R Quiz

Given functions

set.seed(171111)

gen.mat <- function(dim, min = 0, max = 1) {
    mat <- matrix(sample(min:max, dim^2, replace=TRUE), dim)
    rownames(mat) <- LETTERS[1:dim]
    colnames(mat) <- LETTERS[1:dim]
    mat
}

adjmat <- gen.mat(dim = 5)
adjmat

##   A B C D E
## A 0 1 1 1 0
## B 0 0 1 1 1
## C 1 1 1 0 0
## D 0 0 0 0 0
## E 1 0 1 1 1

Test

Rcpp::sourceCpp("leadingone.cpp")

leading.one <- leading_one
leading.num <- leading_one

leading.one(adjmat, 1)

##   A B C D E
## A 0 1 0 0 0
## B 0 0 1 0 0
## C 1 0 0 0 0
## D 0 0 0 0 0
## E 1 0 0 0 0

leading.one(adjmat, 2)

##   A B C D E
## A 0 1 1 1 0
## B 0 0 0 0 1
## C 1 0 0 0 0
## D 0 0 0 0 0
## E 0 0 0 0 0

adjmat <- gen.mat(5, 0, 2)

leading.num(adjmat, 1)

##   A B C D E
## A 2 0 0 0 0
## B 0 2 0 0 0
## C 1 0 0 0 0
## D 2 0 0 0 0
## E 2 0 0 0 0

leading.num(adjmat, 2)

##   A B C D E
## A 2 2 2 1 1
## B 0 0 0 0 0
## C 0 0 0 0 0
## D 0 0 0 0 0
## E 0 0 0 0 0

Full Source Code

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::plugins(cpp1z)]]
// [[Rcpp::export]]
NumericVector find_first_nonzero(const NumericVector& v) {
    NumericVector ret(v.size(), 0);
    for (auto i = 0; i < v.size(); ++i) {
        if (v[i] != 0) {
            // non-zero found
            ret[i] = v[i];
            return ret;
        }
    }
    return ret;
}

// [[Rcpp::export]]
NumericMatrix leading_one(const NumericMatrix& m, int dim) {
    NumericMatrix ret(m);

    int n_count = dim == 1 ? m.nrow() : m.ncol();

    for (auto i = 0; i < n_count; ++i) {
        if (dim == 1) {
            // row by row
            auto row = ret(i, _);
            row = find_first_nonzero(row);
        } else {
            // col by col
            auto col = ret(_, i);
            col = find_first_nonzero(col);
        }
    }

    return ret;
}

Benchmark

library(microbenchmark)

microbenchmark(
    leading.one(adjmat, 1),
    leading.one(adjmat, 2)
)

## Unit: microseconds
##                    expr   min     lq    mean median     uq   max neval
##  leading.one(adjmat, 1) 1.390 1.5435 1.81087  1.684 1.8915 9.887   100
##  leading.one(adjmat, 2) 1.385 1.5025 1.67071  1.587 1.7830 2.396   100