Write a function called halfsum that takes as input an at most two-dimensional array A and computes the sum of the elements of A that are in the lower right triangular part of A, that is, elements in the counter-diagonal (going from the bottom left c

function [sume] = halfsum(A) 
[row,col] = size(A); 
for c = 1 : 3 
  for i = row : -1 :1     
    for j = col : -1 : 1
     sume = sum(A(i,j)); 
    end
  end 
end  
end 

[Emma Sellers]

Can someone explain to me why this doesn't work?

It gives me the sum of the triangle that is in the top right instead of bottom left even though i feel like I have it iindexed to start at the end of the array and work backwards 3 times?

Answer 1

[Yachika Singh]

%for any random matrix
function summa=halfsum(A)
[row, col]=size(A);
summa=0;
for i=1:row
    for j=i:col
        if i<=j
            summa=summa+(A(i,j));
        end
    end
end
end

[Mithra A]

Hi, This code actually worked. But I can't understand how it works. Can you please explain

Answer 2

[Matt J]

The code you've shown will return only the value A(1,1), e.g.,

A =

     1     6    11    16    21
     2     7    12    17    22
     3     8    13    18    23
     4     9    14    19    24
     5    10    15    20    25
     
[row,col] = size(A); 
for c = 1 : 3 
  for i = row : -1 :1     
    for j = col : -1 : 1
     sume = sum(A(i,j)); 
    end
  end 
end  

>> sume

sume =

     1

because sume gets overwritten with every pass through the loops and sum(A(i,j)) is the same as A(i,j).

Answer 3

[Manoj Kumar Sharma]

function [summa] = halfsum(A)
[row, col] = size(A);
for n=1:row
    for i=n:col
 summ(n,i)=(A(n,i));
    end
        end 
summa=sum(summ,'all');
end

[Maryam AlHilali]

can you please explain

summ(n,i)=(A(n,i));