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.

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.

Answer 1

[James Tursa]

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.

[Alexander Villanueva]

Thank you for the clarity, I have updated the code and had another issue regarding giving me the first index if there are consecutive sums but I was able to figure that part out. However another issue popped up. The problem is that if 'n' is greater than numel(v), the answer is supposed to be summa = 0 and index = -1. However, when I do a test run where n is greater than the length(v), that answer does not appear, rather it does give summa = 0 and index = -1 but there is also an error that appears alongside it that says "Index exceeds the number of array elements". I thought the reason for having my first "if' statement was to take care of this issue yet it does not appear to be working. Here's the updated code:

function [summa,index] = max_sum(v,n)

r = 1;

summa = 0;

index = 0;

if n >= length(v)

summa = 0

index = -1

else

while n < numel(v)

sum(v(r:r+n-1));

r = r+1;

if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))

r=r-1;

break

elseif sum(v(r:r+n-1)) > sum(v(r+1:r+1+n-1))

break

end

end

end

index = r;

summa = sum(v(r:r+n-1));

end

[James Tursa]

This:

if n >= length(v)

should be this:

if n > length(v)

This:

    while n < numel(v)

doesn't match my suggestion:

    while( that last index of r+n-1 isn't greater than numel(v) )

And this:

        if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))

doesn't match my suggestion either:

        if( the sum of the v(r:r+n-1) elements is greater than the current value of summa )

Also, your entire if-elseif-end block is more complicated than my suggestion of a simple if-end block, so look at that again.

So, take a look at fixing these up and then get back to us with your progress.

Answer 2

[Mukti Awad]

I am getting this error while executing code

• Assessment result: incorrect[1 2 3 4 5 4 3 2 1]max_sum([ 1 2 3 4 5 4 3 2 1 ], 10) returned the following MATLAB error: Index exceeds the number of array elements (9).
• Assessment result: incorrectrandom vectorsVariable summa has an incorrect value. max_sum([ 71 -54 17 60 -51 -3 98 -65 -61 35 90 94 -11 -69 82 -86 -12 ], 5) returned sum = 121 and index = 3 which is incorrect...

[James Tursa]

Probably best if you open up a new Question. Also, we can't help you correct your code unless we see your code, so post your current code along with your Question.

[Mukti Awad]

Code is here:

function [summa,index] = max_sum(v,n)
r = 1; 
summa = 0; 
index = 0;
if n > length(v)
    summa = 0;
    index = -1;
else
               while n < numel(v)
        sum(v(r:r+n-1)); 
        r = r+1;
        if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))
            r=r-1;
           break
        elseif sum(v(r:r+n-1)) > sum(v(r+1:r+1+n-1))
            break
        end
        end
end
    index = r;
    summa = sum(v(r:r+n-1));
end




Getting error:

Assessment: 0 of 2 Tests Passed
 More Info
 Assessment result: incorrect[1 2 3 4 5 4 3 2 1]
max_sum([  1  2  3  4  5  4  3  2  1  ], 10) returned the following MATLAB error: 
Index exceeds the number of array elements (9).
 Assessment result: incorrectrandom vectors
Variable summa has an incorrect value.
max_sum([  -81  -77  -45  -68  7  85  -9  30  89  62  83  83  94  -77  52  43  -62  -76  -2  26  -80  ], 2) returned sum = 92 and index = 5 which is incorrect...