What command that will erase all but the first three columns of a matrix in matlab?

Question

I am essentially trying to take a matrix a and turn it into a matrix af which has the values in the first three columns of a. I just want to prune a matrix a down to only its first three columns.

Answer 1

a = ones(10,10);
a = a(:,1:3);

Answer 2

newMatrix = oldMatrix(:,1:3)

This says "I have a matrix called oldMatrix. I want to store a subset of that matrix into newMatrix" and the dimensions in the parentheses say what subset you want. The first colon signifies "all rows" and the 1:3 signifies "columns 1 through 3".

Answer 3

If you wish to erase all but the first three columns, then do this...

A(:,4:end) = [];

If you wish to copy over only the first three columns into a new array, then this is right...

B = A(:,1:3);

Of course, you can always do this to essentially erase those last columns...

A = A(:,1:3);

Is one better than the other? There would seem to be little difference.

A = rand(4000);
tic,A(:,4:end) = [];toc
Elapsed time is 0.044124 seconds.

A = rand(4000);
tic,A = A(:,1:3);toc
Elapsed time is 0.040166 seconds.

There seems to be a small (repeatable) difference, but I'd not totally trust that to be consistent. The next release of MATLAB might change those times.