c ++ stringstream太慢了，怎么加速？ [英] c++ stringstream is too slow, how to speed up?

问题描述

可能重复：

从C ++中的文本文件中读取数值的最快方法（在这种情况下为双精度）

#include <ctime>
#include <cstdlib>
#include <string>
#include <sstream>
#include <iostream>
#include <limits>

using namespace std;

static const double NAN_D = numeric_limits<double>::quiet_NaN();

void die(const char *msg, const char *info)
{
    cerr << "** error: " << msg << " \"" << info << '\"';
    exit(1);
}

double str2dou1(const string &str)
{
    if (str.empty() || str[0]=='?') return NAN_D;
    const char *c_str = str.c_str();
    char *err;
    double x = strtod(c_str, &err);
    if (*err != 0) die("unrecognized numeric data", c_str);
    return x;
}

static istringstream string_to_type_stream;

double str2dou2(const string &str)
{
    if (str.empty() || str[0]=='?') return NAN_D;
    string_to_type_stream.clear();
    string_to_type_stream.str(str);
    double x = 0.0;
    if ((string_to_type_stream >> x).fail())
        die("unrecognized numeric data", str.c_str());
    return x;
}

int main()
{
    string str("12345.6789");

    clock_t tStart, tEnd;

    cout << "strtod: ";
    tStart=clock();

    for (int i=0; i<1000000; ++i)
        double x = str2dou1(str);

    tEnd=clock();
    cout << tEnd-tStart << endl;

    cout << "sstream: ";
    tStart=clock();

    for (int i=0; i<1000000; ++i)
        double x = str2dou2(str);

    tEnd=clock();
    cout << tEnd-tStart << endl;

    return 0;
}

strtod：405

sstream：1389

strtod: 405
sstream: 1389

更新：remove underocres，env：win7 + vc10

update: remove undersocres, env: win7+vc10

推荐答案

C / C ++文本到数字格式化非常慢。流非常慢，但即使C号解析很慢，因为很难得到正确的最后一个精度位。

C/C++ text to number formatting is very slow. Streams are horribly slow but even C number parsing is slow because it's quite difficult to get it correct down to the last precision bit.

在一个生产应用程序中，读取速度很重要，而且已知数据最多有三位十进制数字，没有科学记数法，我通过手工编码浮点解析函数仅处理符号，整数部分和任何小数位数（通过浩大我的意思是比 strtod 快10倍）。

In a production application where reading speed was important and where data was known to have at most three decimal digits and no scientific notation I got a vast improvement by hand-coding a floating parsing function handling only sign, integer part and any number of decimals (by "vast" I mean 10x faster compared to strtod).

如果你不需要指数，这个函数的精度就足够了，这就是解析器的代码，类似于我之前写的代码。在我的电脑上，它现在是strtod的6.8倍，比sstream快22.6倍。

If you don't need exponent and the precision of this function is enough this is the code of a parser similar to the one I wrote back then. On my PC it's now 6.8 times faster than strtod and 22.6 times faster than sstream.

double parseFloat(const std::string& input)
{
    const char *p = input.c_str();
    if (!*p || *p == '?')
        return NAN_D;
    int s = 1;
    while (*p == ' ') p++;

    if (*p == '-') {
        s = -1; p++;
    }

    double acc = 0;
    while (*p >= '0' && *p <= '9')
        acc = acc * 10 + *p++ - '0';

    if (*p == '.') {
        double k = 0.1;
        p++;
        while (*p >= '0' && *p <= '9') {
            acc += (*p++ - '0') * k;
            k *= 0.1;
        }
    }
    if (*p) die("Invalid numeric format");
    return s * acc;
}

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