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

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 corner, up and to the right) and elements that are to the right of it. For example, if the input is [1 2 3; 4 5 6; 7 8 9], then the function would return 38.

This is all I have so far:

    function sum=halfsum(v)
    sum=0;
    [row col]=size(v);
    for i= 1:row;
        for j=1:col;

I have spent hours on trying to figure out how to sum the reverse diagonal starting at the bottom left element and everything to the right of it and cannot seem to figure it out. Any help is greatly appreciated!

[nikhil singh]

This program is for calculating sum of upper triangle of a Matrix, hope it helps

function summa=halfsum(M)
[row column]= size(M);
summa=0;
sumdiagonal=0;
sumupper=0;
for i=1:row 
    for j=1:column
    if i==j 
       sumdiagonal = sum(M(i,j))+ sumdiagonal;
    end
    if i<j
        sumupper = sum(M(i,j)) + sumupper;
    end
    summa=sumdiagonal + sumupper;
    end
end
end

[Walter Roberson]

Is there a particular reason to sum the diagonal seperately from summing the upper part?

[Soham Bhosale]

i think this program will help you for summing upper triangular part

function summa = halfsum(x)
[row,col] = size(x);
allsum=0;
for n=1:row
    for c=n:col
        allsum=x(n,c)+allsum;
    end
end
summa=allsum;

[TINOY SANTRA]

function summa=halfsum(A)
b=size(A);
row=b(1);
col=b(2);
summa=0;
for i=1:row
    for j=1:col
        if(i==j)
            summa=summa+sum(A(i,j:end));
        end
    end
end
summa

[TINOY SANTRA]

i did this code and it worked fine

[Image Analyst]

Do not post up here. If you think that none of the 18 answers below and 2 answers above answer the question and that your answer offers something unique, then you can reopen the question to post your answer that is different than the prior 20 solutions.

Answer 1

[James Tursa]

Ask yourself, "what does it mean to be in the lower right triangle" in terms of i and j? I.e., what test could you make on i and j to determine if you were in the lower right triangle? Then code that up inside your for loops. E.g.,

    mysum = 0;  % <-- Changed sum to mysum since sum is the name of a MATLAB function
    [row,col] = size(v);
    for i= 1:row
        for j=1:col
            if( in lower right triangle )  % <-- you put test code inside parentheses
                mysum = mysum + v(i,j);
            end
        end
    end

So, you just need to figure out a test you can make on i and j to determine if you are in the lower right triangle, and insert that code inside the parentheses as noted above. If you can't figure this out immediately, write down all of the i,j pairs for a small matrix on a piece of paper and then look for a pattern that you can exploit and code up that isolates the lower right triangle. Do an example for both a square and non-square matrix to make sure the code you come up with works in both cases.

SIDE NOTE: There are ways to do this without a double for loop, but you should first learn how to do this with for loops.

[Will Gotcher]

Hey James,

Late response I know. Thanks for the reply. That is roughly the code I had at the time of posting the question. Do you have any tips how I can single out the bottom right triangle? I have spent several full days trying to figure this out and just cannot get it. Thanks!

[Image Analyst]

Uh Will, you could use my code. Did you even see it or try it??? It only accesses elements in the lower right triangle.

Answer 2

[Vikrant Prasad]

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

summa =sum;
end

[asad jaffar]

vikrant how to write it for uperright instead of lower right ?

[Image Analyst]

Did you try

if i>=j

and it didn't work??? Or did you do

    summa = sum(A(triu(A)))

You didn't use sum for the name of the variable, contrary to what I said in my answer, did you??

Please attach the code so we can check why it's not working.

[mayank ghugretkar]

another similar way

1. with for loop

function summa=halfsum(A)
[row col]=size(A);
summa=0;
j=0;
for i=1:row 
    for j=1:col
        if j>=i
           summa= A(i,j) +summa;
        end
    end
end
end

2. with While loop

function summa=halfsum(A)
[row col]=size(A);
summa=0;
for i=1:row 
    j=i;
    while (j>=i && j<=col)
        summa= A(i,j)+ summa;
        j=j+1;
    end
end
summa;
end

i love trying this things... simplifying n solving

Answer 3

[Prashanth Reddy Penta Reddy]

 function u = halfsum(P)
    u1 = P(end:-1:1, 1:end);
    u2 = triu(u1);
    u = sum(u2(:));
end

[Jorge Briceño]

Hi everyone,

I solve the problem in a different way. Here it is:

function [counter] = halfsum (A)
% ii = rows ; jj = columns
[m,n]=size(A);
counter = 0;
ii=m;
for jj=1:n
  if ii>0 % This contidion is mandatory, otherwise the ii could be equal zero, which will result in an error
      counter=counter + sum(A(ii,jj:n)); % Function sum(A(ii,jj:n)) sums all the column in a row.
      ii=ii-1; % This is equivalent to a row for loop.
  end
end
counter;
end

Another way using rows and columns for loops would look like:

function [counter] = halfsum (A)
% ii = rows ; jj = columns
[m,n]=size(A);
counter = 0;
col=1;
  for ii=m:-1:1
      for jj=col:n
      counter=counter + A(ii,jj); 

        end
        col=col+1;
    end
counter;
end

[William Gallego]

Great, I understood both ways. Actually, I spent most of my time trying to figure out the loop for ii=m:-1:1. I forgot to count backward :(. Thanks for the help!

[Mukti Awad]

I am getiing this error!!!

Assessment result: incorrect[1 2 3; 4 5 6; 7 8 9]

Variable summa has an incorrect value.

Answer 4

[Nijita Kesavan Namboothiri]

function summa=halfsum(m)
summa=0
[r c]=size(m)
for i=1:r
    for j=1:c
        if i<=j
            summa=summa+m(i,j)
        end
    end
end

Answer 5

[Image Analyst]

Try

theSum = 0;  % DON'T USE sum as a variable name!!!
[rows, columns] = size(v);
for row = 1 : rows
    for col = (columns - row + 1) : columns
        theSum = theSum  + .......

Answer 6

[Justin Whetten]

    For anyone who might be struggling still with this
   function sum=halfsum(A);
   sum = 0;  
    [row,col] = size(A);
    for i= 1:row
        for j=1:col
            if (i+j)>=(row+1)  
                sum = sum + A(i,j);
            end
        end
    end

[Steven Lord]

There's no need for a test inside the loops.

In row 1, how many elements will be included in the sum and in which column are those elements located?

In row 2, how many elements will be included in the sum and in which column are those elements located?

Continue asking this question and see if there is a pattern to the answers that you can exploit in the definition of the second for loop to force only those elements that should be included in the sum to be included.

[Image Analyst]

And of course, NEVER use "sum" as the name of your variable (as I mentioned in my answer) since that's the name of a very important built-in function that you won't be able to use anymore if you redefine "sum" to be your own personal variable.

And if you're puzzled by what Steve said, just see my answer.

Answer 7

[MANAV MALHOTRA]

function summa=halfsum(m)
    summa=0;
    for i=1:size(m,1)
        for j=1:size(m,2)
            
            if(i<=j)
                summa=summa+sum(m(i,j));
            end
        end
    end
for those who are less familiar with matlab

Answer 8

[Roshan Singh]

function summa=halfsum(a)
[r,c]=size(a);
summa=0;
for x=1:r
    for y=1:c
        if (x==y)||(x<y)
            summa=a(x,y)+summa;
        end
    end
end
fprintf('summation of right hand matrix is...%d\n',summa)
end

[Walter Roberson]

(x==y)||(x<y) is the same logical condition as x<=y but x<=y is more efficient.

Answer 9

[Sebastián Añazco]

function summa = halfsum(A)

summa = 0;

for n = 1 : size (A,1) 

   for m = 1:size(A,2)
    if n >m
        a = 0;
    else
    a = A(n, m);
    end
    summa = summa +a;
   end
end

Answer 10

[Shubham Pandey]

function sum = halfsum(a)
sum = 0;
[row,col] = size(a);

    for i=1:row
        for j=1:col
            if i<=j
                sum=sum+a(i,j);
            end 
        end
    end
end

[Shubham Pandey]

this works fine

[Image Analyst]

But every MATLAB expert would advise anyone to never use the name of a built-in function like sum() as the name of a variable.

[Shubham Pandey]

I agree with your feedback. Just a noobie mistake.

Answer 11

[Nitin Kumar soni]

function summa=halfsum(n)
[row,col]=size(n);
summa=0;
for i=1:row
    for j=1:col
        if i<=j
            summa=summa+n(i,j);
        end
    end
end

[Walter Roberson]

You can avoid the if statement if you have j end at i instead of continuing to col.

Answer 12

[Ashay Nagdive]

function a = halfsum(A)
[row col]= size(A);
a = 0;
for r= 1: row
    for c= 1: col
        if c >= r
            a= a + A(r,c)             
        else
            A(r,c)= 0;
        end
    end
end

            
            

Answer 13

[SANTOSH KAMBLE]

function summa=halfsum(x)
[row col]=size(x);
summa=0;
for r=1:row
    for c=1:col
        y=x(r,c);
        if r<=c
         summa=summa+y;
        end
    end
end
end

Answer 14

[ADARSH VISAJI]

function summa = halfsum(A)
[row, col]=size(A);
sum=0;
for i=1:row
    for j=i:col
        if i<=j
            sum= sum + A(i,j);
        end
    end
end
summa =sum;
end
if u guys have any dobts on matlab problem u can contact me through instagram
    my insta id:-    visaji_adarsh
    

[Image Analyst]

This is similar to most of the other answers however you used the function name "sum" as a variable. It is not recommended to overwrite built-in functions with variables because then you can never use the built in function name anymore within that scope. For example if you wanted to do

theSum = sum(1:100);

you would not be able to because you made sum a scalar number -- it's not the built-in function anymore.

Answer 15

[Taif Ahmed BIpul]

function summa=halfsum(A)
[m,n]=size(A);
summ=0;
for i=1:m
    for ii=i:n
        summ=summ+A(i,ii);
    end
end
summa=summ;

Answer 16

[vighnesh rana]

function summa = halfsum(A)

[row col] = size(A);
total = 0;
for r = 1:row
    for c= r:col
        total = total + A(r,c);
    end
end
summa = total;
end

Answer 17

[Geetanjali Alle]

function summa = halfsum(M)
[row column]=size(M);
summa=0;
for i=1:row
    for j=1:column
        if i<j||i==j
            summa=summa + sum(M(i,j));
        end
end
end

Hope this helps!

[Walter Roberson]

i<j||i==j

would be more efficient as

i<=j

There is also an easy change to prevent you from needing the if at all.

sum(M(i,j))

M(i,j) is a scalar. sum() of a scalar is the same scalar. You do not need sum() there.

Answer 18

[AYUSH MISHRA]

function summa=halfsum(A)
    [m,n]=size(A);
    summa=0;
    for i =1:m
        fprintf('active row :%d \n',i);
        for j=1:n
            fprintf('active collumn :%d \n',j);
            if m<=n
                summa=summa+A(i,j);
            end
            fprintf('summa aeter every loop complition : %d \n',summa);
        end
        
end

run it and understand every step working in loop