Pick a random item from a list, prioritize items with lower index.

Random Termial

#!/usr/bin/env python3

import math
import random
import sys


def random_termial(n: int) -> int:
    """Get index to an item from a list of size n.

    The higher index of an item is, the lower probability of picking it.
    """
    assert n >= 1
    termial_of_n = ((n**2) + n) >> 1
    choice = random.randrange(termial_of_n)
    i = math.floor(-0.5 + math.sqrt(0.25 + (choice << 1)))
    return n - 1 - i


def main() -> None:
    if len(sys.argv) == 1:
        print(f"Usage: {sys.argv[0]} [N..]", file=sys.stderr)
        sys.exit(1)

    for item in sys.argv[1:]:
        n = int(item)   # Any exception reported.
        res = random_termial(n)
        print(f"Random index using random termial to a list of size {n} is {res}")


__name__ == "__main__" and main()
                           

A blog post explaining this gist is available on my blog.