- Short-circuit evaluation
-
Evaluation strategies - Strict evaluation
- Applicative order
- Call by value
- Call by reference
- Call by sharing
- Call by copy-restore
- Non-strict evaluation
- Normal order
- Call by name
- Call by need/Lazy evaluation
- Call by macro expansion
- Nondeterministic strategies
- Full β-reduction
- Call by future
- Optimistic evaluation
- Other
- Partial evaluation
- Remote evaluation
- Short-circuit evaluation
v · Boolean operators in some programming languages in which the second argument is only executed or evaluated if the first argument does not suffice to determine the value of the expression: when the first argument of the AND
function evaluates tofalse
, the overall value must befalse
; and when the first argument of theOR
function evaluates totrue
, the overall value must betrue
. In some programming languages (Lisp), the usual Boolean operators are short-circuit. In others (Java, Ada), both short-circuit and standard Boolean operators are available. For some Boolean operations, like XOR, it is not possible to short-circuit, because both operands are always required to determine the result.The short-circuit expression
x Sand y
(usingSand
to denote the short-circuit variety) is equivalent to the conditional expressionif x then y else false
; the expressionx Sor y
is equivalent toif x then true else y
.Short-circuit operators are, in effect, control structures rather than simple arithmetic operators, as they are not strict. ALGOL 68 used "proceduring" to achieve user defined short-circuit operators & procedures.
In loosely-typed languages which have more than the two truth-values
True
andFalse
, short-circuit operators may return the last evaluated subexpression, so thatx Sor y
andx Sand y
are actually equivalent toif x then x else y
andif x then y else x
respectively (without actually evaluatingx
twice). This is called "Last value" in the table below.In languages that use lazy evaluation by default (like Haskell), all functions are effectively "short-circuit", and special short-circuit operators are unnecessary.
Contents
Support in common programming languages
Boolean operators in various languages Language Eager operators Short-circuit operators Result type ABAP none and
,or
Boolean1 Ada, Eiffel and
,or
and then
,or else
Boolean ALGOL 68 and , & , ∧ ; or , ∨ andf , orf (both user defined) Boolean C2 none &&
,||
,?
[1]Numeric ( &&
,||
), opnd-dependent (?
)C++ &
,|
&&
,||
,?
[2]Boolean ( &&
,||
), opnd-dependent (?
)Go, Objective Caml, Haskell none &&
,||
Boolean C#, Java, &
,|
&&
,||
Boolean ColdFusion none AND
,OR
,&&
,||
Boolean Erlang and
,or
andalso
,orelse
Boolean Fortran .and.
,.or.
Boolean JavaScript none &&
,||
Last value Lisp, Lua, Scheme none and
,or
Last value Modula-2 none AND
,OR
Boolean Oberon none &
,OR
Boolean Pascal and
,or
3and_then
,or_else
4Boolean Perl, Ruby &
,|
&&
,and
,||
,or
Last value PHP none &&
,and
,||
,or
Boolean Python none and
,or
Last value Smalltalk &
,|
and:
,or:
Boolean Standard ML Unknown andalso
,orelse
Boolean Visual Basic .NET And
,Or
AndAlso
,OrElse
Boolean VB Script, VB Classic, VBA And
,Or
Select Case
Numeric 1 ABAP does not actually have a distinct boolean type.
2 C, before C99, did not actually have a distinct boolean type; logical operators returned 0 (for false) or 1 (for true).
3 ISO Pascal allows but does not require short-circuiting.
4 ISO-10206 Extended Pascal supportsand_then
andor_else
.[3]Common usage
Avoiding the execution of second expression's side effects
Usual example.
int denom = 0; if (denom && nom/denom) { oops_i_just_divided_by_zero(); // never happens }
Consider the following example using C language:
int a = 0; if (a && myfunc(b)) { do_something(); }
In this example, short-circuit evaluation guarantees that
myfunc(b)
is never called. This is becausea
evaluates to false. This feature permits two useful programming constructs. Firstly, 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. Secondly, 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]); // a) no unneeded isalpha() execution with p == NULL, b) no SEGFAULT risk }
Possible problems
Untested second condition leads to unperformed side effect
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 whetherdo_something()
is executed, such as allocating system resources, andexpressionA
evaluates as false, thenmyfunc(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.[4]
Since minimal evaluation is part of an operator's semantic definition and not an (optional) optimization, many coding styles[which?] rely on it as a succinct (if idiomatic) conditional construct, such as these Perl idioms:
some_condition or die; # Abort execution if some_condition is false some_condition and die; # Abort execution if some_condition is true
Code efficiency
If both expressions used as conditions are simple boolean variables, it can be actually faster to evaluate both conditions used in boolean operation at once, as it always requires a single calculation cycle, as opposed to one or two cycles used in short-circuit evaluation (depending on the value of the first). The difference in terms of computing efficiency between these two cases depends heavily on compiler and optimization scheme used; with proper optimization they will execute at the same speed, as they will get compiled to identical machine code.[5]
Short-circuiting can lead to errors in branch prediction on modern processors, 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 it is not always possible due to possible violations of the C standard. Highly optimized code should use other ways for doing this (like manual usage of assembly code).[citation needed]
References
Categories:
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- Strict evaluation
Short-circuit evaluation
- Short-circuit evaluation
-
Evaluation strategies - Strict evaluation
- Applicative order
- Call by value
- Call by reference
- Call by sharing
- Call by copy-restore
- Non-strict evaluation
- Normal order
- Call by name
- Call by need/Lazy evaluation
- Call by macro expansion
- Nondeterministic strategies
- Full β-reduction
- Call by future
- Optimistic evaluation
- Other
- Partial evaluation
- Remote evaluation
- Short-circuit evaluation
- Strict evaluation