8.3 Logical Operators and Assignments

There are cases where assignments nested inside the condition can actually make a program easier to read. Here is an example using a hypothetical type list which represents a list; it tests whether the list has at least two links, using hypothetical functions, nonempty which is true of the argument is a nonempty list, and list_next which advances from one list link to the next. We assume that a list is never a null pointer, so that the assignment expressions are always “true.”

if (nonempty (list)
    && (temp1 = list_next (list))
    && nonempty (temp1)
    && (temp2 = list_next (temp1)))
  …  /* use temp1 and temp2 */

Here we get the benefit of the ‘&&’ operator, to avoid executing the rest of the code if a call to nonempty says “false.” The only natural place to put the assignments is among those calls.

It would be possible to rewrite this as several statements, but that could make it much more cumbersome. On the other hand, when the test is even more complex than this one, splitting it into multiple statements might be necessary for clarity.

If an empty list is a null pointer, we can dispense with calling nonempty:

if ((temp1 = list_next (list))
    && (temp2 = list_next (temp1)))
 …