Creating classes matrix without for loops

3
votes

I'm trying to create a classes matrix in MATLAB preferably without using for loops. So I have A:

A = [ 1 2 3 4;
      2 5 4 1;
      7 2 3 4];

Now I want to create a classes matrix so I want to look for the max value of each column and whatever that position is will become a "1". So using A, I want B to look like this:

B = [ 0 0 0 1;
      0 1 0 0;
      1 0 0 0];

I've been trying to use the following code to create the B matrix and then search for the max value in each column using the following code:

 [rows_A columns_A] = size(A);
 B = zeros(rows_A, columns_A);
 max(A, [], 2);

Then I'm trying to use ind2sub to get the position within A so I can put a "1" in the B matrix. This approach isn't working out. The matrix A can be of any dimensions. Any help is greatly appreciated.

2
matlabmatrix

2 Answers

1
votes

You can use max and sub2ind to flag the column with the largest value for each row as follows:

[~,indMaxCol] = max(A,[],2);
B = zeros(size(A));
B(sub2ind(size(B),1:size(B,1),indMaxCol.')) = 1  %' flag largest column, each row
B =
     0     0     0     1
     0     1     0     0
     1     0     0     0

Another solution, which does not require sub2ind is to create a sparse matrix:

Bs = sparse(1:size(B,1),indMaxCol,1)
Bs =
   (3,1)        1
   (2,2)        1
   (1,4)        1

This can be converted to a full matrix with full(Bs).

1
votes

One-line solution with bsxfun (compute the maximum for each row and then compare for equality):

bsxfun(@eq, A, max(A,[],2))

This differs from @chappjc's solution in one aspect: if the maximum is achieved more than once in some row, this gives all columns where the maximum is attained. For example:

>> A = [ 4     2     3     4
         2     5     4     1
         7     2     3     4 ];

>> bsxfun(@eq, A, max(A,[],2))

ans =

     1     0     0     1
     0     1     0     0
     1     0     0     0