MATLAB: Help please why ?

sum

A = [ 1 3 2 6 ; 5 7 7 0 ; -2 6 -2 1 ] ;

j = sum(A(1))

j= 1 why ???

Best Answer

the statement

 sum(A(1))

is summing only the first element in A, which is 1.

If you want to sum the first row, it should be

 sum(A(1,:))

and if you want to sum the first column, it is

 sum (A(:,1))

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C = sum([A;B]);

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If you think about how basic matrix, vector multiplication works, you can see that you can sum rows of a matrix by multiplying by a column vector of ones. You can sum selected elements of a column vector by premultplying by a row vector consisting of zeros and ones (with zeros for the elements that you don't want to include in the sum). Putting these ideas together in your case gives

y = [1 0 1]*A*[1;1;1]

You could also accomplish what you want using MATLAB's indexing abilities along with the sum function. Note that A(1,:) means row 1, every column of A. Similarly A(3,:) means row 3 every column of A

y = sum(A(1,:)) + sum(A(3,:))

Best Answer

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