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. The task is to demonstrate how to obtain the contents of a specific line within a file. For the purpose of this task demonstrate how to the contents of the seventh line of a file can be obtained, and store this 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. Note that for functional languages or languages without variables or storage, it is permissible to output the extracted data to standard output.
Aime
<lang aime>void read_line(text &line, text path, integer n) {
file f;
f_affix(f, path);
while (n) {
n -= 1; f_slip(f);
}
f_line(f, line);
}
integer
main(void)
{
if (2 < argc()) {
text line;
read_line(line, argv(1), 6);
o_text(line); o_byte('\n');
}
return 0;
}</lang>
AutoHotkey
<lang AutoHotkey> FileReadLine, OutputVar, filename.txt, 7 if ErrorLevel
MsgBox, There was an error reading the 7th line of the file
</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(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 * garanteed 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>
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() \ 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
The code does not check if the passed file exists.
example: java -cp . LineNbr7 LineNbr7.java
output : line 7: BufferedReader br = new BufferedReader(fr);
<lang java>import java.io.FileReader;
import java.io.BufferedReader;
public class LineNbr7 { public static void main(String[] args) throws Exception { FileReader fr = new FileReader(args[0]); BufferedReader br = new BufferedReader(fr);
String line = null; int lineNbr = 1; while ((line = br.readLine()) != null && lineNbr++ < 7);
if (lineNbr == 1 && line == null) { System.out.println("the file has zero length"); } else if (lineNbr == 7 && line != null) { System.out.println("line 7: " + line); } else { System.out.println("the file has only " + (lineNbr - 1) + " line(s)"); } br.close(); } } </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>
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>
Perl
<lang perl>sub get_line {
my ($line, $file) = @_;
open(my $fh, '<', $file);
my $line_content;
while (<$fh>) {
if ($. == $line) {
if (not /^$/) {
$line_content = $_;
last;
}
else {
warn "Line $. is an empty line\n";
last;
}
}
elsif (eof($fh) and $. < $line) {
warn "File contains only $. line(s)\n";
last;
}
}
close $fh;
return defined $line_content ? $line_content : undef;
}
my $file = '/etc/passwd'; my $wanted_line = get_line(7, $file); print $wanted_line;</lang>
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 $
PicoLisp
<lang PicoLisp>(in "file.txt"
(do 6 (line)) (or (line) (quit "No 7 lines")) )</lang>
Python
<lang python>from itertools import islice
with open('xxx.txt') as f:
linelist = list(islice(f, 7, 8)) assert linelist != [], 'Not 7 lines in file' line = linelist[0]</lang>
Scala
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>
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: result is "";
begin
while lineNum > 1 and hasNext(aFile) do
readln(aFile);
decr(lineNum);
end while;
result := 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>
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.
TUSCRIPT
<lang tuscript> $$ MODE TUSCRIPT file="lines.txt" ERROR/STOP OPEN (file,READ,-std-) line2fetch=7
--> solution 1 ACCESS file: READ/RECORDS/UTF8 $file s,line LOOP n=1,99 READ/NEXT/EXIT file IF (n==line2fetch) PRINT line ENDLOOP ENDACCESS file
--> solution 2 line1to7=FILE (file,#line2fetch) line=SELECT (line1to7,#line2fetch) PRINT line </lang> Output:
line7 line7