How to Loop x Times With Functional Style?

Question

I have this recursive function:

def lifecycle(population):
    ...
    lifecycle(new_population)

I want to perform this loop 100 times. I want to use correct functional style. So, I cannot use a counter? Because assigning new values to a variable is not okay in functional programming. How does one do this functionally? I'm using Python.

[Pythonic way to loop n times](http://stackoverflow.com/questions/2970780/pythonic-way-to-do-something-n-times) — Robert Harvey, Nov 27 '12 at 17:59
[Functional Programming - for and while loops](http://stackoverflow.com/questions/10981037/functional-programming-for-and-while-loops) — Robert Harvey, Nov 27 '12 at 18:03

score 4 · Accepted Answer · answered Nov 27 '12 at 18:03

Technically, like this:

def lifecycle(population, i):
    if i == 100:
        return ...
    ...
    return lifecycle(new_population, i + 1)

But this is a really, really silly thing to do in Python (and it fails for all but the smallest examples, due to tail calls being not optimized). Just use a loop, although that uses mutable state under the hood, the remaining program can be perfectly functional.

Emil Vikström · Answer 2 · 2012-11-27T18:03:42.243

2

You do it recursively, and by adding a decreasing (or increasing) loop variable.

def lifecycle(population, n):
    if n == 0:
        return population
    ...
    return lifecycle(new_population, n-1)

You must return the population if you want it to be truly functional because functional programming frowns upon side-effects.

edited Nov 27 '12 at 18:03

answered Nov 27 '12 at 17:58

Emil Vikström

90,431
16
141
175

score 1 · Answer 3 · answered Nov 27 '12 at 17:58

1

Something like this for example:

def lifecycle(population, n, maxN):
    if n >= maxN: return
    new_population = population
    lifecycle(new_population, n+1, maxN)

lifecycle(['an initial population or something'], 0, 1000)

answered Nov 27 '12 at 17:58

piokuc

25,594
11
72
102

How to Loop x Times With Functional Style?

3 Answers3