# How can I change the elements of a matrix in new one?

3 views (last 30 days)
Ang Vas on 22 Jul 2015
Commented: Ang Vas on 23 Jul 2015
I want to change the elements of a matrix with a loop command. What I have done is, let’s assume that we have a matrix A=[-1 -5 -2 3;-4 -7 8 -3;-2 -3 -1 5;4 8 2 -3]
[m,n]=size(A);
B=[]
for i = 1:m
I = (i-1)*2;
for j = 1:n
J = (j-1)*2 + 0.5;
if(A(i,j)>0)
B=[B [J;m-I]]
end
end
end
and the results that I get is
B =
6.5000
4.0000
B =
4.5000
2.0000
B =
6.5000
0
B =
0.5000
-2.0000
B =
2.5000
-2.0000
B =
4.5000
-2.0000
How can I take all the numbers of B in one Nx2 matrix ?
Stephen23 on 22 Jul 2015
Doing this in a loop is very poor use of MATLAB. Learn how to write vectorized code and take advantage of the neater, faster solutions to problems like this.

Azzi Abdelmalek on 22 Jul 2015
Edited: Azzi Abdelmalek on 22 Jul 2015
A=rand(4)
[m,n]=size(A);
B=[]
for ii = 1:m
I = (ii-1)*2;
for jj=1:n
J = (jj-1)*2 + 0.5;
if(A(ii,jj)>0)
B=[B;J m-I]
end
end
end
##### 2 CommentsShowHide 1 older comment
Ang Vas on 23 Jul 2015
Thank you a lot for your help and I am sorry for the mess!!!

Stephen23 on 22 Jul 2015
Edited: Stephen23 on 22 Jul 2015
Instead of slow and painful loops as if MATLAB was some low-level programming language like C, just use simple vectorized code, which is faster and neater:
A = [-1,-5,-2,3; -4,-7,8,-3; -2,-3,-1,5;4,8,2,-3];
[x,y] = find(A.'>0);
X = (x-1)*2 + 0.5;
Y = size(A,1) - (y-1)*2;
B = [X,Y].'
Which displays this in the command window:
B =
6.5000 4.5000 6.5000 0.5000 2.5000 4.5000
4.0000 2.0000 0 -2.0000 -2.0000 -2.0000

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