Question

How can I convert string to double in C++? I want a function that returns 0 when the string is not numerical.

Answer 1

See C++ FAQ Lite How do I convert a std::string to a number?

See C++ Super-FAQ How do I convert a std::string to a number?

Please note that with your requirements you can't distinguish all the the allowed string representations of zero from the non numerical strings.

 // the requested function
 #include <sstream>
 double string_to_double( const std::string& s )
 {
   std::istringstream i(s);
   double x;
   if (!(i >> x))
     return 0;
   return x;
 } 

 // some tests
 #include <cassert>
 int main( int, char** )
 {
    // simple case:
    assert( 0.5 == string_to_double( "0.5"    ) );

    // blank space:
    assert( 0.5 == string_to_double( "0.5 "   ) );
    assert( 0.5 == string_to_double( " 0.5"   ) );

    // trailing non digit characters:
    assert( 0.5 == string_to_double( "0.5a"   ) );

    // note that with your requirements you can't distinguish
    // all the the allowed string representation of zero from
    // the non numerical strings:
    assert( 0 == string_to_double( "0"       ) );
    assert( 0 == string_to_double( "0."      ) );
    assert( 0 == string_to_double( "0.0"     ) );
    assert( 0 == string_to_double( "0.00"    ) );
    assert( 0 == string_to_double( "0.0e0"   ) );
    assert( 0 == string_to_double( "0.0e-0"  ) );
    assert( 0 == string_to_double( "0.0e+0"  ) );
    assert( 0 == string_to_double( "+0"      ) );
    assert( 0 == string_to_double( "+0."     ) );
    assert( 0 == string_to_double( "+0.0"    ) );
    assert( 0 == string_to_double( "+0.00"   ) );
    assert( 0 == string_to_double( "+0.0e0"  ) );
    assert( 0 == string_to_double( "+0.0e-0" ) );
    assert( 0 == string_to_double( "+0.0e+0" ) );
    assert( 0 == string_to_double( "-0"      ) );
    assert( 0 == string_to_double( "-0."     ) );
    assert( 0 == string_to_double( "-0.0"    ) );
    assert( 0 == string_to_double( "-0.00"   ) );
    assert( 0 == string_to_double( "-0.0e0"  ) );
    assert( 0 == string_to_double( "-0.0e-0" ) );
    assert( 0 == string_to_double( "-0.0e+0" ) );
    assert( 0 == string_to_double( "foobar"  ) );
    return 0;
 }