MATLAB Expanding A Matrix with Zeros

Question

I need a matrix of nxn , where the first pxp of it contains ones and rest are zeros. I can do it with traversing the cells, so I'm not asking a way to do it. I'm looking for "the MATLAB way" to do it, using built-in functions and avoiding loops etc.

To be more clear;

let n=4 and p=2 ,

then the expected result is:

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

There are possibly more than one elegant solution to do it, so I will accept the answer with the shortest and most readable one.

PS The question title looks a bit irrelevant: I put that title because my initial approach would be creating a pxp matrix with ones, then expanding it to nxn with zeros.

Answer 1

The answer is creating a matrix of zeroes, and then setting part of it to 1 using indexing:

For example:

n = 4;
p = 2;
x = zeros(n,n);
x(1:p,1:p) = 1;

If you insist on expanding, you can use:

padarray( zeros(p,p)+1 , [n-p n-p], 0, 'post')

Answer 2

Another way to expand the matrix with zeros :

>> p = 2; n = 4;
>> M = ones(p,p)
M =
     1     1
     1     1
>> M(n,n) = 0
M =
     1     1     0     0
     1     1     0     0
     0     0     0     0
     0     0     0     0

Answer 3

You can create the matrix easily by concatenating horizontally and vertically:

n = 4;
p = 2;
MyMatrix = [ ones(p), zeros(p, n-p); zeros(n-p, n) ];

Answer 4

>> p = 2; n = 4;
>> a = [ones(p, 1); zeros(n - p, 1)]

a =

     1
     1
     0
     0

>> A = a * a'

A =

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