Conditional Expressions

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Conditional Expressions

The statements
if (a > b)
z = a;
else
z = b;
compute in z the maximum of a and b. The conditional expression, written with the ternary operator
``?:'', provides an alternate way to write this and similar constructions. In the expression
expr1 ? expr2 : expr3
the expression expr1 is evaluated first. If it is non-zero (true), then the expression expr2 is evaluated, and
that is the value of the conditional expression. Otherwise expr3 is evaluated, and that is the value. Only
one of expr2 and expr3 is evaluated. Thus to set z to the maximum of a and b,
z = (a > b) ? a : b; /* z = max(a, b) */
It should be noted that the conditional expression is indeed an expression, and it can be used wherever
any other expression can be. If expr2 and expr3 are of different types, the type of the result is determined
by the conversion rules discussed earlier in this chapter. For example, if f is a float and n an int,
then the expression
(n > 0) ? f : n
is of type float regardless of whether n is positive.
Parentheses are not necessary around the first expression of a conditional expression, since the
precedence of ?: is very low, just above assignment. They are advisable anyway, however, since they
make the condition part of the expression easier to see.
The conditional expression often leads to succinct code. For example, this loop prints n elements of an
array, 10 per line, with each column separated by one blank, and with each line (including the last)
terminated by a newline.
for (i = 0; i < n; i++)
printf("%6d%c", a[i], (i%10==9 || i==n-1) ? '\n' : ' ');
A newline is printed after every tenth element, and after the n-th. All other elements are followed by one
blank. This might look tricky, but it's more compact than the equivalent if-else. Another good
example is

printf("You have %d items%s.\n", n, n==1 ? "" : "s");

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