Write a function called halfsum that takes as input an at most two-dimensional matrix A and computes the sum of the elements of A that are in the diagonal or are to the right of it. example, the input is [1 2 3; 4 5 6; 7 8 9],the ans is 26

function s = halfsum(A)
[row col] = size(A);
if row ~= col
error('Expecting a square matrix here people...');
end
s = 0;
for ii = 1:row
for jj = ii:col
s = s + A(ii,jj);
end
end

[Deepanshu Parihar]

but i am getting the error

Problem 3 (halfsum): Feedback: Your program made an error for argument(s) [1 2 3 4 5 6 7 8 9 10] \please help

[Joseph Cheng]

please reformat your question using the {}code button and view the preview section so it is easily readable.

[Deepanshu Parihar]

see now Mr Cheng

[Christos Vyzantios]

You must remove part the code that u don't need:

function s = halfsum(A)
[row col] = size(A);
s = 0;
for ii = 1:row
  for jj = ii:col
    s = s + A(ii,jj); 
  end
end

[asad jaffar]

how you solved it ,now in 2019 i am facing the same problem

[Walter Roberson]

Christos posted complete code 4 years ago.

[asad jaffar]

can you copy paste it again for me ,thanks

[Walter Roberson]

Christos's complete code is just a few comments further up in this thread, at

https://www.mathworks.com/matlabcentral/answers/220468-write-a-function-called-halfsum-that-takes-as-input-an-at-most-two-dimensional-matrix-a-and-comp#comment_288818

You will probably need to click "Show older comments"

[Soham Bhosale]

i think this code will help you for this problem

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;

[Sai Manohar Naidu]

I think this would be the most simplest code to understand well.

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

[Rik]

@Sai Manohar Naidu: why are you posting solutions to homework questions? What is your goal?

[Nadeem U Rehman]

function summa = halfsum(A)
k = size(A);
total = 0;
for i = 1:k(1)
    for j= 1:k(2)
        if i <= j 
            total = total + A(i,j);
        end
    end
end
summa = total;

[Rik]

@Nadeem: this is exactly equivalent to the comment by Sai. Except that in your version, you use the sum function as a variable name. That is a bad idea, as it will lead to strange errors if you later try to use sum as a function.

[Nadeem U Rehman]

thank you @Rik

[Moses Patson]

This code may fail the row and column vector test.

Answer 1

[Sean de Wolski]

f = @(x)sum(sum(triu(x))) % make function
f(magic(3)) % use it

Answer 2

[Buwaneka Dissanayake]

function summa = halfsum(M)
[a b] = size(M);
if a>1
    
 for n = 1:a;
    for m = 1:b;
        if n>m;
            M(n,m) = 0;
            summa = sum(sum(M));
        end
    end
 end 
else 
    summa = sum(M);
end
        
end

[Arshdeep Singh]

can you explain the logic of this function

[DGM]

Of all the inefficient loop-based approaches on this page, this appears to be the most wasteful by far. For every single element of the array, you calculate the sum of the entire array and then discard the result. The only sum that doesn't get discarded is the last one. As a consequence, the time required grows rapidly as the matrix size increases -- all for nothing.

Try feeding this a large array. For a 400x400 matrix, this takes roughly 1000x as much time as the other loop-based examples, and nearly 6000x as much time as @Sean de Wolski's concise and efficient answer. Would you dare to feed it a 4000x4000 matrix?

Answer 3

[Pragyan Dash]

%this worked for me. Happy to help!
function summa = halfsum(M)
[row col] = size(M)
summa = 0;
for ii = 1:row;
    for jj = 1:col;
        if jj >= ii;
            summa = summa + M(ii, jj);
        end
    end
end

[THIERNO AMADOU MOUCTAR BALDE]

very clean thanks

Answer 4

[Joseph Cheng]

your input should be

 input = [1 2 3;4 5 6;7 8 9]

and not

 input = [1 2 3 4 5 6 7 8 9]

[Joseph Cheng]

which if you read carefully the problem statement and the code the input is a 3x3 square matrix. the input you provided it was a 1x9 array.

Answer 5

[Srishti Saha]

This code works perfectly for me:

%function to compute sum of lower most right side triangle in an X*2 matrix
function u = halfsum(P)
  u1 = P(end:-1:1, 1:end);
  u2 = triu(u1);
  u = sum(u2(:));
end

[Muhammad Amir Javed]

How to sum upper triangle of matrix without using triu?

[Image Analyst]

@Muhammad Amir Javed you could use a double for loop. The starting value for the inner for loop would be the current value of the outer for loop.

Answer 6

[Ajith Bharadwaj]

function summa = halfsum(A)
[row col] = size(A);
for ii = 1:row
for jj = ii:col
summa = summa + A(ii,jj);
end
end

[muhammad usman]

%what is wrong with this code? it works for every other test except the one with column vector

function summa = halfsum(M)
[a b] = size(M);

for n = 1:a;
    for m = 1:b;
        if n>m;
            M(n,m) = 0;
            summa = sum(sum(M));
        end
    end
end 

end

[Walter Roberson]

I believe that if you test more carefully, you will find that the failure is in the row vector case.

Suppose you have 1 x 3 vector. Then [a b] = size(M) woud be a=1, b=3. You have for n=1:1 and for m=1:3 . 1>1 is false, 1>2 is false, 1>3 is false, so the if n>m test always fails, so you never assign anything to summa.

[Omkar Gharat]

@Ajith Bharadwaj

Add summa = 0 in your code

[Amit Jain]

function summa = halfsum(M)
[a b] = size(M);
for n = 1:a;
    for m = 1:b;
        if n>m;
            M(n,m) = 0;
            summa = sum(sum(M));
        elseif n ==1
            summa = sum(M);
        end
    end
end 
end

[Amit Jain]

I hope this clears your doubt. You did not tell function what to if there is n ==1 means there is column vector. I have made this correction. Now function works fine. Thanks

Answer 7

[ERTIZA HOSSAIN SHOPNIL]

function summa=halfsum(A)
    t=triu(A);
    list=sum(t);
    s=0;
    for n=list
        s=s+n;
    end
    summa=s;
    
end

[Saurov Baidya]

its brings error at a certain step

[Meron Pawlos]

It restircts the use of keyword triu

[Rik]

That means you will have to copy another answer to cheat on your homework. Or think yourself.

Answer 8

[saurav Tiwari]

function summa=halfsum(a)
[m,n]=size(a)
for i=1:m
j=1:n
x(i)=sum(a(i,j))
end
summa=sum(x)
end

[saurav Tiwari]

cool

awesome coding

[Walter Roberson]

Incorrect code. This calculates sums of an entire row, not the sum of the diagonal and right of the diagonal.

Answer 9

[youssef boudhaouia]

A solution with double For-loop:

function summa=halfsum(M)
summa=0;
s=size(M);
for i=1:s(1)
    for j=1:s(2)
        if j>=i
            summa=summa+M(i,j);
        else
            summa=summa;
        end
    end
end

[Purushottam Shrestha]

Great

[Saurov Baidya]

can you please explain the solution in detail.... i m still confused about it

[youssef boudhaouia]

First we initialize the output summa. Second we assign s as the size of the matrix M . In fact , size(M) returns a vector with two values. The first value s(1) is the row number and the second value s(2) is the column number. The variable i is for the row indexes( from 1 to s(1)) and the variable j is for column indexes(from 1 to s(2)) . In every time where we find a value of the matrix which has a column index ( j ) higher or equal to the row index ( i ), we add this value to summa, and so over, until we find the total sum. If the condition (j>=i) is not true, the variable summa stays the same and doesn't change, that means : summa=summa .

I hope it's clear now . you can ask if there's something u r not convinced with

[Walter Roberson]

You do not need the if condition if you use

for j=i:s(2)

Answer 10

[Abdul Quadir Khan]

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;

Answer 11

[Vishnu V]

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