One of n lines in a file
One of n lines in a file is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
A method of choosing a line randomly from a file:
- Without reading the file more than once
- When substantial parts of the file cannot be held in memory
- Without knowing how many lines are in the file
Is to:
- keep the first line of the file as a possible choice, then
- Read the second line of the file if possible and make it the possible choice if a uniform random value between zero and one is less than 1/2.
- Read the third line of the file if possible and make it the possible choice if a uniform random value between zero and one is less than 1/3.
- ...
- Read the Nth line of the file if possible and make it the possible choice if a uniform random value between zero and one is less than 1/N
- Return the computed possible choice when no further lines exist in the file.
- Task
- Create a function/method/routine called
one_of_n
that givenn
, the number of actual lines in a file, follows the algotrithm above to return an integer - the line number of the line chosen from the file.
The number returned can vary, randomly, in each run. - Use
one_of_n
in a simulation to find what woud be the chosen line of a 10 line file simulated 1,000,000 times. - Print and show how many times each of the 10 lines is chosen as a rough measure of how well the algorithm works.
Note: You may choose a smaller number of repetitions if necessary, but mention this up-front.
Python
<lang python>from random import random as rnd
def one_of_n(n):
# Zero based line numbers choice = 0 for i in range(1, n): if rnd() < 1. / (i + 1.): choice = i return choice
def one_of_n_test(n=10, trials=1000000):
bins = [0] * n if n: for i in range(trials): bins[one_of_n(n)] += 1 return bins
print(one_of_n_test())</lang>
- Sample output
[99833, 100303, 99902, 100132, 99608, 100117, 99531, 100017, 99795, 100762]