Question

This question was given to me during an interview. The interview is long over, but I’m still thinking about hte problem and its bugging me:

You have a language that contains the following tools: a rand() function, while and for loops, if statements, and a readline() method (similar to python’s readline()). Given these tools, write an algorithm that returns a random line in the file. You don’t know the size of the file, and you can only loop over the file’s contents once.

Answer 1

I don’t know the desired answer, but my solution would be the following:

chosen_line = ""
lines = 0

while (current_line = readline()):
    if (rand(0, lines) == 0):
        chosen_line = current_line

    lines++

return chosen_line

Edit: A good explanation why this works was posted in this comment.