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Re: Why is different ^ and .^?


From: Brian Blais
Subject: Re: Why is different ^ and .^?
Date: Mon, 23 May 2005 15:03:15 -0400
User-agent: Mozilla Thunderbird 0.8 (X11/20040913)

Javier Arantegui wrote:
Why the second option (^ instead of .^) doesn't work? Shouldn't be easier if you could use the same operator as when you are working with an scalar? I'd be happy if you could provide me an explanation that I could give to my students.

Octave does math using matrix algebra, not scalar algebra. Thus, if A and B are matrices, then

A*B requires that the number of columns of A be equal to the number of rows of B to do the proper matrix multiply. If you do:

A*A or A^2, then the matrix-math rule says that the matrix A must be square (a special case of the rule above).

Many times, however, you want to do an element-by-element multiply of two matrices. The proper way of doing this is:

A.*B , where A and B must be the same size (same number of elements). The same holds for element-by-element exponents:

A.^2

Note, that the matrix addition/subtraction are the same as the element-by-element addition/subtraction, so you don't need a separate operator.


                        Brian Blais

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