C++ IsFloat function

Question

Does anybody know of a convenient means of determining if a string value \"qualifies\" as a floating-point number?

bool IsFloat( string MyString )
{
   ... e         


        

Answer 1

Inspired by this answer I modified the function to check if a string is a floating point number. It won't require boost & doesn't relies on stringstreams failbit - it's just plain parsing.

static bool isFloatNumber(const std::string& string){
    std::string::const_iterator it = string.begin();
    bool decimalPoint = false;
    int minSize = 0;
    if(string.size()>0 && (string[0] == '-' || string[0] == '+')){
      it++;
      minSize++;
    }
    while(it != string.end()){
      if(*it == '.'){
        if(!decimalPoint) decimalPoint = true;
        else break;
      }else if(!std::isdigit(*it) && ((*it!='f') || it+1 != string.end() || !decimalPoint)){
        break;
      }
      ++it;
    }
    return string.size()>minSize && it == string.end();
  }

I.e.

1
2.
3.10000
4.2f
-5.3f
+6.2f

is recognized by this function correctly as float.

1.0.0
2f
2.0f1

Are examples for not-valid floats. If you don't want to recognize floating point numbers in the format X.XXf, just remove the condition:

&& ((*it!='f') || it+1 != string.end() || !decimalPoint)

from line 9. And if you don't want to recognize numbers without '.' as float (i.e. not '1', only '1.', '1.0', '1.0f'...) then you can change the last line to:

return string.size()>minSize && it == string.end() && decimalPoint;

However: There are plenty good reasons to use either boost's lexical_cast or the solution using stringstreams rather than this 'ugly function'. But It gives me more control over what kind of formats exactly I want to recognize as floating point numbers (i.e. maximum digits after decimal point...).