function with input a matrix and calcualte sum of elements of its diagonal and right of it

Question

Muhammad Usman Saleem 2015-5-27

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回答： Prasanth Venkatesan 2020-8-23

Hi every one, I want to make a function that take two dimensional matrix as input and return sum of its diagonal (index for row and col will be same) and element right to this diagonal(sum of upper triangular elements). I am using that code & to me this is looking fine

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

When i test for

    >> a
      a =
           1     2
           3     4
    >> halfsum(a)
    ans =
         5

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Muhammad Usman Saleem 2015-5-27

Testing answer is 5 , instead of 7 . Kindly check where is need of correction in my code. Thanks in advance for assistance

Nobel Mondal 2015-5-27

在 MATLAB Online 中打开

The bug in your code is in the condition check line. It should be -

if i==j || i<j

However, using in-built functions triu and sum would definitely be the way to go, as Stephen replied.

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Answer 1

Stephen23 2015-5-27

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编辑：Stephen23 2015-5-27

在 MATLAB Online 中打开

>> a = [1,2;3,4]
a =
     1     2
     3     4

Method one:

>> sum(sum(triu(a)))
ans =
     7

Method two:

>> sum(a(triu(true(size(a)))))
ans =
     7

@Muhammad Usman Saleem: you are very busy using MATLAB, now would be a good time to learn to avoid using nested loops for solving everything (as if it was a low-level language), and start to understand the speed and power of vectorized code: faster, neater and less buggy!

If you are doing one of those courses where they insist that the students write everything using loops and basic arithmetic, then try this (note that there is no if statement):

function out = halfsum(A)
[row,col] = size(A);
out = 0;
for rr = 1:row
    for cc = rr:col
        out = out + A(rr,cc);
    end
end
end

and then we can test it to confirm the output (note that it only works with matrices!):

>> halfsum(a)
ans =
   7

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Muhammad Usman Saleem 2015-5-27

编辑：Muhammad Usman Saleem 2015-5-27

@Stephen Cobeldick hahahaha... I admin your knowledge is more than me please suggest me corrections in above codes..

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Answer 2

Anna Rozanska 2017-3-24

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The answer should be 9 not 7 , though.

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Stephen23 2017-3-24

编辑：Stephen23 2017-3-24

Muhammad Usman Saleem, who asked this question, wrote that they wanted the output to be 7: "Testing answer is 5 , instead of 7". Not only that, they also accepted an answer that gives 7 as the output. Note also that the sum of the upper triangle+diagonal of the example matrix is 4+2+1=7 (this sum is what Muhammad Usman Saleem requested).

Please explain why you think that sum should be equal to 9.

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Answer 3

Sumit Kumar Sharma 2020-6-4

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在 MATLAB Online 中打开

function summa=halfsum(x)
[m,n]=size(x);
sum=0
for p=1:m
    for q=1:n
        if p<=q
            sum=sum + x(p,q);
        end
    end
end
summa=sum;
end

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Answer 4

Prasanth Venkatesan 2020-8-23

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在 MATLAB Online 中打开

function summa =halfsum(m)
[row,col]=size(m);
        for r=1:row;
            for c=r:col;
             a(r,c)=m(r,c);
              b=a;
             
             
            end
        end
       summa= sum(b,'all');