Cumulative Sum of a Vector - Syntax

Question

I am trying to resolve why the following Matlab syntax does not work.

1. I have an array A = [2 3 4 5 8 9...]
2. I wish to create an indexed cumulative, for

Answer 1

You are asking two questions, really. One is - how do I compute the cumulative sum. @SouldEc's answer already shows how the cumsum function does that. Your other question is

Can someone please explain why the following does not work

x = 1:10
s(x) = sum(A(1:x))

It is reasonable - you think that the vector expansion should turn

1:x

into

1:1
1:2
1:3
1:4

etc. But in fact the arguments on either side of the colon operator must be scalars - they cannot be vectors themselves. I'm surprised that you say Matlab isn't throwing an error with your two lines of code - I would have expected that it would (I just tested this on Freemat, and it complained...)

So the more interesting question is - how would you create those vectors (if you didn't know about / want to use cumsum)?

Here, we could use arrayfun. It evaluates a function with an array as input element-by-element; this can be useful for a situation like this. So if we write

x = 1:10;
s = arrayfun(@(n)sum(A(1:n)), x);

This will loop over all values of x, substitute them into the function sum(A(1:n)), and voila - your problem is solved.

But really - the right answer is "use cumsum()"...