[Rcpp-devel] problem with sample() implemented in Rcpp/RcppArmadillo

[Christian Gunning]

Dear all,

Code for uniform sampling is below, following the prototype from earlier
post.  Results/unit-tests (not included) are identical to R-core sample().

-Christian

template <class T>
T sample(const T &x, const unsigned int size, const bool replace ) {
    int ii, jj;
    int nOrig = x.size();
    T ret(size);
    if ( size > nOrig && !replace) throw std::range_error( "Tried to sample
more elements than in x without replacement" ) ;
    IntegerVector index(size);
    if (replace) {
        SampleReplace(index, nOrig);
    } else {
        SampleNoReplace(index, nOrig);
    }

    for (ii=0; ii<size; ii++) {
        jj = index[ii];
        ret[ii] = x[jj];
    }
    return(ret);
}

void SampleReplace( IntegerVector index, unsigned int nOrig) {
    int ii;
    int  nSample = index.size();
    for (ii = 0; ii < nSample; ii++) {
        index[ii] = nOrig * unif_rand();
    }
}

void SampleNoReplace( IntegerVector index, unsigned int nOrig) {
    int ii, jj;
    int nSample = index.size();
    IntegerVector sub(nOrig);
    for (ii = 0; ii < nOrig; ii++) {
        sub[ii] = ii;
    }

    for (ii = 0; ii < nSample; ii++) {
        jj = nOrig * unif_rand();
        index[ii] = sub[jj];
        // replace sampled element with last, decrement
        sub[jj] = sub[--nOrig];
    }
}
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