Retrieve row number according to value of another variable in index - stata

Question

This problem is very simple in R, but I can't seem to get it to work in Stata.
I want to use the square brackets index, but with an expression in it that involves another variable, i.e. for a variable with unique values cumul I want:
replace country = country[cumul==20] in 12
cumul == 20 corresponds to row number 638 in the dataset, so the above should replace in line 12 the country variable with the value of that same variable in line 638. The above expression is clearly not the right way to do it: it just replaces the country variable in line 12 with a missing value.

Answer 1

Stata's row indexing does not work in that way. What you can do, however, is a simple two-line solution:
levelsof country if cumul==20
replace country = "`r(levels)'" in 12
If you want to be sure that cumul==20 uniquely identifies just a single value of country, add:
assert `:word count `r(levels)''==1
between the two lines.

Answer 2

It's probably worth explaining why the construct in the question doesn't work as you wish, beyond "Stata is not R!".
Given a variable x: in a reference like x[1] the [1] is referred to as a subscript, despite nothing being written below the line. The subscript is the observation number, the number being always that in the dataset as currently held in memory.
Stata allows expressions within subscripts; they are evaluated observation by observation and the result is then used to look-up values in variables. Consider this sandbox:
clear
input float y
1
2
3
4
5
end
. gen foo = y[mod(_n, 2)]
(2 missing values generated)
. gen x = 3
. gen bar = y[y == x]
(4 missing values generated)
. list
+-------------------+
| y foo x bar |
|-------------------|
1. | 1 1 3 . |
2. | 2 . 3 . |
3. | 3 1 3 1 |
4. | 4 . 3 . |
5. | 5 1 3 . |
+-------------------+
mod(_n, 2) is the remainder on dividing the observation _n by 2: that is 1 for odd observation numbers and 0 for even numbers. Observation 0 is not in the dataset (Stata starts indexing at 1). It's not an error to refer to values in that observation, but the result is returned as missing (numeric missing here, and empty strings "" if the variable is string). Hence foo is x[1] or 1 for odd observation numbers and missing for even numbers.
True or false expressions are evaluated as 1 if true and 0 is false. Thus y == x is true only in observation 3, and so bar is the value of y[1] there and missing everywhere else. Stata doesn't have the special (and useful) twist in R that it is the subscripts for which a true or false expression is true that are used to select zero or more values.
There are ways of using subscripts to get special effects. This example shows one. (It's much easier to get the same kind of result in Mata.)
. gen random = runiform()
. sort random
. gen obs = _n
. sort y
. gen randomsorted = random[obs]
. l
+-----------------------------------------------+
| y foo x bar random obs random~d |
|-----------------------------------------------|
1. | 1 1 3 . .3488717 4 .0285569 |
2. | 2 . 3 . .2668857 3 .1366463 |
3. | 3 1 3 1 .1366463 2 .2668857 |
4. | 4 . 3 . .0285569 1 .3488717 |
5. | 5 1 3 . .8689333 5 .8689333 |
+-----------------------------------------------+
This answer doesn't cover matrices in Stata or Mata.