change vector element by name in rcpp - r

Question

I have a function where I need to make a table (tab, then change one value - the value where tab.names() == k, where k is given in the function call.
Looking at http://dirk.eddelbuettel.com/code/rcpp/Rcpp-quickref.pdf, I've hoped that the following code would work (replacing "foo" with a variable name), but I guess that requires the element name to be static, and mine won't be. I've tried using which but that won't compile (invalid conversion from 'char' to 'Rcpp::traits::storage_type<16>::type {aka SEXPREC*}' - so I'm doing something wrong there.
#include <RcppArmadillo.h>
#include <algorithm>
//[[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
// [[Rcpp::export]]
IntegerVector fun(const arma::vec& assignment, int k) {
// count number of peptides per protein
IntegerVector tab = table(as<IntegerVector>(wrap(assignment)));
CharacterVector all_proteins = tab.names();
char kc = '0' + k;
// what I need a working version of:
tab(kc) = 1; // gets ignored, as does a [] version of the same thing.
// or
tab('0' + k) = 1; // also ignored
int ki = which(all_proteins == kc); // gives me compile errors
// extra credit
// tab.names(k-1) = "-1";
return tab;
}
/*** R
set.seed(23)
x <- rpois(20, 5)
k <- 5
fun(x, k)
# same thing in R:
expected_output <- table(x)
expected_output # before modification
# x
# 3 4 5 6 7 9 10 12
# 2 4 3 3 4 2 1 1
expected_output[as.character(k)] <- 1 # this is what I need help with
expected_output
# x
# 3 4 5 6 7 9 10 12
# 2 4 1 3 4 2 1 1
# extra credit:
names(expected_output)[as.character(k)] <- -1
*/
I'm still learning rcpp, and more importantly, still learning how to read the manual pages and plug in the right search terms into google/stackoverflow. I'm sure this is basic stuff (and I'm open to better methods - I currently think like an R programmer in terms of initial approaches to problems, not a C++ programmer.)
(BTW - The use of arma::vec is used in other parts of the code which I'm not showing for simplicity - I realize it's not useful here. I debated on switching it, but decided against it on the principle that I've tested that part, it works, and the last thing I want to do is introduce an extra bug...)
Thanks!

Answer 1

You can use the .findName() method to get the relevant index:
#include <RcppArmadillo.h>
#include <algorithm>
//[[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
// [[Rcpp::export]]
IntegerVector fun(const arma::vec& assignment, int k) {
// count number of peptides per protein
IntegerVector tab = table(as<IntegerVector>(wrap(assignment)));
CharacterVector all_proteins = tab.names();
int index = tab.findName(std::string(1, '0' + k));
tab(index) = 1;
all_proteins(index) = "-1";
tab.names() = all_proteins;
return tab;
}
/*** R
set.seed(23)
x <- rpois(20, 5)
k <- 5
fun(x, k)
*/
Output:
> Rcpp::sourceCpp('table-name.cpp')
> set.seed(23)
> x <- rpois(20, 5)
> k <- 5
> fun(x, k)
3 4 -1 6 7 9 10 12
2 4 1 3 4 2 1 1

Answer 2

You could write your own function (use String instead of char):
int first_which_equal(const CharacterVector& x, String y) {
int n = x.size();
for (int i = 0; i < n; i++) {
if (x[i] == y) return(i);
}
return -1;
}
Also, it seems that tab(kc) is converting kc to an integer representation.