MATLAB: I need some help in seeing where I am going wrong and how to proceed with writing a particular funciton for a MATLAB course I am taking please.

courserahomeworkwhile loop

The question is this:
Write a function called max_sum that takes v, a row vector of numbers, and n, a positive integer as inputs. The function needs to find the n consecutive elements of v whose sum is the largest possible. In other words, 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. If multiple such sequences exist in v, max_sum returns the first one. 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. If the input n is larger than the number of elements of v, the function returns 0 as the sum and -1 as the index. Here are a few example runs: 
[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],2) 
    summa = 9 
    index = 4
THe code I have written so far is as follows. My problem is that I am stuck on obtaining the largest possible sum. I wrote code and have been able to generate different sums in a vector but I am unable to put those answers in a vector themselves, by putting the answers in a vector I want to be able to use the 'max' function to obtain the largest sum. I haven't even touched finding the 'index' portion yet. Here's my code:
function [summa,index] = max_sum(v,n)
r = 0;
summa = zeros(1,length(v)); %% created a vector of 0's to length of v
if n > length(v)
    summa = 0;
    index = -1;
else
    while n <= length(v)
        summa = sum(v(r+1:n)) % here need to replace 0's with the sum (with logical index) so i can choose max
        r = r+1;
        n = n+1;
    end
    
    summa = max(summa);
    
    index = n;
end
%summa = v;
%index = n;
end
Again, I need some help because I am unsure how to proceed from here. Thank you.

Best Answer

Some issues:
n is a fixed input ... you should not be changing n inside your function. Get rid of that n = n + 1 statement.
The values you are summing v(r+1:n) isn't correct. You need to be summing n consecutive numbers. So, assuming the first index of these numbers is r, the subset would be v(r:r+n-1)
The loop needs to run as long as that last index of r+n-1 isn't greater than numel(v).
Prior to the loop, you should initialize summa and index to default values (e.g., maybe summa = -inf, or perhaps the sum of the first sequence of numbers in the vector). The summa variable is a scalar value (see the examples), not a vector, so you shouldn't be initializing it to a vector of zeroes.
Inside the loop you should have something like this logic:
if( the sum of the v(r:r+n-1) elements is greater than the current value of summa )
    save the sum in summa and save the r in index
end
Give a try at making these changes and then post replies on this thread if you have more problems.
Related Question