c++ - Rcpp: neat way to compare strings derived from an R data frame? -


having headaches in rcpp handling strings, have looked @ "how test rcpp::charactervector elements equality" situation bit more complex that.

to illustrate, suppose have 200-row data frame of names , marks, generated randomly:

df = data.frame(name = paste("person",                              sample(letters[1:10],200,rep=true),sep=""),                  mark = pmax(pmin(round(rnorm(200,60,15)),100),0),                  stringsasfactors=false)

i found following inline code (using rcpp) correctly works out sum of marks rows person named first person given in data frame (i.e. df$name[1] in r, or equivalently name[0] in rcpp code):

library(inline)  fastfunc_good1 <- cxxfunction(     signature(dfin = "data.frame"),     plugin = "rcpp",     body = '         rcpp::dataframe df(dfin);         rcpp::charactervector name = df["name"];         rcpp::integervector mark = df["mark"];         rcpp::charactervector targetname(1);         rcpp::charactervector thisname(1);                int n = name.length();         int tot = 0;         targetname = name[0];         std::string s_targetname = as<std::string>(targetname);          (int = 0; < n; i++) {             thisname=name[i];             std::string s_thisname = as<std::string>(thisname);             if (s_thisname == s_targetname) {                 tot = tot + mark[i];             }         }          return(rcpp::wrap(tot));         ')

now, want simplify as possible, it's messy have define separate variable represent value in name[], coerce std::string, , comparison. there must way of simplifying notation looks more following (which should noted not work!)...

fastfunc_bad1 <- cxxfunction(     signature(dfin = "data.frame"),     plugin = "rcpp",     body = '         rcpp::dataframe df(dfin);         rcpp::charactervector name = df["name"];         rcpp::integervector mark = df["mark"];          int n = name.length();         int tot = 0;          (int = 0; < n; i++) {             if (name[i] == name[0]) {                 tot = tot + mark[i];             }         }          return(rcpp::wrap(tot));         ')

ultimately goal of mini learning project me figure out how iterate through unique names in df$name, compute mark sum each one, , return (the unique names , corresponding sums) neat data frame. i've figured out of nuts , bolts of how build , return final data frame other examples - it's string stuff described above that's causing me headaches. many in advance pointers!

you can use rcpp::as convert r objects c++ containers. following works me. 

fastfunc_good2 <- cxxfunction(     signature(dfin = "data.frame"),     plugin = "rcpp",     body = '         rcpp::dataframe df(dfin);         std::vector<std::string> name = rcpp::as<std::vector<std::string> >(df["name"]);         std::vector<int> mark = rcpp::as<std::vector<int> >(df["mark"]);          int n = name.size();         int tot = 0;          (int = 0; < n; i++) {             if (name[i] == name[0]) {                 tot = tot + mark[i];             }         }          return(rcpp::wrap(tot));         ')   > fastfunc_good1(df) [1] 1040  > fastfunc_good2(df) [1] 1040

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