How to add every nth element of an array to the nth+1 element?

I wonder if there is a short expression for the task above. For example if I had an array like this

a = [1, 2, 3, 3, 4, 5, 5, 6, 7, 7, 8, 9]

How can I add, let's say every 3rd element to every 4th element to get an array like this ?

a = [1, 2, 3+3, 4, 5+5, 6, 7+7, 8, 9]

I can do this with some ugle lines of code, but what if you got a more elegant way?

Answer 1

[Voss]

a = [1, 2, 3, 3, 4, 5, 5, 6, 7, 7, 8, 9];

a(3:3:end-1) = a(3:3:end-1)+a(4:3:end);
a(4:3:end) = []

a = 1x9

1 2 6 4 10 6 14 8 9

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[Niklas Kurz]

haha, this is exactly the same solution that I got, but for me this looks really inconvenient.

Or do you really need to index like that?

[Voss]

Generating a vector of indices using the colon operator is the standard way to get every nth element or every nth+1 element. I can't think of a better way.

Maybe storing the indices would make the code a little neater/clearer in your opinion:

a = [1, 2, 3, 3, 4, 5, 5, 6, 7, 7, 8, 9];

idx = 3:3:numel(a)-1;
a(idx) = a(idx)+a(idx+1);
a(idx+1) = []

a = 1x9

1 2 6 4 10 6 14 8 9

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Or maybe you prefer this:

a = [1, 2, 3, 3, 4, 5, 5, 6, 7, 7, 8, 9];

tmp = a(4:3:end);
a(4:3:end) = [];
a(3:2:end-1) = a(3:2:end-1)+tmp

a = 1x9

1 2 6 4 10 6 14 8 9

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[Niklas Kurz]

Allright thanks. I'll just create a function to make it cleaner and hide the operations.

[Voss]

Sounds good!