How to feed a for loop with its own output in Python?

Question

I have simple problem that I cannot seem to implement. I have a for loop in python that is iterated over a list of tuples and output a list of tuples. All I want to do is to pass the output list of tuples to the aforementioned for loop and keep doing that for a number of times (I decide that).

So, I have a set of individuals in a population, called gen1. It looks like this:

Generation 1 population is [(214, 66, 229, 87, 259), (256, 11, 249, 75, 242), (247, 41, 206, 4, 194), (214, 61, 172, 59, 225), (270, 78, 238, 69, 261), (235, 22, 175, 31, 213), (272, 37, 222, 80, 222), (233, 70, 207, 33, 218), (227, 38, 244, 47, 229), (264, 76, 246, 52, 239), (273, 2, 201, 10, 273), (215, 85, 205, 4, 199), (268, 9, 235, 51, 195), (236, 53, 198, 43, 273), (224, 27, 212, 29, 236), (275, 64, 190, 62, 257), (226, 23, 178, 44, 202), (219, 19, 196, 8, 227), (227, 60, 238, 15, 213), (216, 62, 185, 73, 192)]

Now, I write a for loop as follows:

for Po in gen1:
    *do some operation, including crossover*
     Ouptput gen2

I believe this is similar to recursive function in python, but instead of writing a function, I just want to update the for loop multiple times with its own output. How to achieve this?

Perhaps, try to give an example of what you're actually trying to achieve. — avloss, Sep 06 '21 at 22:47
possible duplicates: [here](https://stackoverflow.com/questions/15363138/scope-of-python-variable-in-for-loop)[, and here](https://stackoverflow.com/questions/15902835/changing-iteration-variable-inside-for-loop-in-python) [, and possibly here](https://stackoverflow.com/questions/14588367/update-iteration-value-in-python-for-loop) (: — coder, Sep 06 '21 at 23:08

score 2 · Answer 1 · answered Sep 06 '21 at 23:12

I mean you can just wrap a for loop around it:

n = ... # insert the number of generations
for i in range(n):
    print(f"generation {i}:")
    for Po in current_generation:
        *do some operation, including crossover*

So after the loop current generation is the updated generation (as you've updated the values in that array). Though if you're unsure about the number of generations you can also use a while True: loop and break once you've reached a certain treshold or what do you intent to do with this?

Rather than using an obfuscated `while True:` with `break`, you could use `while :` where the condition is easily understandable (for instance, the condition could be a boolean variable with an explicit name) — Stef, Sep 07 '21 at 08:00

score 0 · Answer 2 · answered Sep 06 '21 at 23:17

Short answer, use recursion, it is going to be much easier.

def recur(param, curr_depth=0, max_recur=20):
    if(curr_depth >= max_recur or curr_depth > 900): ## Max Depth
       return param

   else:
       ## Do something
       pass
       ## print Output
       pass
       return recur(param, curr_depth + 1, max_recur)

Otherwise if you absolutely need for it not to use recursion, then use a while loop instead of a for, as a for loop cannot be changed mid-execution.

def fun(initial, max_depth=20):

   finished = False

   generation = initial

   depth = 0

   while(not finished):

       for f in generation:
           pass

       # Assign the new result
       generation = []

       if(depth > max_depth or True): ## some condition indicating you are done
           finished = True

How to feed a for loop with its own output in Python?

2 Answers2