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What is the syntax for undefined?

ckaiser524 (2)
Hey guys I can't seem to find the syntax for undefined. For example if I need to say if(x == undefined) or something like that. It is really bothering. Thanks for the help!
Disch (13742)
There is no such thing, because if you have x, then x is defined, otherwise it's a compiler error.

Unless you mean the preprocessor in which case it's #ifndef

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#ifndef x  // if x has not been #defined

  // .. conditionally compiled code

#endif


but note that preprocessor conditionals are different from normal if statements.
Last edited on
ckaiser524 (2)
is there a way to say something that is being divided by zero? for example if(x == defined) where x = 5/0. then dose x have a value if it is being divided by zero?
closed account (1yR4jE8b)
if you divide something by zero in your program, it will crash. So x can never have the value of 5/0.
spycow (6)
You have to tell the program not to divide by zero.

Example:

Say x = num1 / num2

Then you write something like

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if (num2 != 0)
x = num1 / num2;
else
cout << "Error.  Dividing by zero is undefined." << endl;


Hope that's what you're looking for.

Mazd (191)
Your code please.
Kyon (912)
darkestfright wrote:
if you divide something by zero in your program, it will crash. So x can never have the value of 5/0.

Really now?
Then why does this compile?
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#include <iostream>
int main()
{   std::cout << 5.0/0.0;
    return 0;
} // main
Duthomhas (13206)
Your best choice is to use the Boost Math Toolkit's Special Functions: fpclassify
http://www.boost.org/doc/libs/1_44_0/libs/math/doc/sf_and_dist/html/math_toolkit/utils/fpclass.html

It does the right thing on all platforms, and is a header-only (I think). Example:

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#include <iomanip>
#include <iostream>
#include <boost/math/special_functions/fpclassify.hpp>
using namespace std;

void display( const char* name, double d )
  {
  cout << left << setw( 16 ) << name << " :   ";
  cout << ((boost::math::isnan)( d ) ? "NaN" : "   ") << ",  ";
  cout << ((boost::math::isinf)( d ) ? ((d < 0) ? "-INF" : "+INF") : "    ") << ",   ";
  cout << ((boost::math::isnormal)( d ) ? "normal" : "      ") << ", ";
  cout << d << endl;
  }

#define DISPLAY( x ) display( #x, x )

int main()
  {
  double not_a_number     =  0.0/0.0;
  double infinite_number  =  5.0/0.0;
  double ninfinite_number = -5.0/0.0;
  double normal_number    =  5.0;

  cout << "NAME             : ISNAN, ISINF, ISNORMAL, VALUE\n";
  DISPLAY( not_a_number     );
  DISPLAY( infinite_number  );
  DISPLAY( ninfinite_number );
  DISPLAY( normal_number    );

  return 0;
  }
 
NAME             : ISNAN, ISINF, ISNORMAL, VALUE
not_a_number     :   NaN,      ,         , NaN
infinite_number  :      ,  +INF,         , Infinity
ninfinite_number :      ,  -INF,         , Infinity
normal_number    :      ,      ,   normal, 5

As you can see, your other option is simply to use a std::stringstream and try to convert the number to a string and check against "NaN" and "Infinity"...

Hope this helps.
Duthomhas (13206)
Kyon wrote:
Really now?
Then why does this compile?
Because that is done by the compiler, not during the execution of your program. The compiler recognizes that as a special case and assigns the proper INF value to the variable. (In your example the variable is an unnamed temporary sent to cout.)

Hope this helps.
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