How to loop through specific string values of a variable in Stata?

Question

I want to loop through a specific subset vector of a string variable in Stata. I have a data set like:

id  country  effect  period
1   US       0.20    2
2   US       0.25    3
3   Japan    0.37    2
4   Germany  0.22    3
5   US       0.11    3
6   Japan    0.43    1
7   Ireland  0.30    1
...

I don't want to loop through all values of the country-variable, but only through specific values, e.g. US and Japan. I tried:

levelsof country if country=="US" | country=="Japan", local(countrylev)
levelsof period, local(periodlev) //periods are 1,2,3,4
mat m = J(2,4,.)
local i=1
local j=1

foreach x of local countrylev {
 foreach per of local periodlev {
    mat m[`i',`j']=`per' *2
    local ++j
    mat m[`i',`j']=`per' *3
    local ++j 
    mat m[`i',`j']=`per' *3
    local ++j 
    mat m[`i',`j']=`per' *4
    local ++i
    local j=1
 }
 matrix list m
}

However this only loops through "Japan"...

Describing the ultimate purpose of this might get you better advice. — Roberto Ferrer
– Roberto Ferrer, Commented Jun 15, 2015 at 13:19
This question has yet to yield an intelligible, clear and reproducible problem. I've commented on your revised code in an updated answer. As the code reduces to something trivial, I think you edited your real problem out. — Nick Cox
– Nick Cox, Commented Jun 16, 2015 at 11:01
You are totally right. Sorry for that. I'll delete my question as it is not value adding for other users. — jeffrey
– jeffrey, Commented Jun 17, 2015 at 1:50

Nick Cox · Accepted Answer · 2015-06-15 16:30:32Z

3

I have to guess that the answer lies in the data or the code you have represented by dots. For example, check for trailing spaces such as "US ".

For cycling over two distinct values, you can be direct and say

foreach c in US Japan { 
   <stuff> if country == "`c'" 
}

except that (as above) trailing spaces could undermine that. Use trim() if spaces are the problem.

It is often easier to map to a numeric variable and then use the distinct numeric values you want. That is especially true when you have blanks and/or other punctuation to handle too. For more, see (e.g.) this FAQ (which is more relevant than the title implies; see Method 1).

UPDATE

Revised code makes matters more puzzling, not less. Your code seems to boil down to

mat m = J(2,4,.)

forval i = 1/2 {
    forval j = 1/4 {
        mat m[`i',`j']=`j'*2
        mat m[`i',`j']=`j'*3
        mat m[`i',`j']=`j'*3
        mat m[`i',`j']=`j'*4
    }
}

matrix li M

as listing the matrix each time around the loop doesn't seem important.

Indeed, the example seems to collapse to one line

mat M = (2,3,3,4)' * (1,2,3,4)

However, you may have simplified something important for you out of the problem to focus on what is problematic in terms of code.

edited Jun 15, 2015 at 16:30

answered Jun 15, 2015 at 11:36

Nick Cox

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2 Comments

jeffrey Over a year ago

Thanks! You are totally right. The problem is the content in the dots. I edited my question. The issue is that I receive a confirmability error r(503). I think there is a problem with the matrix 'm' after the first loop... Could you help me with this?

user4690969 Over a year ago

At a minimum, your foreach per of local period should be for periodlev, not period.

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