Eliminating part of arrays in matlab

Question

Assume that we have a 100*4 array.

We also have a 100*1 array of 1 and 0. Assume there are n 1's.

We want to create a n*4 array from the 100*4 array, where we only include the columns for which the second array is a 1.

One way to do it is through a double for loop. Is there a simpler method?

So, We have

A = [ [ 332 44 33 22 33 55 33 211 .....
      [ 823 44 12 98 19 23 32 911 .....
      ....
      ....
    ]

and

 B = [1 0 0 1 0 0 0 ....]

and we want

 C = [ [ 332 22 ...
       [ 823 98 ...
       ....
       ....
     ]

Have a look at [this question/answer](http://stackoverflow.com/questions/11419220/matlab-extract-submatrix-with-logical-indexing), I think you are looking for a form of logical indexing. — Schorsch, Jun 20 '13 at 17:35

score 4 · Accepted Answer · answered Jun 20 '13 at 20:14

4

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

answered Jun 20 '13 at 20:14

Shai

1

Concise, correct, and with a reference in case you don't understand the syntax +1. – Floris Jun 20 '13 at 22:22

score 1 · Answer 2 · answered Jun 20 '13 at 18:53

1

First you repmat the logical vector so that it has the exact same size as the matrix A.

idx2keep = repmat(b, [1 4]); % Or [4 1] depending on if it's a col or row vector

Then you can simply index them with

 B = A( idx2keep )

you can then make it into a column vector:

 B = B(:)

That should do the job. Next time please always post some code or notation so it's easier and clearer to answer this.

answered Jun 20 '13 at 18:53

ClojureMostly

`repmat`? haven't you heard of `bsxfun`? – Shai Jun 20 '13 at 20:17
How would bsxfun be used in this situation? Also, this solution does returns a 1x(4n) array instead of a 4xn array. – user e to the power of 2pi Jul 02 '13 at 18:52

2 Answers2