为什么Python的I / O比C ++慢得多？

Question

我已经创建了一些用于在Python和C ++中进行测试的代码，在其中我从文件中读取两个矩阵并打印出一些东西。 似乎Python需要大约两倍的I / O时间：

$ ./test.sh -i Testing/2000.in -p "C++/read-write-only.out" -n 2
Executing: C++/read-write-only.out -i Testing/2000.in > TMPcurrentFileResult.out
It took 8 seconds for 2 executions
MIN: 4 seconds
MAX: 4 seconds

$ ./test.sh -i Testing/2000.in -p "python Python/read-write-only.py" -n 2
Executing: python Python/read-write-only.py -i Testing/2000.in > TMP..Results.out
It took 16 seconds for 2 executions
MIN: 8 seconds
MAX: 8 seconds

这是我用于Python的代码：

#!/usr/bin/python
# -*- coding: utf-8 -*-

from optparse import OptionParser
parser = OptionParser()
parser.add_option("-i", dest="filename", default="bigMatrix.in",
     help="input file with two matrices", metavar="FILE")
(options, args) = parser.parse_args()

def read(filename):
    lines = open(filename, 'r').read().splitlines()
    A = []
    B = []
    matrix = A
    for line in lines:
        if line != "":
            matrix.append(map(int, line.split("\t")))
        else:
            matrix = B
    return A, B

def printMatrix(matrix):
    for line in matrix:
        print "\t".join(map(str,line))

A, B = read(options.filename)
# Do something
printMatrix(B)

这是C ++代码

#include <sstream>
#include <string>
#include <fstream>
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int getMatrixSize(string filename) {
    string line;
    ifstream infile;
    infile.open (filename.c_str());
    getline(infile, line);
    return count(line.begin(), line.end(), '\t') + 1;
}

void read(string filename, vector< vector<int> > &A, vector< vector<int> > &B){
    string line;
    FILE* matrixfile = freopen(filename.c_str(), "r", stdin);

    int i = 0, j, a;
    while (getline(cin, line) && !line.empty()) {
        istringstream iss(line);
        j = 0;
        while (iss >> a) {
            A[i][j] = a;
            j++;
        }
        i++;
    }

    i = 0;
    while (getline(cin, line)) {
        istringstream iss(line);
        j = 0;
        while (iss >> a) {
            B[i][j] = a;
            j++;
        }
        i++;
    }

    fclose (matrixfile);
}

void printMatrix(vector< vector<int> > matrix, int n) {
    for (int i=0; i < n; i++) {
        for (int j=0; j < n; j++) {
            if (j != 0) {
                cout << "\t";
            }
            cout << matrix[i][j];
        }
        cout << endl;
    }
}

int main (int argc, char* argv[]) {
    string filename;
    if (argc < 3) {
        filename = "bigMatrix.in";
    } else {
        filename = argv[2];
    }

    int n = getMatrixSize(filename);
    vector<int> inner (n);
    vector< vector<int> > A(n, inner), B(n, inner), C(n, inner);
    read (filename, A, B);
    // do something with the matrices
    printMatrix(C, n);
    return 0;
}

是否有可能使I / O的Python速度与C ++一样快？ 如何改善Python / C ++的I / O？

（我听说scanf应该比cin更快。为什么应该更快？）

这是包含所有代码的GIT存储库 。

Answer 1

需要一段时间才能启动python解释器。 运行测试时，请考虑到这一点。

Answer 2

当您不混合使用C和C ++文件操作例程时，应关闭与stdio的同步。

http://www.cplusplus.com/reference/iostream/ios_base/sync_with_stdio/

cin和cout通常应比C语言（关闭同步）更快。

就Python的缓慢性而言，那么，为什么不检查I / O功能的实现呢？

Answer 3

您必须以任何语言缓冲I / O。

为什么要比较Python和C ++？

Python是一种解释型语言，而C ++是编译型。