Logical Indexing via multiplication

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Inna Pelloso
Inna Pelloso 2020-12-21
编辑： Stephen23 2020-12-21
Hi,
I have a 3 x 3 matrix, B = [ 1 2 3; 4 5 6; 7 8 9 ]
I have a matrix, A = [ 0 1 0 ]'.
How can I extract only the middle row of matrix B, ie. [ 4 5 6] ?
If I multiply, I get the first and third row with zeros.
This is a simplification of a larger problem. How can I do it via multiplication of A and B?
I want all the rows of B that correspond where the index A equals 1.
Any help would be appreciated!
Thank you,
Inna

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David Goodmanson
David Goodmanson 2020-12-21
编辑：David Goodmanson 2020-12-21
Hi Inna, not done by multiplication, but:
ind = find(A==1)
rows_you_want = B(ind,:)
the colon means to take every column in whatever rows are selected.
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Inna Pelloso
Inna Pelloso 2020-12-21
Thank you. For some reason, I ALWAYS forget about using the find function.

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Walter Roberson
Walter Roberson 2020-12-21
编辑：Walter Roberson 2020-12-21
This is a task that cannot be done by multiplication.
If you use .* elementwise multiplication then the size of the result is max() of the sizes of the inputs provided they are compatible sizes. Saying max() takes into account implicit expansion. The size of output never depends on the content of the data when you use .*
If you use A*B then size(A, 2) must equal size(B, 1) and the size of the output is always size(A, 1) by size(B, 2) no matter what the content of the variables are.
You can create projection matrices that select specific rows, but the size of the matrices depend on the number of rows being selected, so they have to be constructed outside of plain matrix multiplication.
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Walter Roberson
Walter Roberson 2020-12-21
P = eye(size(B))
P = P(logical(A), :)
C = P * B
But you cannot construct P using just matrix multiplication without making decisions based on the content of A

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