Variable number of nested for loops in Python

Question

I am having trouble getting this to work and any help would be greatly appreciated. I want to have a variable number of nested for loops for the following code. The idea is to write every combination possible to a csv file.

here is my code: `

ka = [0.217, 0.445]
kb = [0.03, 0.05]
kc = [10]
kd = [0.15625, 0.7]
ke = [1.02, 0.78]
La = [0.15, 0.25]
Lb = [0.025, 0.075]
tc = [0.002, 0.007]
Ld = [0.025, 0.115]
Le = [0.07, 0.2]

NUMBER_OF_VARIABLES = 10

with open('test.csv', 'w') as file:
    writer = csv.writer(file, lineterminator = '\n')
    row = [0] * len(NUMBER_OF_VARIABLES)
    
    
    for E in Le:
        for D in Ld:                                    
            for C in tc:
                for B in Lb:
                    for A in La:
                        for e in ke:
                            for d in kd:
                                for c in kc:
                                    for b in kb:
                                        for a in ka:
                                            row[0] = a
                                            row[1] = b
                                            row[2] = c
                                            row[3] = d
                                            row[4] = e
                                            row[5] = A
                                            row[6] = B
                                            row[7] = C
                                            row[8] = D
                                            row[9] = E
                                            writer.writerow(row)

` the idea is I would like to be able to add more or remove variables. the k and L of each letter are related. For example to add another variable would include a Lf and kf. I would like to do it without manually adding more loops. The variable structure does not have to remain if it would be better to make it one list.

I feel like I need to write a recursive function but am having trouble figuring this out, any help would be greatly appreciated.

I have tried importing a csv file where each line has a variable but can not figure out the variable number of for loops.

Tim Roberts · Accepted Answer · 2022-11-17 19:36:43Z

1

What you need is itertools.product. It will handle all of this for you.

import itertools
ka = [0.217, 0.445]
kb = [0.03, 0.05]
kc = [10]
kd = [0.15625, 0.7]
ke = [1.02, 0.78]
La = [0.15, 0.25]
Lb = [0.025, 0.075]
tc = [0.002, 0.007]
Ld = [0.025, 0.115]
Le = [0.07, 0.2]

for row in itertools.product(ka,kb,kc,kd,ke,La,Lb,tc,Ld,Le):
    writer.writerow(row)

You can probably even do that in a single line:

writer.writerows(itertools.product(ka,kb,kc,kd,ke,La,Lb,tc,Ld,Le))

answered Nov 17, 2022 at 19:36

Tim Roberts

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2 Comments

Charlie Over a year ago

Thanks, this really cleans up my code. How would I modify this to do the same with a list of lists. ie. vars = [[1, 2], [3,4]...] where each list would be representative of ka, kb, etc.

Tim Roberts Over a year ago

You would read the documentation for itertools.product.... The way you convert a list to a list of positional arguments is through *lst: itertools.product(*lst).

jsbueno · Accepted Answer · 2022-11-17 19:48:56Z

Try using itertools.product:

ka = [0.217, 0.445]
kb = [0.03, 0.05]
kc = [10]
kd = [0.15625, 0.7]
ke = [1.02, 0.78]
La = [0.15, 0.25]
Lb = [0.025, 0.075]
tc = [0.002, 0.007]
Ld = [0.025, 0.115]
Le = [0.07, 0.2]

from itertools import product
with open('test.csv', 'w') as file:
    writer = csv.writer(file, lineterminator = '\n')
    writer.writerows(product(ka, kb, kc, kd, ke, La,Lb, tc, Ld, Le)

As you can see - Python does have built-in tools to deal with this situations. Otherwise, if the iterttols package was not there, the way to do this kind of thing is by using functions, and calling then recursively - something like

def product(*args):
    if not args: return []
    remainder = product(*args[1:])
    result = []
    for item in args[0]:
        if remainder:
            for part in remainder:
                row = [item, *part]
                result.append(row)
        else:
            result.append([item,])
    return result

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