<分区>
写了一个简单的程序来测量STL的速度。以下代码显示在我的 Corei7-2670QM PC(2.2GHz 和 Turbo 3.1GHz)上花费了 1.49 秒。如果我删除循环中的 Employees[buf] = i%1000; 部分，它只需要 0.0132 秒。所以散列部分用了 1.48 秒。为什么这么慢？
#include <string.h>
#include <iostream>
#include <map>
#include <utility>
#include <stdio.h>
#include <sys/time.h>
using namespace std;
extern "C" {
int get(map<string, int> e, char* s){
return e[s];
}
int set(map<string, int> e, char* s, int value) {
e[s] = value;
}
}
double getTS() {
struct timeval tv;
gettimeofday(&tv, NULL);
return tv.tv_sec + tv.tv_usec/1000000.0;
}
int main()
{
map<string, int> Employees;
char buf[10];
int i;
double ts = getTS();
for (i=0; i<1000000; i++) {
sprintf(buf, "%08d", i);
Employees[buf] = i%1000;
}
printf("took %f sec\n", getTS() - ts);
cout << Employees["00001234"] << endl;
return 0;
}
这是您的代码的 C++ 版本。请注意，在 get/set 中传递 map 时，您应该显然通过引用获取 map 。
更新 更进一步，认真优化给定的测试用例:
Live On Coliru
#include <iostream>
#include <boost/container/flat_map.hpp>
#include <chrono>
using namespace std;
using Map = boost::container::flat_map<string, int>;
int get(Map &e, char *s) { return e[s]; }
int set(Map &e, char *s, int value) { return e[s] = value; }
using Clock = std::chrono::high_resolution_clock;
template <typename F, typename Reso = std::chrono::microseconds, typename... Args>
Reso measure(F&& f, Args&&... args) {
auto since = Clock::now();
std::forward<F>(f)(std::forward<Args>(args)...);
return chrono::duration_cast<Reso>(Clock::now() - since);
}
#include <boost/iterator/iterator_facade.hpp>
using Pair = std::pair<std::string, int>;
struct Gen : boost::iterators::iterator_facade<Gen, Pair, boost::iterators::single_pass_traversal_tag, Pair>
{
int i;
Gen(int i = 0) : i(i) {}
value_type dereference() const {
char buf[10];
std::sprintf(buf, "%08d", i);
return { buf, i%1000 };
}
bool equal(Gen const& o) const { return i==o.i; }
void increment() { ++i; }
};
int main() {
Map Employees;
const auto n = 1000000;
auto elapsed = measure([&] {
Employees.reserve(n);
Employees.insert<Gen>(boost::container::ordered_unique_range, {0}, {n});
});
std::cout << "took " << elapsed.count() / 1000000.0 << " sec\n";
cout << Employees["00001234"] << endl;
}
打印
took 0.146575 sec
234
旧答案
这只是在适当的地方使用了 C++
Live On Coliru
#include <iostream>
#include <map>
#include <chrono>
#include <cstdio>
using namespace std;
int get(map<string, int>& e, char* s){
return e[s];
}
int set(map<string, int>& e, char* s, int value) {
return e[s] = value;
}
using Clock = std::chrono::high_resolution_clock;
template <typename Reso = std::chrono::microseconds>
Reso getElapsed(Clock::time_point const& since) {
return chrono::duration_cast<Reso>(Clock::now() - since);
}
int main()
{
map<string, int> Employees;
std::string buf(10, '\0');
auto ts = Clock::now();
for (int i=0; i<1000000; i++) {
buf.resize(std::sprintf(&buf[0], "%08d", i));
Employees[buf] = i%1000;
}
std::cout << "took " << getElapsed(ts).count()/1000000.0 << " sec\n";
cout << Employees["00001234"] << endl;
}
打印:
took 0.470009 sec
234