How to randomly select lines from a text file


 
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# 1  
How to randomly select lines from a text file

I have a text file with 1000 lines, I want to randomly select 200 lines from it and print them as output. How do I go about doing that? Thanks!
# 2  
Your time would be better spent learning awk than asking dozens of questions which could be trivially answered in it.

Code:
awk 'NR==FNR { B++; next }
(NR != FNR) && (!Z) {
        srand();
        if(B < 200) exit(1); # Too few lines
        for(N=1; N<=200; )
        {
                V=sprintf("%d", (rand()*B)+1)+0;
                if(!(V in A)) { A[V]=1; N++ }
        }
        Z=1
} NR in A' inputfile inputfile


Last edited by Corona688; 10-25-2012 at 01:34 PM.. Reason: Fixed typos
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# 3  
For the fun of it here's another way that does not use awk although the awk version will be more efficient. This has the overhead of creating the pipeline repeatedly which should be avoided for good practice. Also I believe the ksh RANDOM built-in has a limit of 32767 that must be considered if the file is large.
Code:
$ cat x
##
## x nbr_of_lines_wanted  filename
##
#!/bin/ksh

iterations=$1
file="$2"

((lines_avail=$(wc -l < "$file")+1))

while (( $iterations > 0 )); do
  head -$((${RANDOM} % $lines_avail)) "$file" | tail -1
  (( iterations=$iterations - 1 ))
done

exit 0

This is actually a good example of how a seemingly simple solution for a small file can end up burning you on performance and system limitations should you need to run it on a much larger file
or a system that may see increased load in the future.
Typically when you see a long command line or pipeline like this being done a large number of times (especially a user-enterable number of times) it should
be a red flag warning that there will most likely be a more efficient way of structuring the program.

Last edited by gary_w; 10-23-2012 at 06:11 PM..
This User Gave Thanks to gary_w For This Post:
# 4  
Hi.

There are a number of commonly-available utilities to do this. Here is a demonstration of two:
Code:
#!/usr/bin/env bash

# @(#) s1	Demonstrate random selection of lines with rl, shuf.

pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && $C rl shuf

pl " Lines selected from:"
cat data0

# Prepare data file from single line of words.
tr ' ' '\n' < data0 > data1

pl " Results from shuf, 1:"
shuf -n 3 data1

pl " Results from shuf, 2:"
shuf -n 3 data1

pl " Results from rl, 1:"
rl -c 3 data1

pl " Results from rl, 2:"
rl -c 3 data1

exit 0

producing:
Code:
% ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0.8 (lenny) 
bash GNU bash 3.2.39
rl 0.2.7
shuf (GNU coreutils) 6.10

-----
 Lines selected from:
foo bar baz qux quux corge grault garble warg fred plugh xyzzy thud

-----
 Results from shuf, 1:
quux
thud
qux

-----
 Results from shuf, 2:
thud
xyzzy
quux

-----
 Results from rl, 1:
corge
qux
grault

-----
 Results from rl, 2:
thud
corge
warg

You may need to install these from your distribution repository. See man pages for details.

See also Algorithm::Numerical::Sample - search.cpan.org if a perl module is desirable.

Best wishes ... cheers, drl
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# 5  
In the order of lines in the file, without all lines in memory:
Code:
awk '
  NR==FNR { next }
  FNR==1{
    srand;
    n=NR-1
    for(i=1; i<=200; i++) {
      line=0
      while(!line || line in A) line=int(rand*n)+1
      A[line]
    }
  } 
  FNR in A
' infile infile


In the order of the selection, with all lines in the file in memory..
Code:
awk '
  { R[NR]=$0 }
  END{
    srand;
    n=NR
    for(i=1; i<=200; i++) {
      line=0
      while(!line || line in A) line=int(rand*n)+1
      A[line]
      print R[line]
    }
  } 
' infile

# 6  
Code:
cat file | head -n 500 | tail -n 200

A very simple idea but not for random lines.
# 7  
Quote:
Originally Posted by Corona688
Code:
awk 'NR==FNR { B++; next }
(NR != FNR) && (!Z) {
        srand();
        if(Z < 200) exit(1); # Too few lines
        for(N=1; N<=200; N++)
        {
                V=sprintf("%d", (rand()*B)+1)+0;
                if(!(V in A)) { A[V]=1; N++ }
        }
        Z=1
} NR in A' inputfile inputfile

Looks like there are a couple of bugs there. The first, a typo of Z for B, always exits before selecting anything. The second, the for-loop post-increment expression, can cause the algorithm to yield anywhere between 1 and 200 lines.

Probably no need to fix it since Scrutinizer's first example in post #5 is a correct implementation of the same approach.

Regards,
Alister

---------- Post updated at 06:28 PM ---------- Previous update was at 06:26 PM ----------

With regard to all of the AWK suggestions, without knowing exactly how the script is to be used, it's possible that all of the recommendations are inadequate. Nearly every awk srand implementation's default seed is the number of seconds since the epoch. Successive or simultaneous runs could yield identical results. May or may not be an issue. We don't have sufficient information to make that determination. Just a head's up for the OP.

If it is an issue, more information would be required to determine a robust seed expression.

Regards,
Alister
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