michigansqllite123 michigansqllite123 - 2 years ago 72
Python Question

How do nested for loops work?

This is the code I wrote, works perfectly fine:

box = [1, 2, 3, 4, 5]

for i in box:
for x in box:
print (i,x)

It outputs the following:

1 1
1 2
1 3
1 4
1 5
2 1
2 2


The funny part is: I got exactly what I want, however I really don't understand the logic. For the code I wrote, I would expect the following output:

1 1
2 2
3 3
4 4
5 5

Why? Because by my idea I stated the following:

For each value
(left column) in box and for each value
(right column) print both.

In fact, the code I believe should act as:

for i,y in zip(box,box):

My question is, why did the nested for-loop in the function behave differently then I expected?

How did it know to create
number * number
How did it order the numbers as it has, what relation acutally happened there?

Answer Source

It is logic. for i in box, i will start being 1 and moving forward until being 5 (all elements from the list). So if you print i inside this loop, you will see that i is first 1, then i is 2 ... until i is 5 (last element from box). But if you nest another for loop, it will do the same (start from 1 and moving forward until 5) for each time i is a different element from the list. So when i is 1, before changing to another different i it will start the second loop (for x in box), so then when iis 1 x will change being x == 1, x == 2... x == 5. When this nested loop finishes, then i changes to another element from the list, so now i would be2, and the nested loop starts again, so x == 1, x == 2..., x == 5.

I think you might understand how it works if you try this

box = [1, 2, 3, 4, 5]

for i in box: #it will go through all elements in the list
    print i, "this is the first loop" #for each different 'i' in box
    for x in box: #it will go through all elements in the list
        print x, "this is the second loop" #you will get all elements from the box
        print (i,x) #this is what you get
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