请连老师百忙之中抽出宝贵时间回答一下学生的问题。多谢！！

clear
  input int id str20 x
  1       "A,C,CSR,F"
  2       "A, C, Ex, F,G,N"
  3       "A,F"
  4       "Ex"
  end
  compress
  
  gen     xx = subinstr(x, ", ", " ", .)
  replace xx = subinstr(x, "," , " ", .)
  local N = _N
  local aa ""
  forvalues i = 1/`N'{
    local a = xx[`i']
    local aa `aa' `a'
        *dis "`aa'"
  }

  foreach c of local aa{
     cap gen `c' = strmatch(xx, "*`c'*")
  }

*-results
   list
   browse

*-General Case:

cd "E:\stata答疑\xiaoshen"
use committee.dta, clear
compress
gen xx = subinstr(commit, ", ", " ", .)

preserve
  keep xx
  duplicates drop
  local N = _N
  local Unique ""
  forvalues i = 1/`N'{
    local a = xx[`i']
        tokenize `a', parse(" ")
        local j = 1
    while "``j''" != ""{
          if strmatch("`Unique'", "*``j''*")==0{
            local Unique `Unique' ``j''
                local j = `j' + 1
          }
          local j = `j' + 1
    }
  }   
restore
  
  dis "`Unique'"
       
  foreach c of local Unique{
        cap gen `c' = strmatch(xx, "*`c'*")
  }