 FIRST */
DECLARE FCB$SIZE LITERALLY '36'; /* SIZE OF A FCB */ DECLARE FCB1 LITERALLY '5CH'; /* ADDRESS OF FIRST FCB */ DECLARE FCB2 LITERALLY '6CH'; /* ADDRESS OF SECOND FCB */ DECLARE DMA$BUFFER LITERALLY '80H'; /* DEFAULT DMA BUFFER ADDRESS */ DECLARE DMA$SIZE LITERALLY '128'; /* SIZE OF THE DMA BUFFER */
DECLARE F$PTR ADDRESS, F$CHAR BASED F$PTR BYTE;
/* GET THE LINE NUMBER FROM THE SECOND PARAMETER */
DECLARE LINE$NUMBER ADDRESS;
LINE$NUMBER = 0;
DO F$PTR = FCB2 + 1 TO FCB2 + 8;
IF F$CHAR >= '0' AND F$CHAR <= '9' THEN DO;
LINE$NUMBER = ( LINE$NUMBER * 10 ) + ( F$CHAR - '0' );
END;
END;
/* CLEAR THE PARTS OF FCB1 OVERLAYED BY FCB2 */
DO F$PTR = FCB1 + 12 TO FCB1 + ( FCB$SIZE - 1 );
F$CHAR = 0;
END;
/* SHOW THE REQUIRED LINE FROM THE FILE, IF THE FILE AND LINE EXIST */
IF NOT FL$EXISTS( FCB1 ) THEN DO; /* THE FILE DOES NOT EXIST */
CALL PR$STRING( .'FILE NOT FOUND$' );CALL PR$NL;
END;
ELSE IF NOT FL$OPEN( FCB1 ) THEN DO; /* UNABLE TO OPEN THE FILE */
CALL PR$STRING( .'UNABLE TO OPEN THE FILE$' );CALL PR$NL;
END;
ELSE DO; /* FILE EXISTS AND OPENED OK - ATTEMPT TO FIND THE LINE */
DECLARE LN ADDRESS, GOT$RCD BYTE, GOT$LINE BYTE, DMA$END ADDRESS;
DMA$END = DMA$BUFFER + ( DMA$SIZE - 1 );
GOT$RCD = FL$READ( FCB1 ); /* GET THE FIRST RECORD */
GOT$LINE = FALSE;
F$PTR = DMA$BUFFER;
LN = 1;
CALL PR$NUMBER( LINE$NUMBER ); CALL PR$STRING( .': <$' );
DO WHILE( GOT$RCD AND LN <= LINE$NUMBER );
IF F$PTR > DMA$END THEN DO;
/* AT THE END OF THE BUFFER - GET THE NEXT RECORD */
GOT$RCD = FL$READ( FCB1 );
F$PTR = DMA$BUFFER;
END;
ELSE IF F$CHAR = NL$CHAR THEN DO; /* END OF LINE */
LN = LN + 1;
IF LN = LINE$NUMBER THEN GOT$LINE = TRUE;
END;
ELSE IF F$CHAR = EOF$CHAR THEN GOT$RCD = FALSE; /* END OF FILE */
ELSE IF LN = LINE$NUMBER THEN CALL PR$BYTE( F$CHAR );
F$PTR = F$PTR + 1;
END;
CALL PR$CHAR( '>' ); CALL PR$NL;
/* SHOULD NOW HAVE EOF OR THE END OF THE REQUIRED LINE */
IF NOT GOT$LINE THEN DO;
/* COULDN'T READ THE SPECIFIED LINE */
CALL PR$STRING( .'CANNOT READ LINE $' );
CALL PR$NUMBER( LINE$NUMBER );
END;
/* CLOSE THE FILE */
IF NOT FL$CLOSE( FCB1 ) THEN DO;
CALL PR$STRING( .'UNABLE TO CLOSE THE FILE$' ); CALL PR$NL;
END;
END;
CALL EXIT;
EOF</lang>
PowerShell
<lang Powershell> $file = Get-Content c:\file.txt if ($file.count -lt 7) {Write-Warning "The file is too short!"} else {
$file | Where Readcount -eq 7 | set-variable -name Line7
} </lang>
PureBasic
<lang purebasic>Structure lineLastRead
lineRead.i line.s
EndStructure
Procedure readNthLine(file, n, *results.lineLastRead)
*results\lineRead = 0 While *results\lineRead < n And Not Eof(file) *results\line = ReadString(file) *results\lineRead + 1 Wend If *results\lineRead = n ProcedureReturn 1 EndIf
EndProcedure
Define filename.s = OpenFileRequester("Choose file to read a line from", "*.*", "All files (*.*)|*.*", 0) If filename
Define file = ReadFile(#PB_Any, filename)
If file
Define fileReadResults.lineLastRead, lineToRead = 7
If readNthLine(file, lineToRead, fileReadResults)
MessageRequester("Results", fileReadResults\line)
Else
MessageRequester("Error", "There are less than " + Str(lineToRead) + " lines in file.")
EndIf
CloseFile(file)
Else
MessageRequester("Error", "Couldn't open file " + filename + ".")
EndIf
EndIf </lang>
Python
Using only builtins (note that enumerate is zero-based):
<lang python>with open('xxx.txt') as f:
for i, line in enumerate(f):
if i == 6:
break
else:
print('Not 7 lines in file')
line = None</lang>
Using the islice iterator function from the itertools standard library module, which applies slicing to an iterator and thereby skips over the first six lines:
<lang python>from itertools import islice
with open('xxx.txt') as f:
try:
line = next(islice(f, 6, 7))
except StopIteration:
print('Not 7 lines in file')</lang>
Similar to the Ruby implementation, this will read up to the first 7 lines, returning only the last. Note that the 'readlines' method reads the entire file contents into memory first as opposed to using the file iterator itself which is more performant for large files. <lang python> print open('xxx.txt').readlines()[:7][-1] </lang>
R
<lang R>> seven <- scan('hw.txt', , skip = 6, nlines = 1, sep = '\n') # too short Read 0 items > seven <- scan('Incoming/quotes.txt', , skip = 6, nlines = 1, sep = '\n') Read 1 item </lang>
Racket
<lang Racket>
- lang racket
- simple, but reads the whole file
(define s1 (list-ref (file->lines "some-file") 6))
- more efficient
- read and discard n-1 lines
(define s2
(call-with-input-file "some-file" (λ(i) (for/last ([line (in-lines i)] [n 7]) line))))
</lang>
Raku
(formerly Perl 6) <lang perl6>say lines[6] // die "Short file";</lang> Without an argument, the lines function reads filenames from the command line, or defaults to standard input. It then returns a lazy list, which we subscript to get the 7th element. Assuming this code is in a program called line7:
$ cal 2011 > cal.txt $ line7 cal.txt 16 17 18 19 20 21 22 20 21 22 23 24 25 26 20 21 22 23 24 25 26 $
This works even on infinite files because lists are lazy:
$ yes | line7 y $
REBOL
<lang rebol> x: pick read/lines request-file/only 7 either x [print x] [print "No seventh line"] </lang>
Red
<lang Red>>> x: pick read/lines %file.txt 7
case [
x = none [print "File has less than seven lines"] (length? x) = 0 [print "Line 7 is empty"] (length? x) > 0 [print append "Line seven = " x]
]</lang>
REXX
for newer REXXes
<lang REXX>/*REXX program reads a specific line from a file (and displays the length and content).*/ parse arg FID n . /*obtain optional arguments from the CL*/ if FID== | FID=="," then FID= 'JUNK.TXT' /*not specified? Then use the default.*/ if n== | n=="," then n=7 /* " " " " " " */
if lines(FID)==0 then call ser "wasn't found." /*see if the file exists (or not). */
call linein FID, n-1 /*read the record previous to N. */ if lines(FID)==0 then call ser "doesn't contain" N 'lines.'
/* [↑] any more lines to read in file?*/
$=linein(FID) /*read the Nth record in the file. */
say 'File ' FID " line " N ' has a length of: ' length($) say 'File ' FID " line " N 'contents: ' $ /*display the contents of the Nth line.*/ exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ ser: say; say '***error!*** File ' FID " " arg(1); say; exit 13</lang>
for older REXXes
Some older REXXes don't support a 2nd argument for the linein BIF, so here is an alternative: <lang REXX>/*REXX program reads a specific line from a file (and displays the length and content).*/ parse arg FID n . /*obtain optional arguments from the CL*/ if FID== | FID=="," then FID= 'JUNK.TXT' /*not specified? Then use the default.*/ if n== | n=="," then n=7 /* " " " " " " */
if lines(FID)==0 then call ser "wasn't found." /*see if the file exists (or not). */
do n-1 call linein FID /*read all the lines previous to N. */ end /*n-1*/
if lines(FID)==0 then call ser "doesn't contain" N 'lines.'
/* [↑] any more lines to read in file?*/
$=linein(FID) /*read the Nth record in the file. */
say 'File ' FID " line " N ' has a length of: ' length($)
say 'File ' FID " line " N 'contents: ' $ /*display the contents of the Nth line.*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
ser: say; say '***error!*** File ' FID " " arg(1); say; exit 13</lang>
Ring
<lang ring> fp = fopen("C:\Ring\ReadMe.txt","r") n = 0
r = "" while isstring(r)
while n < 8
r = fgetc(fp)
if r = char(10) n++ see nl
else see r ok
end
end fclose(fp) </lang>
Ruby
The each_line method returns an Enumerator, so no more than seven lines are read. <lang ruby> seventh_line = open("/etc/passwd").each_line.take(7).last </lang>
Run BASIC
<lang runbasic>fileName$ = "f:\sample.txt" requiredLine = 7 open fileName$ for input as #f
for i = 1 to requiredLine
if not(eof(#f)) then line input #f, a$
next i close #f print a$ end</lang>
Rust
<lang rust>use std::fs::File; use std::io::BufRead; use std::io::BufReader; use std::io::Error; use std::path::Path;
fn main() {
let path = Path::new("file.txt");
let line_num = 7usize;
let line = get_line_at(&path, line_num - 1);
println!("{}", line.unwrap());
}
fn get_line_at(path: &Path, line_num: usize) -> Result<String, Error> {
let file = File::open(path).expect("File not found or cannot be opened");
let content = BufReader::new(&file);
let mut lines = content.lines();
lines.nth(line_num).expect("No line found at that position")
}</lang>
Alternate implementation with argument parsing. First argument is the path to the file and is required. Second argument is the line number and is optional. By default the first line will be printed. <lang rust>use std::env; use std::fs::File; use std::io::BufRead; use std::io::BufReader; use std::path::Path;
fn main() {
if env::args().len() <= 1 {
println!("At least a path to a file is needed: No file path given");
return;
} else {
let path = &env::args().nth(1).expect("could not parse the path");
let path = Path::new(&path);
let mut line_num = 1usize;
if let Some(arg) = env::args().nth(2) {
line_num = arg.parse::<usize>().expect("Parsing line number failed");
}
print_line_at(&path, line_num);
}
}
fn print_line_at(path: &Path, line_num: usize) {
if line_num < 1 {
panic!("Line number has to be > 0");
}
let line_num = line_num - 1;
let file = File::open(path).expect("File not found or cannot be opened");
let content = BufReader::new(&file);
let mut lines = content.lines();
let line = lines.nth(line_num).expect("No line found at given position");
println!("{}", line.expect("None line"));
}</lang>
Scala
Discussion
The code will throw a NoSuchElementException if the file doesn't have 7 lines.
<lang scala>val lines = io.Source.fromFile("input.txt").getLines val seventhLine = lines drop(6) next</lang>
Imperative version
Solving the task to the letter, imperative version:
<lang scala>var lines: Iterator[String] = _ try {
lines = io.Source.fromFile("input.txt").getLines drop(6)
} catch {
case exc: java.io.IOException =>
println("File not found")
} var seventhLine: String = _ if (lines != null) {
if (lines.isEmpty) println("too few lines in file")
else seventhLine = lines next
} if ("" == seventhLine) println("line is empty")</lang>
Functional version (Recommanded)
<lang scala>val file = try Left(io.Source.fromFile("input.txt")) catch {
case exc => Right(exc.getMessage)
} val seventhLine = (for(f <- file.left;
line <- f.getLines.toStream.drop(6).headOption.toLeft("too few lines").left) yield
if (line == "") Right("line is empty") else Left(line)).joinLeft</lang>
sed
To print the seventh line: <lang sed>sed -n 7p</lang> To print an error message, if no such line: <lang sed>sed '7h;$!d;x;s/^$/Error: no such line/'</lang> That is we remember (h) the line, if any, in hold space. At the last line ($) we exchange (x) pattern space and hold space. If the hold space was empty, replace it by an error message. (Does not work with empty input, because there is no last line.)
Seed7
The function getLine skips lines with readln and reads the requested line with getln afterwards:
<lang seed7>$ include "seed7_05.s7i";
const func string: getLine (inout file: aFile, in var integer: lineNum) is func
result
var string: line is "";
begin
while lineNum > 1 and hasNext(aFile) do
readln(aFile);
decr(lineNum);
end while;
line := getln(aFile);
end func;
const proc: main is func
local
var string: fileName is "input.txt";
var file: aFile is STD_NULL;
var string: line is "";
begin
aFile := open(fileName, "r");
if aFile = STD_NULL then
writeln("Cannot open " <& fileName);
else
line := getLine(aFile, 7);
if eof(aFile) then
writeln("The file does not have 7 lines");
else
writeln("The 7th line of the file is:");
writeln(line);
end if;
end if;
end func;</lang>
SenseTalk
SenseTalk's chunk expressions and ability to treat a file directly as a container make this trivial. If the file has fewer than 7 lines, theLine will be empty. <lang sensetalk>put line 7 of file "example.txt" into theLine </lang>
Sidef
<lang ruby>func getNthLine(filename, n) {
var file = File.new(filename);
file.open_r.each { |line|
Num($.) == n && return line;
}
warn "file #{file} does not have #{n} lines, only #{$.}\n";
return nil;
}
var line = getNthLine("/etc/passwd", 7); print line if defined line;</lang>
Smalltalk
<lang smalltalk> line := (StandardFileStream oldFileNamed: 'test.txt') contents lineNumber: 7. </lang>
SPL
<lang spl>lines = #.readlines("test.txt")
- .output("Seventh line of text:")
? #.size(lines,1)<7
#.output("is absent")
!
#.output(lines[7])
.</lang>
Stata
See use in Stata help, to load a dataset or a part of it.
<lang stata>* Read rows 20 to 30 from somedata.dta . use somedata in 20/30, clear
- Read rows for which the variable x is positive
. use somedata if x>0, clear</lang>
If there are not enough lines, an error message is print. It's possible to capture the error and do something else:
<lang stata>capture use somedata in 7, clear if _rc { display "Too few lines" }</lang>
Tcl
This code can deal with very large files with very long lines (up to 1 billion characters in a line should work fine, provided enough memory is available) and will return an empty string when the nth line is empty (as an empty line is still a valid line). <lang tcl>proc getNthLineFromFile {filename n} {
set f [open $filename]
while {[incr n -1] > 0} {
if {[gets $f line] < 0} {
close $f
error "no such line"
}
}
close $f
return $line
}
puts [getNthLineFromFile example.txt 7]</lang>
Where it is necessary to provide very fast access to lines of text, it becomes sensible to create an index file describing the locations of the starts of lines so that the reader code can seek directly to the right location. This is rarely needed, but can occasionally be helpful.
TorqueScript
%file = new fileObject();
%file.openForRead("File/Path.txt");
$seventhLine = "";
while(!%file.isEOF())
{
%line++;
if(%line == 7)
{
$seventhLine = %file.readLine();
if($seventhLine $= "")
{
error("Line 7 of the file is blank!");
}
}
}
%file.close();
%file.delete();
if(%line < 7)
{
error("The file does not have seven lines!");
}
TUSCRIPT
<lang tuscript>$$ MODE TUSCRIPT file="lines.txt" ERROR/STOP OPEN (file,READ,-std-) line2fetch=7</lang>
TXR
From the top
Variable "line" matches and takes eighth line of input: <lang txr>@(skip nil 7) @line</lang>
From the bottom
Take the third line from the bottom of the file, if it exists.
<lang txr>@(skip)
@line
@(skip 1 2)
@(eof)</lang>
How this works is that the first skip will skip enough lines until the rest of the query successfully matches the input. The rest of the query matches a line, then skips two lines, and matches on EOF. So @line can only match at one location: three lines up from the end of the file. If the file doesn't have at least three lines, the query fails.
UNIX Shell
<lang bash>get_nth_line() {
local file=$1 n=$2 line
while ((n-- > 0)); do
if ! IFS= read -r line; then
echo "No such line $2 in $file"
return 1
fi
done < "$file"
echo "$line"
}
get_nth_line filename 7</lang>
Ursa
<lang ursa>decl string<> lines decl file f f.open "filename.txt" set lines (f.readlines) f.close
if (< (size lines) 7)
out "the file has less than seven lines" endl console
stop
end if
out "the seventh line in the file is:" endl endl console out lines<6> endl console</lang>
VBScript
<lang vb> Function read_line(filepath,n) Set objFSO = CreateObject("Scripting.FileSystemObject") Set objFile = objFSO.OpenTextFile(filepath,1) arrLines = Split(objFile.ReadAll,vbCrLf) If UBound(arrLines) >= n-1 Then If arrLines(n-1) <> "" Then read_line = arrLines(n-1) Else read_line = "Line " & n & " is null." End If Else read_line = "Line " & n & " does not exist." End If objFile.Close Set objFSO = Nothing End Function
WScript.Echo read_line("c:\temp\input.txt",7) </lang>
- Input:
1st 22d 3rd 4th 5th 6th 7th 8th
- Output:
7th
Vedit macro language
This example reads the 7th line (including newline character(s)) into text register 10.
<lang vedit>File_Open("example.txt", BROWSE) Goto_Line(7) if (Cur_Line < 7) {
Statline_Message("File contains too few lines")
} else {
if (At_EOL) {
Statline_Message("Empty line")
}
Reg_Copy(10, 1)
} Buf_Close(NOMSG) </lang>
If the file does not exist, the buffer will be empty and you get "File contains too few lines" error.
If the line is too long (more than about 230,000 characters), Vedit displays error message "Block too large for text registers, try clipboard"). This error could be avoided by reading the line to clipboard (which has larger size limit) or by copying the line to another edit buffer using a tmp file (in which case there is no size limit).
Wren
<lang ecmascript>import "io" for File
var lines = File.read("input.txt").replace("\r", "").split("\n") if (lines.count < 7) {
System.print("There are only %(lines.count) lines in the file")
} else {
var line7 = lines[6].trim()
if (line7 == "") {
System.print("The seventh line is empty")
} else {
System.print("The seventh line is : %(line7)")
}
}
/* Note that 'input.txt' contains the eight lines: Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8
- /</lang>
- Output:
The seventh line is : Line 7
XPL0
Filename.ext must be terminated with an EOF character (hex 1A). Usage: readline <filename.ext
<lang XPL0>include c:\cxpl\codes; \intrinsic 'code' declarations def MaxLen = 82; \maximum length of line that can be stored (incl CR+LF)
func ReadLine(N, L); \Read line N from input file and return it in string L int N; char L; int I, C; [for I:= 1 to N-1 do \skip to start of specified line
repeat C:= ChIn(1);
if C = $1A\EOF\ then
[Text(0, "File only has "); IntOut(0, I);
Text(0, " lines^M^J"); return false];
until C = $0A\LF\;
I:= 0; repeat C:= ChIn(1);
if C = $1A\EOF\ then
[Text(0, "Line is empty (EOF)^M^L"); return false];
L(I):= C; I:= I+1;
until C=$0A\LF\ or I>=MaxLen; if I >= MaxLen then Text(0, "Line might be truncated^M^J"); if I = 2 then Text(0, "Line is empty^M^J"); L(I-1):= L(I-1) ! $80; \terminate string return true; ];
char LineN(MaxLen); if ReadLine(7, LineN) then Text(0, LineN)</lang>
zkl
Many zkl sequence objects contain a readln method, some contain a seek (or equivalent) method. However, File only has readln. If, for some, reason, the nth line can't be read, an exception is thrown. <lang zkl>reg line; do(7){line=File.stdin.readln()} println(">>>",line);</lang> Or, suck in lines and take the last one: <lang zkl>lines:=File.stdin.readln(7); println(">>>",line[-1]);</lang>
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