I have a matrix A=[1 2 3; 1 2 9; 2 3 4]. I want a matrix B whose column 1 is equal to 1. How can I do this? B=[1 2 3; 1 2 9]

I have a matrix A=[1 2 3; 1 2 9; 2 3 4]. I want a matrix B whose column 1 is equal to 1. How can I do this? B=[1 2 3; 1 2 9]

Answer 1

[dpb]

B=A(A(:,1)==1,:);

Lookup "logical addressing" in documentation for details on how this works--it's important Matlab syntax.

[nabin]

How about if I want to get B=[1 2 3; 1 2 9; 0 0 0] i.e to get a matrix of same size but with 0 entry for whose column 1 are not equal to 1.

[dpb]

Ah, come on-- think a little. You've seen how to find one set, extrapolating that to the other problem isn't difficult. Let's at least see an attempt (big hint: if you can dispense w/ keeping original A it's also a one-liner, working on the existing content of A; if not, copy A, then go to work).

[Image Analyst]

Why are you comparing A to 1? That was not a specification he gave, at least not explicitly.

[dpb]

Implied interpretation of the request based on the value of A and the desired answer, B. B in his first ? is the two rows from A whose first columns are 1. That that was the correct interpretation would be inferred by his acceptance of the answer. :)

That his specification was indirect and implicit rather than explicit is, as you note, also true. :)

A precise specification never hurts; oftentimes is the crux of much difficulty in endeavoring to answer a query but in this case it was possible to infer what was desired with less than perfect case statement.

[Image Analyst]

He should take my comment as a hint to make his problem statement more precise.

[dpb]

That's really tilting at windmills if were to let this level of mis- or incomplete specification stop us in our tracks! (vbg)

Answer 2

[Image Analyst]

You say "I have a matrix A=[1 2 3; 1 2 9; 2 3 4]. I want a matrix B whose column 1 is equal to 1. How can I do this? B=[1 2 3; 1 2 9]" You just gave the answer in the question itself! The answer is

B=[1 2 3; 1 2 9];

Read your question. The matrix A is not involved AT ALL in how you came up with B. No criteria or tests were to be applied to A, so A can just be ignored.