Replacing part of a matrix without looping element by element

[Sean Higgins]

Is there an efficient way to replace a sub-matrix within a matrix with another matrix? I know I can do it by looping element by element but this is not very efficient.

matrix A = J(20,4,.) // matrix of missing values
matrix B = (1,2 \ 3,4 \ 5,6 \ 7,8 \ 9,10) // smaller matrix
* and I want to do something like
matrix A[10..14,2..3] = B
* which gives
* 14 invalid name
* r(198);
* I know I can do it element by element as follows but 
* this is not very efficient with big matrices
local rowcount = 1
forval row = 10/14 {
     local colcount = 1
     forval col = 2/3 {
          matrix A[`row',`col'] = B[`rowcount',`colcount']
          local ++colcount
     }
     local ++rowcount
}

[Christophe Kolodziejczyk]

matrix A[10,2] = B

http://www.stata.com/help.cgi?matrix+substitution

[Nick Cox]

Use Mata:

 
. mata 
------------------------------------------------- mata (type end to exit) --------------
: A = J(20,4,.) 

: B = (1,2 \ 3,4 \ 5,6 \ 7,8 \ 9,10) 

: A[|10,1 \14, 2|] = B

: A 
         1    2    3    4
     +---------------------+
   1 |   .    .    .    .  |
   2 |   .    .    .    .  |
   3 |   .    .    .    .  |
   4 |   .    .    .    .  |
   5 |   .    .    .    .  |
   6 |   .    .    .    .  |
   7 |   .    .    .    .  |
   8 |   .    .    .    .  |
   9 |   .    .    .    .  |
  10 |   1    2    .    .  |
  11 |   3    4    .    .  |
  12 |   5    6    .    .  |
  13 |   7    8    .    .  |
  14 |   9   10    .    .  |
  15 |   .    .    .    .  |
  16 |   .    .    .    .  |
  17 |   .    .    .    .  |
  18 |   .    .    .    .  |
  19 |   .    .    .    .  |
  20 |   .    .    .    .  |
     +---------------------+

: end

[Sean Higgins]

Thanks to both -- both solutions work.

[Nick Cox]

Amusingly, at least to me, I had forgotten about Stata's indulgence here, as pointed out by Christophe: I am now more used to Mata for this kind of thing.

[Christophe Kolodziejczyk]

Thanks Nick. I thought for a moment I had missed something . That being said Mata is far more powerful when it comes to work with matrices