这是C ++中输入速度最快的方法 [英] Which is the fastest method of input in C++

问题描述

我正在编写各种编程奥林匹克运动，因此提高时间效率我寻找最快的方法获得输入，而不需要添加额外的库，即在标准的可用目录下的gcc编译器，因为在比赛我们显然不能向编译器添加库。直到现在在C + +，我使用cin和cout，但发现scanf和printf比它更快，所以任何人都可以告诉我一个更快的方式，它可能是一个自定义函数，因为我不介意空间复杂性，但我更喜欢时间更多。
谢谢你在高级。

I am coding for various programming olympiads and thus to improve time efficiency i am looking for the fastest method for getting input without need of adding extra library , ie , within the standard available directory as per gcc compilers as during the competitions we obviously cant add libraries to compiler . Till now in C++ , i used cin and cout , but found that scanf and printf are much faster than it , so can anybody tell me a more faster way , it maybe a custom function , for i don't mind space complexity much , but i prefer time more . Thankyou in advanced .

推荐答案

流总是慢于C-API函数是一个很常见的误解，因为默认情况下，它们与C层同步。

That streams are always slower than the C-API functions is a pretty common misconception because by default, they synchronize with the C-layer. So yeah, that's a feature, not a bug.

在不牺牲类型安全性（和可读性，取决于您的品味）的情况下，您可以通过使用以下方式获得流的性能： / p>

Without sacrificing type safety (and readability, depending on your taste), you possibly gain performance with streams by using:

std::ios_base::sync_with_stdio (false);

---

一点指示符：

---

A little indicator:

#include <cstdio>
#include <iostream>

template <typename Test> 
void test (Test t)
{
    const clock_t begin = clock();
    t();
    const clock_t end = clock();
    std::cout << (end-begin)/double(CLOCKS_PER_SEC) << " sec\n";
}

void std_io() {
    std::string line;
    unsigned dependency_var = 0;

    while (!feof (stdin)) {
        int c;
        line.clear();
        while (EOF != (c = fgetc(stdin)) && c!='\n')
            line.push_back (c);
        dependency_var += line.size();
    }

    std::cout << dependency_var << '\n';
}

void synced() {
    std::ios_base::sync_with_stdio (true);
    std::string line;
    unsigned dependency_var = 0;
    while (getline (std::cin, line)) {
        dependency_var += line.size();
    }
    std::cout << dependency_var << '\n';
}

void unsynced() {
    std::ios_base::sync_with_stdio (false);
    std::string line;
    unsigned dependency_var = 0;
    while (getline (std::cin, line)) {
        dependency_var += line.size();
    }
    std::cout << dependency_var << '\n';
}

void usage() { std::cout << "one of (synced|unsynced|stdio), pls\n"; }

int main (int argc, char *argv[]) {
    if (argc < 2) { usage(); return 1; }

    if (std::string(argv[1]) == "synced") test (synced);
    else if (std::string(argv[1]) == "unsynced") test (unsynced);
    else if (std::string(argv[1]) == "stdio") test (std_io);
    else { usage(); return 1; }

    return 0;
}

使用g ++ -O3和一个大文本文件：

With g++ -O3, and a big text file:

cat testfile | ./a.out stdio
...
0.34 sec

cat testfile | ./a.out synced
...
1.31 sec

cat testfile | ./a.out unsynced
...
0.08 sec

适用于您的情况。修改此玩具基准，添加更多测试，并比较例如 std :: cin>> a>> b>> c 与 scanf（％d％d％d，& a，& b，& c）; 。我保证，通过优化（即不处于调试模式），性能差异将是微妙的。

How this applies to your case depends. Modify this toy-benchmark, add more tests, and compare e.g. something like std::cin >> a >> b >> c with scanf ("%d %d %d", &a, &b, &c);. I guarantee, with optimizations (i.e. without being in debug mode), performance differences will be subtle.

如果这不会使您的需求饱和，您可以尝试其他方法，例如。首先读取整个文件（可能带来更多性能）或内存映射（这是一个非便携式解决方案，但是大型桌面有它们）。

If that does not saturate your needs, you might try other approaches, e.g. reading the whole file first (may or may not bring more performance) or memory maps (which is a non-portable solution, but the big desktops have them).

#include <cstdio>
#include <iostream>

template <typename Test> 
void test (Test t)
{
    const clock_t begin = clock();
    t();
    const clock_t end = clock();
    std::cout << (end-begin)/double(CLOCKS_PER_SEC) << " sec\n";
}

void scanf_() {
    char x,y,c;
    unsigned dependency_var = 0;

    while (!feof (stdin)) {
        scanf ("%c%c%c", &x, &y, &c);
        dependency_var += x + y + c;
    }

    std::cout << dependency_var << '\n';
}

void unsynced() {
    std::ios_base::sync_with_stdio (false);
    char x,y,c;
    unsigned dependency_var = 0;
    while (std::cin) {
        std::cin >> x >> y >> c;
        dependency_var += x + y + c; 
    }
    std::cout << dependency_var << '\n';
}

void usage() { std::cout << "one of (scanf|unsynced), pls\n"; }

int main (int argc, char *argv[]) {
    if (argc < 2) { usage(); return 1; }

    if (std::string(argv[1]) == "scanf") test (scanf_);
    else if (std::string(argv[1]) == "unsynced") test (unsynced);
    else { usage(); return 1; }

    return 0;
}

结果：

scanf: 0.63 sec
unsynced stream: 0.41 

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