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Python for each element in a list add the value of previous index and next index

November 30, 2022

For each element in a list I want to add the value before and after the element and append the result to an empty list. The problem is that at index 0 there is no index before and at the end there is no index next. At index 0 I want to add the value of index 0 with value of index 1, and in the last index I want to add the value of the last index with the same index value. As following:

vec = [1,2,3,4,5]
newVec = []

for i in range(len(vec)):
    newValue = vec[i] + vec[i+1] + vec[i-1]
    # if i + 1 or i - 1 does now exist pass 
    newVec.append(newValue)

 Expected output: newVec = [1+2, 2+1+3, 3+2+4,4+3+5,5+4]

 # newVec = [3, 6, 9, 12, 9]

>Solution :

You have possible exceptions here, I think this code will do the trick and manage the exceptions.

vec = [1, 2, 3, 4, 5]
new_vec = []
for index, number in enumerate(vec):
    new_value = number
    if index != 0:
        new_value += vec[index - 1]
    try:
        new_value += vec[index + 1]
    except IndexError:
        pass
    new_vec.append(new_value)

Your output will look like this: