MATLAB: Find the first element that satisfy a condition in a vector

Question

Hello! I have a homework: if v is [1 2 3 4 5 4 3 2 1] and n is 3, it will find 4 5 and 4 because their sum of 13 is the largest of any 3 consecutive elements of v. The function returns summa, the sum as the first output argument and index, the index of the first element of the n consecutive ones as the second output.

for example

• [summa, index] = max_sum([1 2 3 4 5 4 3 2 1],3)
• summa = 13
• index = 4
• [summa, index] = max_sum([1 2 3 4 5 4 3 2 1],1)
• summa = 5
• index = 5
• [summa, index] = max_sum([1 2 3 4 5 4 3 2 1],9)
• summa = 25
• index = 1

Then I have written the code. The summa Part is correct but the index part ist wrong. index is always equal to v(k-n+1). Could someone give me some idea?

Here is the result of my code:

• max_sum([1 2 3 4 5 4 3 2 1],1)
• summa =5
• index =1

function [summa,index] = max_sum(v,n)
k = length(v);
a = zeros(1,k-n+1);
if isscalar(n) == 1 && n > 0 && n == round(n)
    if n <= k
        for ii = 1:(k-n+1)
            b = sum(v(ii:ii+(n-1))); 
            a(ii) = b;
            summa = max(a);
        end
        index = v(ii);
    else
        summa = 0;
        index = -1;
    end
else
    fprintf('wrong');
end

Answer 1

function [summa, index] = max_sum(A,n)
    B = movsum(A(:),[0 n-1]);
    summa = max(B);
    index = find(summa == B);
end

or without movsum:

function [summa, index] = max_sum(A,n)
    B = A((1:n) + (0:numel(A)-n)');
    summa = max(B);
    index = find(summa == B);
end