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How do I find the dot product of 2 matrices without the same number of elements?

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Christian Jaramillo 2018-10-24

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编辑： Matt J 2018-10-25

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I have the following vectors that I need to find the dot product for using the dot function. here is what I have so far;

A = [1 2 3]
B = [12 20 15 7]
dot(A, B)

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Ghetto blaster 2018-10-24

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编辑：Image Analyst 2018-10-24

To perform dot product function it is mathematical need that you have 2 matrices where dimensions of matrices agree. There needs to be the same amount of rows of first matrix as the number of columns of second matrix. You cannot make a dot product function that would multiply two matrices where dimensions don't agree. In that situation you wouldn't be talking about dot product.

But let me ask you. Given your example 2 matrices, how would you "dot product" them? Seeing how you posted 2 vector matrices, you should have preformulated the question, but the need for dimensions of matrices to agree still stays. Lengths of vector matrices need to agree.

Dot Product

Christian Jaramillo 2018-10-24

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I tried the meshgrid function to make them equal and used the dot product on that however it's still give an answer of [54 108 162]

Matt J 2018-10-24

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We cannot tell you how to do undefined things. Provide a definition for "product of 2 matrices without the same number of elements", and then the conversation can start.

John D'Errico 2018-10-24

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编辑：John D'Errico 2018-10-24

This is not even a question about MATLAB. A dot product here is mathematically meaningless. So unless you can define what it should mean to you to compute that dot product, you need to start over again.

What is it that you think you need to do here? Why do you want to compute that dot product? Only when you have a mathematically defined operation can you even think of trying to write MATLAB code to do the computation.

Perhaps, are you wanting to compute an outer product?

Christian Jaramillo 2018-10-24

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using a simple problem this is what I'm trying to do;

A = [ 1 2 3];
B = [ 4 5 6];
y = A.*B
y =
    4  10  18
sum(y)
ans =
      32
% which is also the same as
dot(A,B)
ans =
     32

Bruno Luong 2018-10-24

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Everyone tell you that there is no such thing as dot product with unequal length vectors. At least not universally defined.

Image Analyst 2018-10-24

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Christian, that example works because they ARE the same size. What are you expecting with your first example where the sizes don't match?

Christian Jaramillo 2018-10-24

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I understand that it's not mathematically possible as is it would have to involve having to matrices that have the same amount of elements. That being said how can I structure the problem to yield the result? This is for my homework in the Text Book MatLab for Engineers I'm attaching a printout that shows the question also. Any help is appreciated

Bruno Luong 2018-10-24

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I guess the book has a typo error.

Image Analyst 2018-10-24

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I don't think 1 and 2 can be done. 3 can be done because for each food item there is a count and a price so you just do sum(count.*price).

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Haritha 2018-10-25

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编辑：Matt J 2018-10-25

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Hi,

To create the equal size of a vector you have to do padding.

Example:

    A = [1 2 3 0];
    B = [12 20 15 7];
    dot(A, B)
ans =
    97

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How do I find the dot product of 2 matrices without the same number of elements?

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