 # Python for loop

old = 0
list1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
list2 =
for i in list1:
list2.append(i + old)
old = i
print(list2)

I do not understand why the answer below is correct.

[0, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

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For example (this is not code, it’d be by hand):

``````first iteration:
old = 0
i = 0
list2.append(0) #0+0
(list2=0)

2nd iteration:
old = 0
i = 1
list2.append(1) #0+1
(list=[0,1])

3rd iteration:
old=1
i=2
list2.append(3) #1+2
(list=[0,1,3])
``````
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How did you know when old = 1 in the third iteration?

Can you explain why we equal to zero in the if statement?

Each time the `for` loop iterates, `old` is assigned the value of `i`. The first time the loop iterates, `i = 0` (the first element in `list1`) and `old = 0` as defined in the first line, so the value of `0 + 0` is appended. Then, we assign `old` the value of `i`, which was `0` since it is still the first iteration of the loop.

The second time the loop iterates `i = 1` and `old = 0`, so the value of `1 + 0` is appended. Then, we once again assign `old` the value of `i`, where now `i = 1` (the second element in `list1`).

At this point, `old = 1`. The third time the loop iterates, `i = 2` and `old = 1` so the value of `2 + 1` is appended. This continues until the last element of `list1` has been iterated over.

1 Like

I presume you are referring to the following line.

``````if i % 10 == 0:
``````

The modulo operator returns the remainder of one number divided by another. In this case, it returns the remainder of `i` divided by `10`. If a number is divisible by 10, it should have no remainder.

``````30 / 10 == 3       ->       # is divisible by 10 (remainder is 0)
22 / 10 = 2.2      ->       # is not divisible by 10 (remainder is 2)
``````

Since we are trying to find out if a certain number is divisible by 10, and numbers that are divisible by 10 have a remainder of 0 when divided by 10, we can use an `if` statement to check if a number modulo 10 is equal to 0.

2 Likes

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