Tuesday 12 April 2011

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.

2 comments:

  1. very nice blog with very useful examples. great work keep it up!

    ReplyDelete
  2. Actually, there is something that supposedly does this..

    Though I haven't tested it...
    Header File: ctype.h
    Function Name: isdigit()

    This function accepts an ASCII value, and returns whether or not it is a digit (0 to 9) when converted to its equivalent ASCII character. It returns a zero if it is not a digit, and non-zero if it is.

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