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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

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