Find the first element that satisfy a condition in a vector

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

[dpb]

You don't set index until after the loop finishes so ii is always the max it takes on in the loop...

Would be easier to compute all the sums first, then figure out which one of those is the winner...

Points for having "shown your work!" and having made the effort...

[paul mary]

could you give me some details?

[paul mary]

I have written another code, but the result of my code is also wrong, when n = 2

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

I ' m block on this part

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);
            if b == summa
                index = v(ii);
            end
        end
    else
        summa = 0;
        index = -1;
    end
else
    fprintf('wrong');
end

[paul mary]

finally! I finished

[Stephen23]

See also earlier question (different OP):

https://www.mathworks.com/matlabcentral/answers/444784-write-a-function-called-max_sum-that-takes-v-a-row-vector-of-numbers-and-n-a-positive-integer-as

[Eva Petrossova]

Hey, I can't manage with it, how did you do ?

Answer 1

[Andrei Bobrov]

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

[dpb]

I'm guessing if OP turns this one in, the instructor eyebrows will raise -- or at least they should! :)

Answer 2

[Sritama Mitra]

function [summa,index]=max_sum(v,n)
k=length(v);
index=ones(1,1);
if n<=k
    for i=1:(k-n+1)
        b=sum(v(i:i+n-1));
        a(i)=b;
        summa=max(a);
        
    end
    x=find(a==summa);
    [o,p]=size(x);
   
    if p~=1
        index=x(1);
    else
        index=x;
    end
        
else
    summa=0;
    index=-1;
end
end