How do I resize a matrix in MATLAB?

Question

Suppose I had a 1-by-12 matrix and I wanted to resize it to a 4-by-3 matrix. How could I do this?

My current solution is kind of ugly:

for n=1:length(mat)/3
    out(n,1:3) = mat((n-1)*3+1:(n-1)*3+3);
end

Is there a better way to do this?

Answer 1

Try the RESHAPE function:

A = (1-by-12 matrix);
B = reshape(A,4,3);

Note that the matrix B will be filled with elements from A in a columnwise fashion (i.e. columns will be filled from top to bottom, moving left to right).

Example:

>> A = 1:12;
>> B = reshape(A,4,3)

B =

     1     5     9
     2     6    10
     3     7    11
     4     8    12

Answer 2

to extend gnovice's solution:

If you need a different order of matrix construction, use transpose (the ' operator) or permute() to change the dimension ordering after you have called reshape().

Answer 3

reshape is of course the proper solution, as stated by gnovice.

A nice feature of reshape is that it allows this:

A = 1:12;
B = reshape(A,4,[]);
B =
     1     5     9
     2     6    10
     3     7    11
     4     8    12

So if you ddon't know how many columns there will be, reshape will compute it for you. Likewise, reshape will fill in the number of rows, if you leave that out.

C = reshape(A,[],4)
C =
     1     4     7    10
     2     5     8    11
     3     6     9    12