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20.8.6 Floating-Point Comparison Functions

The standard C comparison operators provoke exceptions when one or other of the operands is NaN. For example, 

     int v = a < 1.0;

will raise an exception if a is NaN. (This does not happen with == and !=; those merely return false and true, respectively, when NaN is examined.) Frequently this exception is undesirable. ISO C99 therefore defines comparison functions that do not raise exceptions when NaN is examined. All of the functions are implemented as macros which allow their arguments to be of any floating-point type. The macros are guaranteed to evaluate their arguments only once.

— Macro: int isgreater (real-floating x, real-floating y)

This macro determines whether the argument x is greater than y. It is equivalent to (x) > (y), but no exception is raised if x or y are NaN.

— Macro: int isgreaterequal (real-floating x, real-floating y)

This macro determines whether the argument x is greater than or equal to y. It is equivalent to (x) >= (y), but no exception is raised if x or y are NaN.

— Macro: int isless (real-floating x, real-floating y)

This macro determines whether the argument x is less than y. It is equivalent to (x) < (y), but no exception is raised if x or y are NaN.

— Macro: int islessequal (real-floating x, real-floating y)

This macro determines whether the argument x is less than or equal to y. It is equivalent to (x) <= (y), but no exception is raised if x or y are NaN.

— Macro: int islessgreater (real-floating x, real-floating y)

This macro determines whether the argument x is less or greater than y. It is equivalent to (x) < (y) || (x) > (y) (although it only evaluates x and y once), but no exception is raised if x or y are NaN.

This macro is not equivalent to x != y, because that expression is true if x or y are NaN.

— Macro: int isunordered (real-floating x, real-floating y)

This macro determines whether its arguments are unordered. In other words, it is true if x or y are NaN, and false otherwise.

Not all machines provide hardware support for these operations. On machines that don't, the macros can be very slow. Therefore, you should not use these functions when NaN is not a concern.

Note: There are no macros isequal or isunequal. They are unnecessary, because the == and != operators do not throw an exception if one or both of the operands are NaN.