How do nested for loops work?

Question

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

etc.

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

In fact, the code I believe should act as:

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

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 rows? How did it order the numbers as it has, what relation acutally happened there?

Answer 1

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 i is 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 be 2 , 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

Answer 2

You've got a loop within your loop, so this inner loop will run in its entirety for each iteration of the outer loop. So for each number of i, you need to go through the entire loop for x.

Answer 3

This:

box = [1, 2, 3, 4, 5]
for i in box:
    for x in box:
        print (i,x)

Is the same as this:

i = 1
for x in box:
    print (i,x)
i = 2
for x in box:
    print (i,x)
i = 3
for x in box:
    print (i,x)
i = 4
for x in box:
    print (i,x)
i = 5
for x in box:
    print (i,x)

Answer 4

If this is what you want

1 1
2 2
3 3
4 4
5 5

then do this:

for i in box:
    print(i,i)

If you do this:

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

then after the outer loop is executed once, it will execute the inner loop until the inner loop is COMPLETE. That means for 1 outer loop, inner loop will be executed for 5 times.
the logic is like this:

• i will retrieve the value of the first element of the box arrray
• x will retrieve the value of the first element of the box arrray
• Then 1 1 will be printed
• x will retrieve the value of the second element of the box array
• Then 1 2 will be printed
• x will retrieve the value of the third element of the box array
• Then 1 3 will be printed. And so on until 1 5 is printed
• i will retrieve the value of the second element of the box array
• x will retrieve the value of the first element of the box arrray
• Then 2 1 will be printed
• x will retrieve the value of the second element of the box array
• Then 2 2 will be printed and so on