Could you please explain me?

Question

Why is the output [1,0,0,0]6

What is the output of this code?
A=[1,2,3,4]
pos, sum=0,0
for val in A[:Len(A)-1]:
A[pos+1]=0
sum=sum+val
pos=pos+1
print(A,sum)

Answer 1

1. ok I understood how we got "3" but what is 0 about?

Arrays (or lists) start counting from index 0. It can be omitted when slicing a list. I just mentioned it to get a full picture.

2. how do you know that it relates to EVERY element beyond 0, why not only to "[1]" cz "+1"

It is inside a for loop. You access each element by its index of A[pos+1]. So you work with A[1], A[2] and A[3] that correspond to [2, 3, 4].
For every element in this range you assign element's value to 0, thus making a list [0, 0, 0]. When you print the list, you concatate the unchanged value of [1], to the 'new' list [0, 0, 0], getting [1, 0, 0, 0].

3. I didn't get it at all, are these indexes of the list? And how do you know then that it's about indexes and not about the [1,2,3,4] itself?

When you want to access an index of a list, you use the syntax element[index]. That's why you access elements by index in a previous step, with A[pos].
When you initiate a for loop, you name elements as val. Values or vals in list correspond to 1, 2, 3, 4 respectively.
You declare a variable sum with value of 0 first.
First iteration: 0 = 0 + 1 (element with index A[0] has a value of 1)
Second iteration: 1 = 1 + 2 (element with index A[1] has a value of 2, which add to a sum that we got from before)
Third iteration: 3 = 3 + 3
There should've been a fourth iteration but we limit our range to range(len(A) - 1). Thus limiting the number of iterations to - 1, in our case 3.
That's how you get the sum of 6.

Answer 2

1) For val in A[0:3] # ok I understood how we got "3" but what is 0 about?
2) A[pos+1] # how do you know that it relates to EVERY element beyond 0, why not only to "[1]" cz "+1"
3) sum=sum+val is the same as sum=0+1+2+3=>6 # I didn't get it at all, are these indexes of the list? And how do you know then that it's about indexes and not about the [1,2,3,4] itself?

Sorry for bothering and thank you for your help

Answer 3

Oh, man, thank you very much!!!

Answer 4

len(A) = 4 # 4 elements in a list.
This way it can be presented as:
for val in A[0:3]
sum = sum+val is the same as sum = 0 + 1 + 2 + 3 => 6
A[pos+1] = 0 # You assign the value of 0 to every element with index beyond 0
A[0] = 1, A[1] = 0, A[2] = 0, A[3] = 0
Combine that and we get:
[1, 0, 0, 0] and a sum, which is 6.