isNumeric()

For some reason many C++ programmers believe there is an isNumeric() function available as part of the language. In fact I was searching for the header file to include to get a piece of code to work.

Its not difficult to create a function that will be able to check if the string is numeric or not. Here is the code.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include <iostream>
#include <string>
#include <sstream>

using namespace std;
 
bool isNumeric1(string stringToCheck)
{
  bool numeric = false;
 
  if(stringToCheck.find_first_not_of("0123456789.") == string::npos)
    numeric = true;
 
  return numeric;
}
 
bool isNumeric2(string stringToCheck)
{
  bool numeric = true;
  unsigned i = 0;
 
  while(numeric && i < stringToCheck.length())
  {
    switch (stringToCheck[i])
    {
    case '0': case '1': case '2': case '3': case '4': case '5': 
    case '6': case '7': case '8': case '9': case '.':
      //do nothing
      break;
    default:
      numeric = false;
    }
    i++;
  }
 
  return numeric;
}
 
bool isNumeric3(string stringToCheck)
{
  stringstream streamVal(stringToCheck);
  double tempVal;
 
  streamVal >> tempVal; //If numeric then everything transferred to tempVal

  if(streamVal.get() != EOF)
    return false;
  else
    return true;
}
 
int main()
{
  string str1("123"), str2("134.567"), str3("12AB");
  cout << "Approach 1" << endl;
  cout << str1 << (isNumeric1(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric1(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric1(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  cout << "\nApproach 2" << endl;
  cout << str1 << (isNumeric2(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric2(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric2(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  cout << "\nApproach 3" << endl;
  cout << str1 << (isNumeric3(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric3(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric3(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  return 0;
}
 

Output is as follows:

There are few problems in the program above:

1. It does not cater for negative numbers

2. It will return wrong result when you have incorrect string like 12.34.56.78

I have intentionally left it as an exercise.

Other approaches are possible as well. Why not try and think of an approach yourself.