What is the syntax for undefined?

[ckaiser524]

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]

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

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

  // .. conditionally compiled code

#endif 

but note that preprocessor conditionals are different from normal if statements.

[ckaiser524]

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?

[1yR4jE8b]

if you divide something by zero in your program, it will crash. So x can never have the value of 5/0.

[spycow]

You have to tell the program not to divide by zero.

Example:

Say x = num1 / num2

Then you write something like

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

Hope that's what you're looking for.

[Mazd]

Your code please.

[Kyon]

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?

#include <iostream>
int main()
{   std::cout << 5.0/0.0;
    return 0;
} // main 

[Duthomhas]

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:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

#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]

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.