how to add zeros around a matrix?

for example i have a matrix A = [1 2 3; 2 3 4; 5 6 7] I am working on image processing and I want to add zeros around this matrix like below. How can i do that?

                   0 0 0 0 0
                   0 1 2 3 0
                   0 2 3 4 0
                   0 5 6 7 0
                   0 0 0 0 0

Answer 1

[Stephen23]

>> conv2(A,[0,0,0;0,1,0;0,0,0])
ans =
   0   0   0   0   0
   0   1   2   3   0
   0   2   3   4   0
   0   5   6   7   0
   0   0   0   0   0

[Mucahid Akyar]

thank u so much for answer. for example think a picture I = imread('pic.png'); Ig = rgb2gray(I); can i use this method for this Ig matrix;

[Stephen23]

The conv2 help states the inputs must be vectors or matrices. As long as Ig is a matrix then it should work.

[Mucahid Akyar]

thank u so much again :) can i ask something else ? if i want to write a for loop like

for(i=0 : 1: 5 && j=0 :1 : 5) matlab is allow that or not ?

[Stephen23]

It is not clear what you would expect the result to be. MATLAB does not currently have a syntax for looping over two variables at once. You could define the loop to be an index and use that to select elements from those vectors:

iV = 0:1:5;
jV = 0:1:5;
for k = 1:numel(iV)
    iN = iV(k);
    jN = jV(k);
end

[Eren Ozcelik]

Hi. How can I use this method for non-square matrix? For example creating a border for 1000x1500 matrix.

Answer 2

[KL]

newA = zeros(size(A)+2);
newA(2:end-1,2:end-1)=A

[Medyan]

Thx man.

Answer 3

[Andrei Bobrov]

padarray(A,[1 1],0)

[Stephen23]

Note: requires the image toolbox.

[DGM]

While padarray() requires IPT, MIMT has padarrayFB(), which uses, but does not require IPT. That's a possibility for those who don't have IPT. ... but then MIMT also has other options for adding borders to images.

Answer 4

[noam gridish]

% create bigger mat of zeros

[m,n]=size(A);

paded_mat=zeros(m+1,n+1);

paded_mat(2:end-1,2:end-1)=A;

A=paded_mat;

[DGM]

Have you tested this? I ask because it will throw an error. Your preallocated array is too small.