ANSI C: How to split a string by newline and get a random line

Question

I'm a new to C and got stuck with subj. I can split string with strtok but I don't know how to get a random token.

Thanks.

Answer 1

You can parse it twice, then get a random number and pick one, which you collect on the second pass of the same string.

Or, you can do it in a single pass if you use reservoir sampling.

Mastering reservoir sampling will be a very useful way of learning C as a side to learning some maths! :)

Answer 2

The following pseudocode shows how to return a candidate uniformly selected among the tokens of the string:

string result = null;
int tokens = 0;
while (true) {
  string candidate = next token;
  if (candidate does not exist) break;
  tokens = tokens + 1;
  if ((a random integer selected between 0 and tokens-1) == 0) result = token;
}
return result;

This is a special case of Algorithm R from section 3.4.2 of Volume II of Knuth's The Art of Computer Programming.