Read a specific line from a file

From Rosetta Code
Jump to: navigation, search
Task
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 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. Note that for functional languages or languages without variables or storage, it is permissible to output the extracted data to standard output.

Contents

[edit] Ada

A github repository was created for this solution https://github.com/mulander/read_a_specific_line_from_a_file

The repository contains test cases for specific file types the program might be run against.

Based on the task description the following requirements were identified

  • Display a specific message for a file having fewer than 7 lines
  • Display a specific message if the 7th line is empty
  • Display a specific message if the 7th line is too big to be retrieved and stored in memory
  • Acquire the 7th line of a file, using built-in language features/standard libraries if accessible
  • Store the retrieved line in a variable/memory location

The following undefined behavior was identified and addressed

  • The task description does not define an empty line. The program assumes that an empty line contains only a line break character. A line containing whitespace (' ', '\t') is not considered empty.
  • Things considered an error in execution are printed out to stderr
  • In case of an encountered error the return code of the program is changed to Failure (1)

In order to ease testing - the program expects the file name to be provided as the first argument on the command line.

Additional checks are performed by the program

  • Tests if the file name was provided on the command line
  • Tests if the it is possible to open the provided file with read access
with Ada.Command_Line,
Ada.Text_IO;
 
procedure Rosetta_Read is
use Ada.Command_Line, Ada.Text_IO;
 
Source : File_Type;
begin
 
if Argument_Count /= 1 then
Put_Line (File => Standard_Error,
Item => "Usage: " & Command_Name & " file_name");
Set_Exit_Status (Failure);
return;
end if;
 
declare
File_Name : String renames Argument (Number => 1);
begin
Open (File => Source,
Mode => In_File,
Name => File_Name);
exception
when others =>
Put_Line (File => Standard_Error,
Item => "Can not open '" & File_Name & "'.");
Set_Exit_Status (Failure);
return;
end;
 
Set_Line (File => Source,
To => 7);
 
declare
Line_7 : constant String := Get_Line (File => Source);
begin
if Line_7'Length = 0 then
Put_Line ("Line 7 is empty.");
else
Put_Line (Line_7);
end if;
end;
exception
when End_Error =>
Put_Line (File => Standard_Error,
Item => "The file contains fewer than 7 lines.");
Set_Exit_Status (Failure);
return;
when Storage_Error =>
Put_Line (File => Standard_Error,
Item => "Line 7 is too long to load.");
Set_Exit_Status (Failure);
return;
end Rosetta_Read;

[edit] 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;
}

[edit] ALGOL 68

Works with: ALGOL 68G version Any - tested with release 2.8.win32
# 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:
BEGIN
 
FILE input file;
 
line := "";
err := "";
 
IF 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 #
 
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
 
END; # 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
 
)
Output:
line seven is: "                          , INT        number        # line 7 #"

[edit] AutoHotkey

FileReadLine, OutputVar, filename.txt, 7
if ErrorLevel
MsgBox, There was an error reading the 7th line of the file

[edit] 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"}

[edit] BBC BASIC

      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$

[edit] 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.

#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;
}

[edit] 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.

#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;
}
 

[edit] C++

#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 ;
}
}

[edit] 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))))
Output:
user=> (read-nth-line "/tmp/test.txt" 7)
"foo"

[edit] Common 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))))
 

Example call:

CL-USER> (read-nth-line "/tmp/test1.text" 7)
"foo"

[edit] C#

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.");
}
}
}

[edit] 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);
}
}
line 7:     foreach (char[] line; f.byLine()) {

[edit] 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?

 
-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.
 
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)

[edit] F#

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

[edit] 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
}

[edit] Groovy

def line = null
new File("lines.txt").eachLine { currentLine, lineNumber ->
if (lineNumber == 7) {
line = currentLine
}
}
println "Line 7 = $line"

[edit] 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"

[edit] 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.

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

[edit] J

readLine=: 4 :0
(x-1) {:: <;.2 ] 1!:1 boxxopen y
)

Thus:

$ cal 2011 > cal.txt
   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

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

require 'files'     NB. required for versions before J701
readLineT=: <:@[ {:: 'b'&freads@]

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.

[edit] Java

example: java -cp . LineNbr7 LineNbr7.java
output : line 7: public static void main(String[] args) throws Exception {;

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)");
}
}
}
}

[edit] 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.

open(readlines, "path/to/file")[7]

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.

function read_nth_lines(stream, num)
for i = 1:num-1
readline(stream)
end
result = readline(stream)
print(result != "" ? result : "No such line.")
end
Output:
julia> open(line -> read_nth_lines(line, 7), "path/to/file")
"Hi, I am the content of the seventh line\n"


[edit] 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
 

[edit] Lua

iter = io.lines 'test.txt'
for i=0, 5 do
if not iter() then
error 'Not 7 lines in file'
end
end
 
line = iter()

[edit] 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.

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$

[edit] Mathematica

 If[# != EndOfFile , Print[#]]& @ ReadList["file",  String, 7] 

[edit] MATLAB / Octave

 
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);
 

Insert non-formatted text here

[edit] MoonScript

Translation of: Lua
iter = io.lines 'test.txt'
for i=0, 5
error 'Not 7 lines in file' if not iter!
 
print iter!

[edit] 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)
 

[edit] Nimrod

var
line: TaintedString
f = open("test.txt", fmRead)
 
for x in 0 .. 6:
try:
line = readLine f
except EIO:
echo "Not 7 lines in file"

[edit] 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:

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))

[edit] 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.

[edit] Pascal

Works with: Free_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.

Output:

line 7 from file test

[edit] Perl

#!/usr/bin/perl -s
# invoke as <scriptname> -n=7 [input]
while (<>) { $. == $n and print, exit }
die "file too short\n";

[edit] Perl 6

say lines[6] // die "Short file";

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
$

[edit] PicoLisp

(in "file.txt"
(do 6 (line))
(or (line) (quit "No 7 lines")) )

[edit] PL/I

 
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: ;
 

[edit] PowerShell

Works with: PowerShell version 3.0
 
$file = Get-Content c:\file.txt
if ($file.count -lt 7)
{Write-Warning "The file is too short!"}
else
{
$file | Where Readcount -eq 7 | set-variable -name Line7
}
 

[edit] Python

Using only builtins (note that enumerate is zero-based):

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

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:

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')

[edit] 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

[edit] 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
 

[edit] 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))))
 

[edit] REXX

/*REXX program to read a  specific line  from a file.                   */
parse arg fileId n . /*get the user args: fileid n */
if fileID=='' then fileId='JUNK.TXT' /*assume fileID default: JUNK.TXT*/
if n=='' then n=7 /*assume N default: 7 */
L=lines(fileid) /*first, see if the file exists. */
if L==0 then do; say 'error, fileID not found:' fileId; exit; end
q=linein(fileId, n) /*read the Nth line, store in Q.*/
if length(q)==0 then say 'line' n "not found."
else say 'file' fileId "record" n '=' q
/*stick a fork in it, we're done.*/

[edit] Ruby

The each_line method returns an Enumerator, so no more than seven lines are read.

 seventh_line = open("/etc/passwd").each_line.take(7).last
 

[edit] Run BASIC

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

[edit] Scala

The code will throw a NoSuchElementException if the file doesn't have 7 lines.

val lines = io.Source.fromFile("input.txt").getLines
val seventhLine = lines drop(6) next

Solving the task to the letter, imperative version:

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")

Functional version:

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

[edit] sed

To print seventh line

 
sed -n 7p
 

To print error message if no such line

 
sed -n '7{p;h;}; ${x;/^$/s/^/Error: no such line/p}'
 

That is we remember (h) the line, if any, in hold space. At last line ($) we exchange (x) pattern space and hold space. If hold space was empty -- print error message.

[edit] Seed7

The function getLine skips lines with readln and reads the requested line with getln afterwards:

$ 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;

[edit] Sidef

func getNthLine(filename, n) {
var file = File.new(filename);
file.open_r.each { |line|
$. == n && return line;
};
Sys.warn("file #{file} does not have #{n} lines, only #{$.}\n");
return nil;
}
 
var wantedLine = getNthLine("/etc/passwd", 7);
wantedLine != nil && print wantedLine;

[edit] Smalltalk

 
line := (StandardFileStream oldFileNamed: 'test.txt') contents lineNumber: 7.
 

[edit] 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).

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]

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.

[edit] 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!");
 }

[edit] TUSCRIPT

$$ MODE TUSCRIPT
file="lines.txt"
ERROR/STOP OPEN (file,READ,-std-)
line2fetch=7

[edit] TXR

[edit] From the top

Variable "line" matches and takes eighth line of input:

@(skip nil 7)
@line

[edit] From the bottom

Take the third line from the bottom of the file, if it exists.

@(skip)
@line
@(skip 1 2)
@(eof)

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.

[edit] UNIX Shell

Translation of: Tcl
Works with: 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

[edit] Vedit macro language

This example reads the 7th line (including newline character(s)) into text register 10.

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)

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).

[edit] XPL0

Filename.ext must be terminated with an EOF character (hex 1A). Usage: readline <filename.ext

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)

[edit] 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.

reg line; do(7){line=File.stdin.readln()} println(">>>",line);

Or, suck in lines and take the last one:

lines:=File.stdin.readln(7); println(">>>",line[-1]);
Personal tools
Namespaces

Variants
Actions
Community
Explore
Misc
Toolbox