Conditional Execution

Because a do clause can contain arbitrary Lisp forms, you can use any Lisp expressions you want, including control constructs such as **IF** and **WHEN**. So, the following is one way to write a loop that prints only the even numbers between one and ten:

(loop for i from 1 to 10 do (when (evenp i) (print i)))

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:

(loop for i from 1 to 10 when (evenp i) sum i) ==> 30

**LOOP** provides three conditional constructs, and they all follow this basic pattern:

conditional test-form loop-clause

The conditional can be if, when, or unless. 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.

As an extra bit of syntactic sugar, within the first loop clause, after the test form, you can use the variable it to refer to the value returned by the test form. For instance, the following loop collects the non-**NIL** values found in some-hash when looking up the keys in some-list:

(loop for key in some-list when (gethash key some-hash) collect it)

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 unless 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.

All three conditional clauses can also take an else branch, which is followed by another loop clause or multiple clauses joined by and. When conditional clauses are nested, the set of clauses connected to an inner conditional clause can be closed with the word end. The end is optional when not needed to disambiguate a nested conditional—the end of a conditional clause will be inferred from the end of the loop or the start of another clause not joined by and.

The following rather silly loop demonstrates the various forms of **LOOP** 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.

(loop for i from 1 to 100
      if (evenp i)
        minimize i into min-even and 
        maximize i into max-even and
        unless (zerop (mod i 4))
          sum i into even-not-fours-total
        end
        and sum i into even-total
      else
        minimize i into min-odd and
        maximize i into max-odd and
        when (zerop (mod i 5)) 
          sum i into fives-total
        end
        and sum i into odd-total
      do (update-analysis min-even
                          max-even
                          min-odd
                          max-odd
                          even-total
                          odd-total
                          fives-total
                          even-not-fours-total))