conditional indexing of mata matrix

[Max TabordMeehan]

Hello,

I have been struggling to figure out a way to index elements of a mata matrix conditionally. My ultimate goal is as follows:

Consider two n by k matrices A and B, where B contains only 0s and 1s. I would like to place missing values into A at those indices where B is equal to 0.

If the goal was to place something other than missing values (for example say we wanted to place the number 23), then something like:

C = A:*B + J(n,k,23):*(J(n,k,1):-B)

would do the trick. Unfortunately replacing J(n,k,23) by J(n,k,.) does not work.

Any ideas on how to accomplish this?

Regards,

Max

[Rafal Raciborski (StataCorp)]

How about

set seed 12345
mata:
    n = 5
    k = 3
    A = ceil(10*runiform(n,k))
    B = runiform(n,k) :> .3
    A
    B
    A :/ B
end

Here is the output:

:         A
       1   2   3
    +-------------+
  1 |  4   5   7  |
  2 |  6   6   3  |
  3 |  1   7   5  |
  4 |  3   1   1  |
  5 |  5   7   9  |
    +-------------+

:         B
       1   2   3
    +-------------+
  1 |  1   0   0  |
  2 |  0   1   0  |
  3 |  1   1   1  |
  4 |  1   1   1  |
  5 |  0   1   1  |
    +-------------+

:         A :/ B
       1   2   3
    +-------------+
  1 |  4   .   .  |
  2 |  .   6   .  |
  3 |  1   7   5  |
  4 |  3   1   1  |
  5 |  .   7   9  |
    +-------------+

[Max TabordMeehan]

Hi Rafal,

That is great. Using division by zero is perfect here, wish I had thought of it.

Regards,

Max