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()"...

Answer 2

Actually what you are doing is s(1:10)= sum(A(1:[1,2,3...10])) what you should do is

for i=1:10
    s(i)=sum(A(1:i))
end

hope it will help you

Answer 3

For calculating the cumulative sum, you should be using cumsum:

>> A = [2 3 4 5 8 9]

A =

     2     3     4     5     8     9

>> cumsum(A)

ans =

     2     5     9    14    22    31

The issue is that 1:x is 1 and that sum reduces linear arrays. To do this properly, you need a 2d array and then sum the rows:

s(x)=sum(triu(repmat(A,[prod(size(A)) 1])'))