(C++) ToDouble

 

ToDouble is a conversion code snippet to convert a std::string to double.

 

ToDouble can be implemented in multiple equivalent ways (incomplete list):

1. ToDouble using std::atof
2. ToDouble using std::strtod
3. ToDouble using std::istringstream

 

To conversion a std::string to any data type one can use LexicalCast and boost::lexical_cast.

 

 

 

 

 

ToDouble using std::atof

 

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#include <cstdlib> #include <string> //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDouble(const std::string& s) {   return std::atof(s.c_str()); }

 

 

 

 

 

ToDouble using std::istringstream

 

The implementation below is similar to the C++ FAQ Lite's convertToDouble code snippet.

 

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#include <sstream> #include <stdexcept> #include <string> //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDouble(std::string const& s) {   std::istringstream i(s);   double x;   if (!(i >> x))   {     throw std::logic_error("std::string cannot be converted to double");   }   return x; }

 

 

 

 

 

ToDouble using std::strtod

 

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#include <cstdlib> #include <string> //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDouble(const std::string& s) {   return std::strtod(s.c_str(),0); }

 

 

 

 

 

ToDouble tests

 

In the code below, all ToDouble flavors are compared and found out equivalent.

 

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#include <algorithm> #include <cassert> #include <cstdlib> #include <iostream> #include <sstream> #include <stdexcept> #include <string> //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDoubleUsingAtof(const std::string& s) {   return std::atof(s.c_str()); } //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDoubleUsingStrtod(const std::string& s) {   return std::strtod(s.c_str(),0); } //From http://www.richelbilderbeek.nl/CppToDouble.htm double ToDoubleUsingStream(std::string const& s) {   std::istringstream i(s);   double x;   if (!(i >> x))   {     throw std::logic_error("std::string cannot be converted to double");   }   return x; } const char GetRandomDigit() {   switch (std::rand() % 10)   {     case 0: return '0';     case 1: return '1';     case 2: return '2';     case 3: return '3';     case 4: return '4';     case 5: return '5';     case 6: return '6';     case 7: return '7';     case 8: return '8';     case 9: return '9';   } } const std::string GetRandomDouble() {   const std::string sign = ((std::rand() >> 4) % 2 ? "+" : "-");   std::string s1;   s1.resize(1 + (std::rand() % 3));   std::generate(s1.begin(),s1.end(),GetRandomDigit);   std::string s2;   s2.resize(1 + (std::rand() % 5));   std::generate(s2.begin(),s2.end(),GetRandomDigit);   const std::string s = sign + s1 + "." + s2;   return s; } int main() {   for (int i=0; i!=20; ++i)   {     const std::string s = GetRandomDouble();     std::cout << s << ":\t" << ToDoubleUsingAtof(s) << '\n';     assert(ToDoubleUsingAtof(s)==ToDoubleUsingStrtod(s));     assert(ToDoubleUsingAtof(s)==ToDoubleUsingStream(s));   } }

 

Screen output:

 

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+67.3: 67.3 +29.27: 29.27 +36.6: 36.6 -18.920: -18.92 +75.22897: 75.229 +12.31947: 12.3195 +503.10: 503.1 -2.6: -2.6 -5.76569: -5.76569 +452.4: 452.4 -430.868: -430.868 +3.49: 3.49 +6.9266: 6.9266 -504.7172: -504.717 +2.10: 2.1 +59.90917: 59.9092 -159.767: -159.767 +5.39: 5.39 +12.39088: 12.3909 +96.8561: 96.8561