Meaning of the if statement

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Muhtasim Haque Nahian 2021-4-19

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评论： Muhtasim Haque Nahian 2021-5-9

function z = in_prod(x,y)
if size(x,2)==size(y,1)
z=x*y
else
z = "The inner dimensions are " + size(x,2) + " and " + size(y,1) + ". Matrix multiplication is not possible"; 
end

Can anyone explain me the meaning of this if statement please?
More specifically, why size(x,2) and size(y,1)? Why not size(x,any other value)?

TIA!

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Stephen23 2021-4-19

"More specifically, why size(x,2) and size(y,1)?"

Because matrix multiplication is only defined when those dimensions are equal:

https://en.wikipedia.org/wiki/Matrix_multiplication

http://mathworld.wolfram.com/MatrixMultiplication.html

"Why not size(x,any other value)?"

Because matrix multiplication is only defined for 2D matrices.

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Answer 1

David Fletcher 2021-4-19

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编辑：David Fletcher 2021-4-19

The if statement is checking that the number of columns in x (the number two in the argument of size is specifying that the function returns the number of columns) matches the number of rows in y (the number one in the argument of size is specifying that the function returns the number of rows). This ensures that matrix multiplication can be applied to x and y. If the inner dimensions do not match, the matrices cannot be multiplied.