Filter a matrix by some column values

I have a matrix A and a vector b. I don't know their sizes, the size varies because it is the output of another function. What I want to do is filter A by a column (let's say j th column) which has at least one value that is in b.

How do I do this without measuring the size of b and concatenating every filtered result. Right now, the code is like this (assume j is a given value)

    bsize=size(b,1);
    for i=1:bsize
        if i==1
            a=A(A(:,j)==b(i),:);
        else
            a=[a; A(A(:,j)==b(i),:)];
        end
    end

I want to code a faster solution.

I am adding a numerical example just to make it clear. Let's say

    A=[2  4
       7 14
      11 13
      15 14]

and

b=[4 14]

What I'm trying to do is filter to obtain the A matrix whose values are 4 and 14 in the second column, the elements of b to obtain the following output.

    A=[2  4
       7 14
      15 14]

In my data A has more than 12000 rows and b has more than 100 elements. It doesn't always have to be the second column, sometimes the column index changes but that's not the problem now.

Answer 1

[Walter Roberson]

filtered_A = A( ismember(A(:,j), B), :);