making a matrix from another one

i havematrix a=

                 1  2  3

                 4  5  6

                 7  8  9

i need to make a new matrix from a, i used the following code:

b=[];

for 1=1:3

for j=1:3

d=[a(i,j)]

b=[b;d];

end

end

but it gives me a 1x9 matrix,

i need b as a 3x3 matrix, with whole contents of a, i need to shape b like this: b=

                                1  2  3

                                4  5  6

                                7  8  9

where is my fault?

Answer 1

[Wayne King]

I'm not sure why you want to do this with a for loop since you are just creating a copy of the original matrix. You'd be much better off to do just:

 b = a;

But if you must use a for loop:

 b = zeros(3,3);
 for ii =1:3
 for jj =1:3
 b(ii,jj)= a(ii,jj);
 end
 end

If you insist on doing it the way you did, then you have to do:

 b = reshape(b,3,3)';

after you exit the loop:

 b=[];

 for ii =1:3

 for jj =1:3

 d=[a(ii,jj)];

 b=[b;d];

 end

 end
 b = reshape(b,3,3)';

[babak]

actually its a simplified example. my exact work is :

>> enz=[];

>> enz=zeros(8,3);

>> for ii=1:14

for jj=1:3

match=ismember(file1,rxn)

if match(ii)==1

enz(ii,jj)=rxn(ii,jj)

end

end

end

i corrected my code according to your advise, but it doesont work, rxn is 14x3, and file1 is 8x1 . i found the contents of the file1 in rxn and now i'm going to copy the whole contents of related rows(the rows that contains the same contents) in rxn to a new matrix, which should creat a 8x3 matrix, but i cant shape this matrix

Answer 2

[nah]

    for i=1:3
      for j=1:3
         b(i,j) = a(i,j);
      end
    end

The fault is that you haven't defined the end the a row anywhere.

   d=[a(i,j)]
   b=[b;d];

d becomes a(i,j) and the element goes into b, which becomes a vector of 9 elements;

what is preventing you from simply doing ,

   b = a; ?
or b(i,j) = a(i,j) ?