how to change values in matrix by indices

Hi, i need to change all the values in the index where i,j are both even or odd to zero and multiple by 2 all the other. for example:

change

       1,2,3
       4,5,6
       7,8,9

to

       0,4,0 
       8,0,12
       0,16,0

[James Tursa]

Sounds like homework. Are you instructed to use any particular method (for loop, logical indexing, etc)?

Answer 1

[James Tursa]

Check out these links for patterns you could use:

http://www.mathworks.com/matlabcentral/answers/196631-how-can-i-create-a-matrix-of-alternating-1s-and-0s-for-any-size-matrix

http://www.mathworks.com/matlabcentral/newsreader/view_thread/340389#933451

Answer 2

[Stephen23]

You might find toeplitz useful for this:

>> n = 4;
>> toeplitz(mod(1:n,2))
ans =
   1     0     1     0
   0     1     0     1
   1     0     1     0
   0     1     0     1

You could use MATLAB's rather powerful indexing ability to solve this problem, in which case you can convert that matrix of ones and zeros to a logical array using logical.

Answer 3

[Andrei Bobrov]

variant:

a = [1,2,3
     4,5,6
     7,8,9];

[i1,i2] = ndgrid(1:size(a,1));
out = rem(i1+i2,2)*2.*a;

Answer 4

[Thorsten]

 index = 1:9;

 ieven = mod(index, 2) ==0;

 index(ieven) = 2*index
 index(~ieven) = 0