Matlab - vector divide by vector, use loop

Question

I have to two evenly sized very large vectors (columns) A and B. I would like to divide vector A by vector B. This will give me a large matrix AxB filled with zeros, except the last column. This column contains the values I'm interested in. When I simple divide the vectors in a Matlab script, I run out of memory. Probably because the matrix AxB becomes very large. Probably I can prevent this from happening by repeating the following:

calculating the first row of matrix AxB
filter the last value and put it into another vector C.
delete the used row of matrix AxB
redo step 1-4 for all rows in vector A

How can I make a loop which does this?

What is B^-1? Are you multiplying A by the element-wise inverse of B? — Jonas
– Jonas, Commented Feb 12, 2013 at 17:45
Can you update your question with the code you are using to divide A by B — slayton
– slayton, Commented Feb 12, 2013 at 18:59

slayton · Accepted Answer · 2013-02-12 18:57:55Z

0

You're question doesn't make it clear what you are trying to do, although it sounds like you want to do an element wise division.

Try:

C = A./B

answered Feb 12, 2013 at 18:57

slayton

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felipe kopel · Accepted Answer · 2017-09-16 18:21:52Z

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"Matrix product AxB" and "dividing vectors" are distinct operations. If we understood this correctly, what you do want to calculate is "C = last column from AxB", such that:

lastcolsel=zeros(size(B,2),1)
C=(A*B)*lastcolsel

If that code breaks your memory limit, recall that matrix product is associative (MxN)xP = Mx(NxP). Simplifying your example, we get:

lastcolsel=zeros(size(B,2),1)
simplifier=B*lastcolsel
C=A*simplifier

answered Sep 16, 2017 at 18:21

felipe kopel

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