Summation in a for loop without cumulitive

I have a very simple question.

I want to summate the variable A for a certain interval + and - B.

for i=(b+1):length((a)-b)

C(i) = sum(a(i-b):(i+b));

end

This loop works, however the output that I get is a cumulative sum of all the previous. So I get an exponential increase. However I would like to just have the sum over the interval -b:b for all values of i. So no cummulation.

Answer 1

[Iain]

 C(i) = sum( a([i-b]:[i+b]) )

That puts into C(i), the sum of the (i-b)th index of a, to the (i+b)th index of a.

What you had, put into C(i), the sum of the (i-b)th index of a to (i+b)

[Ot]

Based on your remark I changed it to:

for i=(b+1):length((a)-b)

C(i) = sum(a([i-b]:[i+b]))

end

However now I get an error: ??? Index exceeds matrix dimensions.

[Iain]

If i+b is bigger than the length of a, you'll get that error.

length((a)-b) should be (length(a)-b)

Answer 2

[Jos (10584)]

Another solution:

% hide the for-loop
% A is the array, B is a scalar
A = randperm(10)
B = 2
C1 = arrayfun(@(x) sum(A(x-B:x+B)), B+1:numel(A)-B)

% and yet another solution
cumA = cumsum(A)
C2 = cumA(2*B+1:end) - [0 cumA(1:end-2*B)]