Objects First

More Advanced Features of C

A Ternary Operator

C has many unary operators (operators that take one operand):

Operator	Name	Example
!	Logical not	!((x>a)&&(y>b))
~	Complement	~b
&	Address of	&x

and plenty of binary ones!

But it is almost unique among programming languages in having a ternary one - one that takes three operands.

cond ? exp_T : exp_F The ? operator evaluates cond, if it is true, it evaluates exp_T and returns it as the value of the ? expression. If cond evaluates to false, it evaluates exp_F and returns it as the value of the expression. This operator is, of course, entirely redundant: the same effect can be achieved with an if( cond ) { .. } else { .. }. However it does allow one to write some very compact expressions and thus is quite popular with C programmers (many of whom seem to have chosen C for its terse syntax, which allows them to

be lazy and
appear incredibly clever.

One common use of the ? operator is the definition of MAX and MIN "functions":

#define MAX(a,b) (((a)>(b))?(a):(b))
#define MIN(a,b) (((a)<(b))?(a):(b))

Remembering that the pre-processor will expand MAX(a,b) as text, it can be seen that this MAX "function" works equally well for int, float and double.

Although it is somewhat cryptic and one could argue that the ? operator merely adds to the difficulty of deciphering the meaning of the average C program with all its single character operators and other short-cuts, the ? operator does allow some compact coding which would be unnecessarily verbose otherwise. For example, if you're annoyed by programs that print:

10 cats and 1 dogs

instead of

10 cats and 1 dog

You can fix this by the following artifice:

printf("%d cat%c", n_cats, (n_cats>1)?'s':'\0');
(n_dogs>0)?printf("%d dog%c", n_dogs, (n_dogs>1):'s':'\0'):printf("");

These statements rely on the fact that a null character ('\0') will be ignored by any output device. The second statement, while perfectly valid C, is starting to move into questionable waters. Many would argue that:

printf("%d cat%c", n_cats, (n_cats>1)?'s':'\0');
if (n_dogs>0) printf("%d dog%c", n_dogs, (n_dogs>1)?'s':'\0');

is better - and I would probably agree with them! However the extra if .. necessary to avoid an unneeded 's' in the output definitely adds to the sheer volume of the code compared to this convenient use of the ? operator.