However, sometimes you’ll want conditional control at the level of loop clauses. For instance, suppose you wanted to sum only the even numbers between one and ten using a summing clause. You couldn’t write such a loop with a do clause because there’d be no way to “call” the sum i in the middle of a regular Lisp form. In cases like this, you need to use one of **LOOP**‘s own conditional expressions like this:

The conditional can be if, when, or . The test-form is any regular Lisp form, and loop-clause can be a value accumulation clause (count, collect, and so on), an unconditional execution clause, or another conditional execution clause. Multiple loop clauses can be attached to a single conditional by joining them with and.

A conditional clause is executed each time through the loop. An if or when clause executes its loop-clause if test-form evaluates to true. An reverses the test, executing loop-clause only when test-form is **NIL**. Unlike their Common Lisp namesakes, **LOOP**‘s if and when are merely synonyms—there’s no difference in their behavior.

The following rather silly loop demonstrates the various forms of conditionals. The update-analysis function will be called each time through the loop with the latest values of the various variables accumulated by the clauses within the conditionals.