bool Double::isNan() { return this->nan; } void Double::NaN(string s) { int pos;

Question

bool Double::isNan()
   {
       return this->nan;
   }
   void Double::NaN(string s)
   {
       int pos;
       pos = s.find(" . ", 0);
       if (pos != string::npos)
       {
           pos = s.find(" . ", pos + 1);
           if (pos != string::npos)
           {
               this->nan = true;
               return;
           }
       }

       string::iterator p;
       for (p = s.begin(); p < s.end(); p++)
       {
           if (!isdigit(*p) && *p != ' . ')
           {
               this->nan = true;
               return;
           }
       }
       this->nan = false;
       return;
   }
}

modify function to use recursion to check if a string contains a valid number

Explanation / Answer

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bool Double::isNan()
{
return this->nan;
}
void Double::NaN(string s)
{
// Makes sure a string is not sized 0 (for recursion)
if (s.size() == 0) {
this->nan = false;
return;
}
// Checks for 0 or 1 dots
int pos;
pos = s.find(".", 0);
if (pos != string::npos)
{
pos = s.find(".", pos + 1);
if (pos != string::npos)
{
this->nan = true;
return;
}
}

// Recursion step
if (!isdigit(s[0]) && s[0] != '.')
{
this->nan = true;
return;
} else {
return NaN(s.substr(1, string::npos));
}
}

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