Section 9. Boolean Expressions and Variables

9.1:    What is the right type to use for Boolean values in C?  Why
	isn't it a standard type?  Should I use #defines or enums for
	the true and false values?

A:      C does not provide a standard Boolean type, in part because
	picking one involves a space/time tradeoff which can best be
	decided by the programmer.  (Using an int may be faster, while
	using char may save data space.  Smaller types may make the
	generated code bigger or slower, though, if they require lots of
	conversions to and from int.)

	The choice between #defines and enumeration constants for the
	true/false values is arbitrary and not terribly interesting (see
	also questions 2.22 and 17.10).  Use any of

		#define TRUE  1                 #define YES 1
		#define FALSE 0                 #define NO  0

		enum bool {false, true};        enum bool {no, yes};

	or use raw 1 and 0, as long as you are consistent within one
	program or project.  (An enumeration may be preferable if your
	debugger shows the names of enumeration constants when examining
	variables.)

	Some people prefer variants like

		#define TRUE (1==1)
		#define FALSE (!TRUE)

	or define "helper" macros such as

		#define Istrue(e) ((e) != 0)

	These don't buy anything (see question 9.2 below; see also
	questions 5.12 and 10.2).

9.2:    Isn't #defining TRUE to be 1 dangerous, since any nonzero value
	is considered "true" in C?  What if a built-in logical or
	relational operator "returns" something other than 1?

A:      It is true (sic) that any nonzero value is considered true in C,
	but this applies only "on input", i.e. where a Boolean value is
	expected.  When a Boolean value is generated by a built-in
	operator, it is guaranteed to be 1 or 0.  Therefore, the test

		if((a == b) == TRUE)

	would work as expected (as long as TRUE is 1), but it is
	obviously silly.  In fact, explicit tests against TRUE and
	FALSE are generally inappropriate, because some library
	functions (notably isupper(), isalpha(), etc.) return,
	on success, a nonzero value which is not necessarily 1.
	(Besides, if you believe that "if((a == b) == TRUE)" is
	an improvement over "if(a == b)", why stop there?  Why not
	use "if(((a == b) == TRUE) == TRUE)"?)  A good rule of thumb
	is to use TRUE and FALSE (or the like) only for assignment
	to a Boolean variable or function parameter, or as the return
	value from a Boolean function, but never in a comparison.

	The preprocessor macros TRUE and FALSE (and, of course, NULL)
	are used for code readability, not because the underlying values
	might ever change.  (See also questions 5.3 and 5.10.)

	Although the use of macros like TRUE and FALSE (or YES
	and NO) seems clearer, Boolean values and definitions can
	be sufficiently confusing in C that some programmers feel
	that TRUE and FALSE macros only compound the confusion, and
	prefer to use raw 1 and 0 instead.  (See also question 5.9.)

	References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41; K&R2 Sec. 2.6
	p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206; ISO
	Sec. 6.3.3.3, Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14,
	Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S Sec. 7.5.4 pp. 196-7,
	Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8,
	Sec. 7.8 pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4;
	"What the Tortoise Said to Achilles".

9.3:    Is if(p), where p is a pointer, a valid conditional?

A:      Yes.  See question 5.3.