c++ stringstream is too slow, how to speed up? [duplicate]

Question

Possible Duplicate:
Fastest way to read numerical values from text file in C++ (double in this case)

#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

update: remove undersocres, env: win7+vc10

Answer 1

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.

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).

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; }