Although D does not provide support for if-then-else constructs, it does provide support for simple conditional expressions by using the ? and : operators. These operators enable a triplet of expressions to be associated, where the first expression is used to conditionally evaluate one of the other two.

For example, the following D statement could be used to set a variable x to one of two strings, depending on the value of i:

x = i == 0 ? "zero" : "non-zero";

In the previous example, the expression i == 0 is first evaluated to determine whether it is true or false. If the expression is true, the second expression is evaluated and its value is returned. If the expression is false, the third expression is evaluated and its value is returned.

As with any D operator, you can use multiple ?: operators in a single expression to create more complex expressions. For example, the following expression would take a char variable c containing one of the characters 0-9, a-f, or A-F, and return the value of this character when interpreted as a digit in a hexadecimal (base 16) integer:

hexval = (c >= '0' && c <= '9') ? c - '0' : (c >= 'a' && c <= 'f') ? c + 10 - 'a' : c + 10 - 'A';

To be evaluated for its truth value, the first expression that is used with ?: must be a pointer or integer. The second and third expressions can be of any compatible types. You may not construct a conditional expression where, for example, one path returns a string and another path returns an integer. The second and third expressions also may not invoke a tracing function such as trace or printf. If you want to conditionally trace data, use a predicate instead. See Section 2.4, “Predicate Examples” for more information.