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

i want loop through specific subset vector of string variable in stata. have data set like:

id  country  effect  period 1         0.20    2 2         0.25    3 3   japan    0.37    2 4   germany  0.22    3 5         0.11    3 6   japan    0.43    1 7   ireland  0.30    1 ... 

i don't want loop through values of country-variable, through specific values, e.g. , japan. tried:

levelsof country if country=="us" | country=="japan", local(countrylev) levelsof period, local(periodlev) //periods 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 loops through "japan"...

i have guess answer lies in data or code have represented dots. example, check trailing spaces such "us ".

for cycling on 2 distinct values, can direct ,

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

except (as above) trailing spaces undermine that. use trim() if spaces problem.

it easier map numeric variable , use distinct numeric values want. true when have blanks and/or other punctuation handle too. more, see (e.g.) this faq (which more relevant title implies; see method 1).

update

revised code makes matters more puzzling, not less. code seems boil down

mat m = j(2,4,.)  forval = 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 matrix each time around loop doesn't seem important.

indeed, example seems collapse 1 line

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

however, may have simplified important out of problem focus on problematic in terms of code.