how to test for 0
Major Section:  PROGRAMMING

Below are six commonly used idioms for testing whether x is 0. Zip and zp are the preferred termination tests for recursions down the integers and naturals, respectively.

idiom       logical              guard              primary
            meaning                              compiled code*

(equal x 0)(equal x 0) t (equal x 0) (eql x 0) (equal x 0) t (eql x 0) (zerop x) (equal x 0) x is a number (= x 0) (= x 0) (equal x 0) x is a number (= x 0) (zip x) (equal (ifix x) 0) x is an integer (= x 0) (zp x) (equal (nfix x) 0) x is a natural (int= x 0) (zpf x) (equal (nfix x) 0) x is a fixnum >= 0 (eql (the-fixnum x) 0)

*See guards-and-evaluation, especially the subsection titled ``Guards and evaluation V: efficiency issues''. Primary code is relevant only if guards are verified. The ``compiled code'' shown is only suggestive.

The first four idioms all have the same logical meaning and differ only with respect to their executability and efficiency. In the absence of compiler optimizing, (= x 0) is probably the most efficient, (equal x 0) is probably the least efficient, and (eql x 0) is in between. However, an optimizing compiler could always choose to compile (equal x 0) as (eql x 0) and, in situations where x is known at compile-time to be numeric, (eql x 0) as (= x 0). So efficiency considerations must, of course, be made in the context of the host compiler.

Note also that (zerop x) and (= x 0) are indistinguishable. They have the same meaning and the same guard, and can reasonably be expected to generate equally efficient code.

Note that (zip x) and (zp x) do not have the same logical meanings as the others or each other. They are not simple tests for equality to 0. They each coerce x into a restricted domain, zip to the integers and zp to the natural numbers, choosing 0 for x when x is outside the domain. Thus, 1/2, #c(1 3), and 'abc, for example, are all ``recognized'' as zero by both zip and zp. But zip reports that -1 is different from 0 while zp reports that -1 ``is'' 0. More precisely, (zip -1) is nil while (zp -1) is t.

Note that the last five idioms all have guards that restrict their Common Lisp executability. If these last five are used in situations in which guards are to be verified, then proof obligations are incurred as the price of using them. If guard verification is not involved in your project, then the first five can be thought of as synonymous.

Zip and zp are not provided by Common Lisp but are ACL2-specific functions. Why does ACL2 provide these functions? The answer has to do with the admission of recursively defined functions and efficiency. Zp is provided as the zero-test in situations where the controlling formal parameter is understood to be a natural number. Zip is analogously provided for the integer case. We illustrate below.

Here is an admissible definition of factorial

(defun fact (n) (if (zp n) 1 (* n (fact (1- n)))))
Observe the classic recursion scheme: a test against 0 and recursion by 1-. Note however that the test against 0 is expressed with the zp idiom. Note also the absence of a guard making explicit our intention that n is a natural number.

This definition of factorial is readily admitted because when (zp n)

is false (i.e., nil) then n is a natural number other than 0 and so (1- n) is less than n. The base case, where (zp n) is true, handles all the ``unexpected'' inputs, such as arise with (fact -1) and (fact 'abc). When calls of fact are evaluated, (zp n) checks (integerp n) and (> n 0). Guard verification is unsuccessful for this definition of fact because zp requires its argument to be a natural number and there is no guard on fact, above. Thus the primary raw lisp for fact is inaccessible and only the :logic definition (which does runtime ``type'' checking) is used in computation. In summary, this definition of factorial is easily admitted and easily manipulated by the prover but is not executed as efficiently as it could be.

Runtime efficiency can be improved by adding a guard to the definition.

(defun fact (n)
  (declare (xargs :guard (and (integerp n) (>= n 0))))
  (if (zp n) 1 (* n (fact (1- n)))))
This guarded definition has the same termination conditions as before -- termination is not sensitive to the guard. But the guards can be verified. This makes the primary raw lisp definition accessible during execution. In that definition, the (zp n) above is compiled as (= n 0), because n will always be a natural number when the primary code is executed. Thus, by adding a guard and verifying it, the elegant and easily used definition of factorial is also efficiently executed on natural numbers.

Now let us consider an alternative definition of factorial in which (= n 0) is used in place of (zp n).

(defun fact (n) (if (= n 0) 1 (* n (fact (1- n)))))
This definition does not terminate. For example (fact -1) gives rise to a call of (fact -2), etc. Hence, this alternative is inadmissible. A plausible response is the addition of a guard restricting n to the naturals:
(defun fact (n)
 (declare (xargs :guard (and (integerp n) (>= n 0))))
 (if (= n 0) 1 (* n (fact (1- n)))))
But because the termination argument is not sensitive to the guard, it is still impossible to admit this definition. To influence the termination argument one must change the conditions tested. Adding a runtime test that n is a natural number would suffice and allow both admission and guard verification. But such a test would slow down the execution of the compiled function.

The use of (zp n) as the test avoids this dilemma. Zp provides the logical equivalent of a runtime test that n is a natural number but the execution efficiency of a direct = comparison with 0, at the expense of a guard conjecture to prove. In addition, if guard verification and most-efficient execution are not needed, then the use of (zp n) allows the admission of the function without a guard or other extraneous verbiage.

While general rules are made to be broken, it is probably a good idea to get into the habit of using (zp n) as your terminating ``0 test'' idiom when recursing down the natural numbers. It provides the logical power of testing that n is a non-0 natural number and allows efficient execution.

We now turn to the analogous function, zip. Zip is the preferred 0-test idiom when recursing through the integers toward 0. Zip considers any non-integer to be 0 and otherwise just recognizes 0. A typical use of zip is in the definition of integer-length, shown below. (ACL2 can actually accept this definition, but only after appropriate lemmas have been proved.)

(defun integer-length (i)
  (declare (xargs :guard (integerp i)))
  (if (zip i)
    (if (= i -1)
      (+ 1 (integer-length (floor i 2))))))
Observe that the function recurses by (floor i 2). Hence, calling the function on 25 causes calls on 12, 6, 3, 1, and 0, while calling it on -25 generates calls on -13, -7, -4, -2, and -1. By making (zip i) the first test, we terminate the recursion immediately on non-integers. The guard, if present, can be verified and allows the primary raw lisp definition to check (= i 0) as the first terminating condition (because the primary code is executed only on integers).