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Asked: 2026-05-10T21:42:01+00:00

What’s the best way to return a random line in a text file using

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What’s the best way to return a random line in a text file using C? It has to use the standard I/O library (<stdio.h>) because it’s for Nintendo DS homebrew.

Clarifications:

  • Using a header in the file to store the number of lines won’t work for what I want to do.
  • I want it to be as random as possible (the best being if each line has an equal probability of being chosen as every other line.)
  • The file will never change while the program is being run. (It’s the DS, so no multi-tasking.)
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  1. Read each line, and use a random number to choose whether to keep that line or ignore it. For the first line, you want odds of 1:1 to keep; for the second, you want odds of 1:2, etc.

    count = 0; while (fgets(line, length, stream) != NULL) {     count++;     if ((rand() * count) / RAND_MAX == 0)         strcpy(keptline, line); }

    I haven’t verified that this has the proper random qualities, but it seems right at first glance.

    It has been pointed out that integer overflow would quickly become a problem with the way the comparison is coded, and I had independently reached the same conclusion myself. There are probably many ways to fix it, but this is the first that comes to mind:

    if ((rand() / (float)RAND_MAX) <= (1.0 / count))
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