C++ 文件读入速度测试

C++ 文件读入速度测试

在信息学竞赛中,输入数据规模可能会很大,这时候就需要注意文件读取的效率。本文在 Linux 环境下测试了 C++ 几种常见读入方式的效率。

1. 系统环境

Arch Linux x86_64

默认 Linux 内核,版本 6.1.3

gcc 12.2.0

ext4

2. 测试代码

编译命令(省略文件):g++ -std=c++20不开优化

2.0 随机整数(32位有符号)生成

#include <climits>
#include <fstream>
#include <random>
using namespace std;

using i64 = long long;

int main()
{
    constexpr int N = 10000;
    ofstream fout("in");
    random_device rd;
    mt19937 gen(rd());

    for (int i = 0; i < N; ++i) {
        for (int j = 0; j < N; ++j)
            fout << (i64)gen() + (i64)INT_MIN << " ";
        fout << "
";
    }
    return 0;
}

2.1 freopen + scanf

#include <cstdio>
using namespace std;

int main()
{
    freopen("in", "r", stdin);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            scanf("%d", &x);
    return 0;
}

2.2 FILE*

#include <cstdio>
using namespace std;

int main()
{
    FILE *fp = fopen("in", "r");
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fscanf(fp, "%d", &x);
    return 0;
}

2.3 ifstream

#include <fstream>
using namespace std;

int main()
{
    ifstream fin("in");
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fin >> x;
    return 0;
}

2.4 freopen + cin

#include <cstdio>
#include <iostream>
using namespace std;

int main()
{
    freopen("in", "r", stdin);
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            cin >> x;
    return 0;
}

2.5 快读

从洛谷模板题快速排序的最优解复制的快读模板(做了少量修改)

#include <cctype>
#include <cstdio>
#include <cstring>
using namespace std;

namespace IO
{
class qistream
{
    static const size_t SIZE = 1 << 16, BLOCK = 32;
    FILE *fp;
    char buf[SIZE];
    int p;

  public:
    qistream(FILE *_fp = stdin) : fp(_fp), p(0)
    {
        fread(buf + p, 1, SIZE - p, fp);
    }

    void flush()
    {
        memmove(buf, buf + p, SIZE - p), fread(buf + SIZE - p, 1, p, fp), p = 0;
    }

    qistream &operator>>(char &str)
    {
        str = getch();
        while (isspace(str))
            str = getch();
        return *this;
    }

    template <class T> qistream &operator>>(T &x)
    {
        x = 0;
        p + BLOCK >= SIZE ? flush() : void();
        bool flag = false;
        for (; !isdigit(buf[p]); ++p)
            flag = buf[p] == "-";
        for (; isdigit(buf[p]); ++p)
            x = x * 10 + buf[p] - "0";
        x = flag ? -x : x;
        return *this;
    }

    char getch() { return buf[p++]; }

    qistream &operator>>(char *str)
    {
        char ch = getch();
        while (ch <= " ")
            ch = getch();
        for (int i = 0; ch > " "; ++i, ch = getch())
            str[i] = ch;
        return *this;
    }
};
} // namespace IO

int main()
{
    FILE *fp = fopen("in", "r");
    IO::qistream fin(fp);
    constexpr int N = 10000;
    int x;
    for (int i = 0; i < N; ++i)
        for (int j = 0; j < N; ++j)
            fin >> x;
    return 0;
}

3. 测试方法

  1. 运行一遍随机整数生成。输出的 in 文件作为 5 份代码共同的输入文件。
  2. 依次运行 5 份代码,每份代码运行 5 次,使用 bash 内置 time 命令进行计时,取最后 3 次运行时间取平均数。

4. 测试结果

freopen + scanf FILE* ifstream freopen + cin 快读
10.471s 10.485s 6.767s 6.889s 4.967s

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