Since you need to know the number of the lines in the file,
No, you don't. There's a fair and linear way to pick a random element from a stream of unknown length, and its algorithm (which can be implemented as a one-liner) has been displayed in a post before yours.

Detailed analysis can be found in The Art of Computer Programming, by Donald Knuth. Volume 1, IIRC.

In reply to Re^2: Opening random files then reading random lines from file. by JavaFan
in thread Opening random files then reading random lines from file. by fame

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