C++におけるstringのsplit

c++にはstring内にsplitメソッドが定義されていない。そのため自分で実装しなければならない。
##実装
###stringstreamを用いる

split.cpp

#include <vector>
#include <string>
#include <sstream>

using namespace std;

vector<string> split(const string &s, char delim) {
	vector<string> elems;
	stringstream ss(s);
	string item;
	while (getline(ss, item, delim)) {
    if (!item.empty()) {
			elems.push_back(item);
		}
	}
	return elems;
}

###愚直に実装

split_naive.cpp

#include <vector>
#include <string>

using namespace std;

vector<string> split_naive(const string &s, char delim) {
	vector<string> elems;
	string item;
	for (char ch: s) {
		if (ch == delim) {
			if (!item.empty())
				elems.push_back(item);
			item.clear();
		}
		else {
			item += ch;
		}
	}
	if (!item.empty())
		elems.push_back(item);
	return elems;
}

##ベンチマーク
###bench1: 入力文字は短く、関数の実行回数が多い

bench1.cpp

void bench1() {
	string input = "This is  the part when I say I don't want ya";
	char delim = ' ';
	
	cout << "split: " << measeure(split, input, delim, 100000) / (CLOCKS_PER_SEC / 1000) << "ms" << endl;
	cout << "naive: " << measeure(split_naive, input, delim, 100000) / (CLOCKS_PER_SEC / 1000) << "ms" << endl;
}

###bench2: 入力文字は長く、関数の実行回数は少ない。

bench2.cpp

void bench2() {
	string random_input = "";
	int len = 1000000;
	mt19937 engine;
	uniform_int_distribution<int> distribution(0, 25);
	char delim = distribution(engine) + 'a';
	for (int p=0; p<len; ++p) {
		int ch = distribution(engine);
    random_input += ch + 'a';
	}
	
	cout << "split: " << measeure(split, random_input, delim, 10) / (CLOCKS_PER_SEC / 1000) << "ms" << endl;
	cout << "naive: " << measeure(split_naive, random_input, delim, 10) / (CLOCKS_PER_SEC / 1000) << "ms" << endl;
}

結果:

	bench1[ms]	bench2[ms]
split	604	477
naive	384	188

##Reference
[1]http://stackoverflow.com/questions/236129/how-to-split-a-string-in-c