replacing subset of data

[charlie wong]

Hi all

I am new to Mata. Wondering what would be the Mata way (the fastest I suppose) of doing the following 'replace':

clear
set obs 10
g a=1
g b=1
g c=1
g d=1
g e=1
g id=1 if _n>5

foreach var of varlist a-e{
replace `var'=. if id==1
}

Grateful for your help.

[German Rodriguez]

st_data() and st_store() have an argument called "selectvar" to select elements. Just make sure you replace with a matrix of the correct dimension. For your data

. replace id = _n > 5
(5 real changes made)

. mata n = sum(st_data(., "id"))

. mata st_store(., 1..5, "id", J(n, 5, .))

. list

     +------------------------+
     | a   b   c   d   e   id |
     |------------------------|
  1. | 1   1   1   1   1    0 |
  2. | 1   1   1   1   1    0 |
  3. | 1   1   1   1   1    0 |
  4. | 1   1   1   1   1    0 |
  5. | 1   1   1   1   1    0 |
     |------------------------|
  6. | .   .   .   .   .    1 |
  7. | .   .   .   .   .    1 |
  8. | .   .   .   .   .    1 |
  9. | .   .   .   .   .    1 |
 10. | .   .   .   .   .    1 |
     +------------------------+

Note that I changed your id variable to take values 1 and 0 rather than 1 and missing, so it corresponds to true and false. I would also call id something else, like flag. Instead of hardcoding variables 1..5 you could define vars = ("a", "b", "c", "d", "e") and then code st_store(., vars, "id", J(n, length(vars), .)), which is more general.

[charlie wong]

German Rodriguez thanks a lot!