Take a uniform sampling of whitespace delimted strings from the standard input. algorithm R

sample.cpp

#include <iostream>
#include <random>
#include <algorithm>
#include <iterator>
#include <numeric>
#include <iomanip>
#include <string>

int main(int argc, char *argv[]) {
	if (argc < 2 || argv[1] == "--help" || argv[1] == "-h") {
		std::cout << "Usage: " << argv[0] << "<sample size>\n";
		std::cout << "       " << argv[0] << "[--help|-h]\n\n";
		std::cout << "Description: Take a uniform sampling of whitespace delimted strings from the standard input.\n";
		return argc < 2 ? EXIT_FAILURE : EXIT_SUCCESS;
	}

	int sample_size;
	try {
		sample_size = std::stoi(argv[1]);
	}
	catch (...) {
		std::cerr << "Unable to parse sample size '" << argv[1] << "'\n";
		return EXIT_FAILURE;
	}

	if (sample_size < 1) {
		std::cerr << "Invalid sample size '" << argv[1] << "'. Sample size must be larger than zero.\n";
		return EXIT_FAILURE;
	}

	std::default_random_engine eng{ std::random_device{}() };
	std::uniform_real_distribution<> dist;

	std::vector<std::string> sample(sample_size);

	std::istream_iterator<std::string> input(std::cin);
	std::istream_iterator<std::string> input_end;

	// initialize the resevoir with the first k elements
	for (int i = 0; i < sample_size && input != input_end; ++i) {
		sample[i] = *input++;
	}

	std::string s;
	long long input_count = sample_size;
	while (input != input_end) {
		s = *input++;
		if (input_count == std::numeric_limits<decltype(input_count)>::max()) {
			std::cerr << "Error: input too long. Overflowing internal count.";
			return EXIT_FAILURE;
		}
		input_count++;

		// for each element thereafter, add it to the resevoir with probability k / (i+1)
		// input_count = i + 1
		// i would be the zero based index of the input
		if (std::uniform_int_distribution<>(1, input_count)(eng) <= sample_size) {
			sample[std::uniform_int_distribution<>(0, sample.size() - 1)(eng)] = s; // Choose the element in the resevoir to replace randomly
		}
	}

	std::copy(begin(sample), end(sample), std::ostream_iterator<std::string>(std::cout, "\n"));
}