How to replace values of certain rows in a matrix?

Hi,

I have a matrix "a"

a =

     1     1
     2     2
     3     3
     4     4

I would like to replace the 2nd and 3rd row in "a" with "b" = [0 0]. So I want to change "a" to

     a =

     1     1
     0     0
     0     0
     4     4

Do I have to do it in a loop? Can I avoid the loop?

Answer 1

[Adam]

a( [2 3], : ) = repmat( b, 2, 1 )

A bit more generally:

indicesToReplace = [2 3];
a( [2 3], : ) = repmat( b, indicesToReplace, 1 );

[dpb]

For the OP's specific case of replacing with a constant, automagic expansion negates need for repmat.

a(indicesToReplace,:)=0;

Side note to OP--work through the example tutorials in the "Getting Started" documentation link to get a feel for basic MATLAB syntax and addressing modes including colon and friends...will be an investment in time paid back many times over.