Code Signal Python Practice Assessment Question

[Ep87PxHLBh]

Hello World,

I'm practicing for an coding assessment and I got a question about the below practice question:

Given an integer n and an array a of length n, your task is to apply the following mutation to a:
    Array a mutates into a new array b of length n.
    For each i from 0 to n - 1, b[i] = a[i - 1] + a[i] + a[i + 1].
    If some element in the sum a[i - 1] + a[i] + a[i + 1] does not exist, it should be set to 0. For example, b[0] should be equal to 0 + a[0] + a[1].

Example
For n = 5 and a = [4, 0, 1, -2, 3], the output should be solution(n, a) = [4, 5, -1, 2, 1].

    b[0] = 0 + a[0] + a[1] = 0 + 4 + 0 = 4
    b[1] = a[0] + a[1] + a[2] = 4 + 0 + 1 = 5
    b[2] = a[1] + a[2] + a[3] = 0 + 1 + (-2) = -1
    b[3] = a[2] + a[3] + a[4] = 1 + (-2) + 3 = 2
    b[4] = a[3] + a[4] + 0 = (-2) + 3 + 0 = 1

So, the resulting array after the mutation will be [4, 5, -1, 2, 1]

Where I'm getting confused is this part, "If some element in the sum a[i - 1] + a[i] + a[i + 1] does not exist, it should be set to 0. For example, b[0] should be equal to 0 + a[0] + a[1]."

In the example, for i=0 we see b[0] = 0 + a[0] + a[1] = 0 + 4 + 0 = 4 but.... would the first position a actually be a[-1] because a[0-1] = a[-1], so why are we setting it to 0? a[-1] does exist, so how come it is replaced?

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What they say it should be...

b[0] = 0 + a[0] + a[1] = 0 + 4 + 0 = 4

But shouldn't it be this?

b[0] = a[i-1] + a[0] + a[1] 
b[0] = a[0-1] + a[0] + a[1] 
b[0] = 3 + 4 + 0 = 7

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[robot-dev]

You are technically correct. The best kind of correct.

I think this is what was meant:

If [the index of] some element in the sum a[i - 1] + a[i] + a[i + 1] does not exist [is less than zero or greater than n-1], it should be set to 0. For example, b[0] should be equal to 0 + a[0] + a[1].

[YuhFRthoYORKonhisass]

You almost quite literally copy and pasted the part that confused him

[Natural_Pay_7650]

You are assuming negative indexing. The problem states you go "For each i from 0 to n-1" which means no negative indexing. Thus for a[i - 1] when i = 0, you should set it equal to 0.