Read a specific line from a file

Read a specific line from a file is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

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,

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.

<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.

Perl 6

<lang perl6>say lines[6];</lang> This works even on infinite files because lists are lazy:

$ yes | line7
y
$

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>

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.