Short-circuit evaluation

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Short-circuit evaluation" – news · newspapers · books · scholar · JSTOR (August 2013) (Learn how and when to remove this message)

Short-circuit evaluation, minimal evaluation, or McCarthy evaluation (after John McCarthy) is the semantics of some Boolean operators in some programming languages in which the second argument is executed or evaluated only if the first argument does not suffice to determine the value of the expression: when the first argument of the AND function evaluates to false, the overall value must be false; and when the first argument of the OR function evaluates to true, the overall value must be true.

In programming languages with lazy evaluation (Lisp, Perl, Haskell), the usual Boolean operators short-circuit. In others (Ada, Java, Delphi), both short-circuit and standard Boolean operators are available. For some Boolean operations, like exclusive or (XOR), it is impossible to short-circuit, because both operands are always needed to determine a result.

Short-circuit operators are, in effect, control structures rather than simple arithmetic operators, as they are not strict. In imperative language terms (notably C and C++), where side effects are important, short-circuit operators introduce a sequence point: they completely evaluate the first argument, including any side effects, before (optionally) processing the second argument. ALGOL 68 used proceduring to achieve user-defined short-circuit operators and procedures.

The use of short-circuit operators has been criticized as problematic:

The conditional connectives — "cand" and "cor" for short — are ... less innocent than they might seem at first sight. For instance, cor does not distribute over cand: compare

(A cand B) cor C with (A cor C) cand (B cor C);

in the case ¬A ∧ C , the second expression requires B to be defined, the first one does not. Because the conditional connectives thus complicate the formal reasoning about programs, they are better avoided.

— Edsger W. Dijkstra^[1]

In any programming language that implements short-circuit evaluation, the expression x and y is equivalent to the conditional expression if x then y else x, and the expression x or y is equivalent to if x then x else y. In either case, x is only evaluated once.

The generalized definition above accommodates loosely typed languages that have more than the two truth-values True and False, where short-circuit operators may return the last evaluated subexpression. This is called "last value" in the table below. For a strictly-typed language, the expression is simplified to if x then y else false and if x then true else y respectively for the boolean case.

Although AND takes precedence over OR in many languages, this is not a universal property of short-circuit evaluation. An example of the two operators taking the same precedence and being left-associative with each other is POSIX shell's command-list syntax.^{[2]: §2.9.3}

The following simple left-to-right evaluator enforces a precedence of AND over OR by a continue:

function short-circuit-eval (operators, values)     let result := True     for each (op, val) in (operators, values):         if op = "AND" && result = False             continue         else if op = "OR" && result = True             return result         else             result := val     return result 

Short-circuit logic, with or without side-effects, have been formalized based on Hoare's conditional. A result is that non-short-circuiting operators can be defined out of short-circuit logic to have the same sequence of evaluation.^[3]

As you look at the table below, keep in mind that bitwise operators often do not behave exactly like logical operators, even if both arguments are of 0, 1 or Boolean type.

Examples:

• In JavaScript, each of the following 3 expressions evaluates to false:
  (true & true) === (true && true),
  (false | false) === (false || false),
  (1 & 2) === (1 && 2).
• In PHP, each of the following 3 expressions evaluates to false:
  (true & true) === (true && true),
  (0 | 0) === (0 || 0),
  (1 & 2) === (1 && 2).

Usual example, using a C-based language:

int denom = 0; if (denom != 0 && num / denom) {     ... // ensures that calculating num/denom never results in divide-by-zero error    } 

Consider the following example:

int a = 0; if (a != 0 && myfunc(b)) {     do_something(); } 

In this example, short-circuit evaluation guarantees that myfunc(b) is never called. This is because a != 0 evaluates to false. This feature permits two useful programming constructs.

1. If the first sub-expression checks whether an expensive computation is needed and the check evaluates to false, one can eliminate expensive computation in the second argument.
2. It permits a construct where the first expression guarantees a condition without which the second expression may cause a run-time error.

Both are illustrated in the following C snippet where minimal evaluation prevents both null pointer dereference and excess memory fetches:

bool is_first_char_valid_alpha_unsafe(const char *p) {     return isalpha(p[0]); // SEGFAULT highly possible with p == NULL }  bool is_first_char_valid_alpha(const char *p) {     return p != NULL && isalpha(p[0]); // 1) no unneeded isalpha() execution with p == NULL, 2) no SEGFAULT risk } 

Since minimal evaluation is part of an operator's semantic definition and not an optional optimization, a number of coding idioms rely on it as a succinct conditional construct. Examples include:

Perl idioms:

some_condition or die;    # Abort execution if some_condition is false some_condition and die;   # Abort execution if some_condition is true 

POSIX shell idioms:^[13]

modprobe -q some_module && echo "some_module installed" || echo "some_module not installed" 

This idiom presumes that echo cannot fail.

Despite these benefits, minimal evaluation may cause problems for programmers who do not realize (or forget) it is happening. For example, in the code

if (expressionA && myfunc(b)) {     do_something(); } 

if myfunc(b) is supposed to perform some required operation regardless of whether do_something() is executed, such as allocating system resources, and expressionA evaluates as false, then myfunc(b) will not execute, which could cause problems. Some programming languages, such as Java, have two operators, one that employs minimal evaluation and one that does not, to avoid this problem.

Problems with unperformed side effect statements can be easily solved with proper programming style, i.e., not using side effects in boolean statements, as using values with side effects in evaluations tends to generally make the code opaque and error-prone.^[14]

Short-circuiting can lead to errors in branch prediction on modern central processing units (CPUs), and dramatically reduce performance. A notable example is highly optimized ray with axis aligned box intersection code in ray tracing.^{[clarification needed]} Some compilers can detect such cases and emit faster code, but programming language semantics may constrain such optimizations.^{[citation needed]}

An example of a compiler unable to optimize for such a case is Java's Hotspot virtual machine (VM) as of 2012.^[15]

• Don't-care term