• C

Random line from text file


I just need to read a random line from a text file, say quotes.txt for exmaple. This is for a program I'm going to be running under Linux in the Bash shell, so I imagine using /dev/urandom or /dev/random to seed would work well enough. Also, this does not need to be an entire program, I only require a simple function for this, which would return only one line each time the function is called.

Thank you.
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Jaime OlivaresSoftware ArchitectCommented:
You have 2 paths for this, in my opinion:
- Non-indexed, to produce a random number, you have to know how many lines are present. To do this, you need to scan all text file counting newline characters. Then you can produce a random number. To retrieve selected line, you have to read all the file, line by line, up to reach the desired line.
- Indexed, more complex but far faster. You need to create a binary index first, where each record is the offset where first character of each line is located at text file. Number of lines is the division of index_file_size by sizeof_record (let's say 4), don't need to scan all text file to know. Chosse a random number, go to index file, obtain desired line index, go to text file and directly read line at proper location.

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devolved99Author Commented:
Thanks jaime.
/* a one pass method */
FILE *f = fopen("file","r");
if( !f ){ perror("file"); exit(1); }
char in[1000],out[1000];
int line = 0;
while( fgets(rand()%++line?in:out,100,f) ){}
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Kent OlsenDBACommented:

I know that I'm a late arrival, but since you're in a unix/linus environment, why not let the shell do this for you?

# Pick a line number

#Get the line
head -n $LINE SomeFile|tail -n 1

devolved99Author Commented:

Indeed Kent, that is what I tried doing, which worked just as well. However, in the end I chose a simple Perl script for the job.

Here is what I am now using:


for($x=0;$x<=4;$x++) {
 open(plist,"proxylist.txt") or die "$!";
  $rprox = $file[int rand $nlines];
  print "$rprox";
Kent OlsenDBACommented:


Getting a random number within a range is not straight-forward in KSH.  The rest is a piece of cake.

Glad it worked out for you,

You can do it without opening proxylist.txt 4 times.
Do you want to allow the possibility of repeated lines?
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