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 <code>AND</code> function evaluates to <code>false</code>, the overall value must be <code>false</code>; and when the first argument of the <code>OR</code> function evaluates to <code>true</code>, the overall value must be <code>true</code>.
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:
In any programming language that implements short-circuit evaluation, the expression <code>x and y</code> is equivalent to the conditional expression <code>if x then y else x</code>, and the expression <code>x or y</code> is equivalent to <code>if x then x else y</code>. In either case, x is only evaluated once.
The generalized definition above accommodates loosely typed languages that have more than the two truth-values <code>True</code> and <code>False</code>, 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 <code>if x then y else false</code> and <code>if x then true else y</code> respectively for the Boolean case.
Although takes precedence over 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.
The following simple left-to-right evaluator enforces a precedence of over by a :
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.
<code>&</code> and <code>|</code> are bitwise operators that occur in many programming languages. The major difference is that bitwise operations operate on the individual bits of a binary numeral, whereas conditional operators operate on logical operations. Additionally, expressions either side of a bitwise operator are always evaluated. In some languages, including Java and C#, they can be used on Boolean operands to force both sides to be evaluated. If expression 1 is true, expressions 2 and 3 are not checked. This checks expressions 2 and 3, even if expression 1 is true.
Short-circuit operators can thus reduce run times by avoiding unnecessary calculations. They can also avoid null exceptions when expression 1 checks whether an object is valid.
The following table is restricted to common programming languages and the basic Boolean operators for logical conjunction <code>AND</code> and logical disjunction <code>OR</code>. Bitwise operators are shown only for languages that allow them to be used as eager Boolean operators and have the same return type.
Note that there are more short-circuit operators, for example the ternary conditional operator, which is <code>cond ? e1 : e2</code> (C, C++, Java, PHP), <code>if cond then e1 else e2</code> (ALGOL, Haskell, Kotlin, Rust), <code>e1 if cond else e2</code> (Python). Please take a look at ternary conditional operator#Usage.
Usual example, using a C-based language:
Consider the following example:
In this example, short-circuit evaluation guarantees that <code>myfunc(b)</code> is never called. This is because <code>a != 0</code> evaluates to false. This feature permits two useful programming constructs.
Both are illustrated in the following C snippet where minimal evaluation prevents both null pointer dereference and excess memory fetches:
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:
POSIX shell idioms:
This idiom presumes that <code>echo</code> 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 <code>myFunc(b)</code> is supposed to perform some required operation regardless of whether <code>doSomething()</code> is executed, such as allocating system resources, and <code>expressionA</code> evaluates as false, then <code>myFunc(b)</code> 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.
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. Some compilers can detect such cases and emit faster code, but programming language semantics may constrain such optimizations.
An example of a compiler unable to optimize for such a case is Java's Hotspot virtual machine (VM) as of 2012.