View source for Conditional (computer programming)

{{Short description|Control flow statement that executes code according to some condition(s)}}
[[File:If-Then-Else-diagram.svg|thumb|If-then-else flow diagram]]
[[File:IF-THEN-ELSE-END flowchart.svg|thumb|right|A nested ''if–then–else'' flow diagram]]
In [[Computer science|computer science]], '''conditionals''' (that is, '''conditional statements''', '''conditional expressions''' and '''conditional constructs''') are [[Programming language|programming language]] commands for handling decisions. Specifically, conditionals perform different computations or actions depending on whether a programmer-defined Boolean ''condition'' evaluates to true or false. In terms of [[Control flow|control flow]], the decision is always achieved by selectively altering the control flow based on some condition (apart from the case of branch predication).
Although [[Dynamic dispatch|dynamic dispatch]] is not usually classified as a conditional construct, it is another way to select between alternatives at [[Run time (program lifecycle phase)|runtime]]. Conditional statements are the checkpoints in the programe that determines behaviour according to situation.

== Terminology ==
In [[Imperative programming|imperative programming]] languages, the term "conditional statement" is usually used, whereas in [[Functional programming|functional programming]], the terms "conditional expression" or "conditional construct" are preferred, because these terms all have distinct meanings.

== If–then(–else) ==
The <code>if–then</code> construct (sometimes called <code>if–then–else</code>) is common across many programming languages. Although the syntax varies from language to language, the basic structure (in [[Pseudocode|pseudocode]] form) looks like this:

 '''If''' (boolean condition) '''Then'''
    (consequent)
 '''Else'''
    (alternative)
 '''End If'''

For example:

 '''If''' stock=0 '''Then'''
    message= order new stock
 '''Else'''
    message= there is stock
 '''End If'''

In the example code above, the part represented by ''(boolean condition)'' constitutes a conditional ''expression'', having intrinsic value (e.g., it may be substituted by either of the values <code>True</code> or <code>False</code>) but having no intrinsic meaning. In contrast, the combination of this expression, the <code>If</code> and <code>Then</code> surrounding it, and the consequent that follows afterward constitute a conditional ''statement'', having intrinsic meaning (e.g., expressing a coherent logical rule) but no intrinsic value.

When an [[Interpreter (computing)|interpreter]] finds an <code>If</code>, it expects a Boolean condition – for example, <code>x > 0</code>, which means "the variable x contains a number that is greater than zero" – and evaluates that condition. If the condition is <code>true</code>, the statements following the <code>then</code> are executed. Otherwise, the execution continues in the following branch – either in the <code>else</code> [[Block (programming)|block]] (which is usually optional), or if there is no <code>else</code> branch, then after the <code>end If</code>.

After either branch has been executed, [[Control flow|control]] returns to the point after the <code>end If</code>.

=== History and development ===
In early programming languages, especially some dialects of [[BASIC]] in the 1980s [[Engineering:Home computer|home computer]]s, an <code>if–then</code> statement could only contain <code>GOTO</code> statements (equivalent to a [[Branch (computer science)|branch]] instruction). This led to a hard-to-read style of programming known as spaghetti programming, with programs in this style called ''spaghetti code''.  As a result, [[Structured programming|structured programming]], which allows (virtually) arbitrary statements to be put in statement blocks inside an <code>if</code> statement, gained in popularity, until it became the norm even in most BASIC programming circles. Such mechanisms and principles were based on the older but more advanced [[ALGOL]] family of languages, and ALGOL-like languages such as [[Pascal (programming language)|Pascal]] and [[Modula-2]] influenced modern BASIC variants for many years. While it is possible while using only <code>GOTO</code> statements in <code>if–then</code> statements to write programs that are not spaghetti code and are just as well structured and readable as programs written in a structured programming language, structured programming makes this easier and enforces it.  Structured <code>if–then–else</code> statements like the example above are one of the key elements of structured programming, and they are present in most popular high-level programming languages such as [[C (programming language)|C]], [[Java (programming language)|Java]], [[JavaScript]] and [[Visual Basic]] .

==== The "dangling else" problem ====
{{Main|Dangling else}}
The <code>else</code> keyword is made to target a specific <code>if–then</code> statement preceding it, but for [[Nesting (computing)|nested]] <code>if–then</code> statements, classic programming languages such as [[ALGOL 60]] struggled to define which specific statement to target. Without clear boundaries for which statement is which, an <code>else</code> keyword could target any preceding <code>if–then</code> statement in the nest, as parsed.
 '''if''' a '''then''' '''if''' b '''then''' s '''else''' s2
can be parsed as
 '''if''' a '''then''' ('''if''' b '''then''' s) '''else''' s2
or
 '''if''' a '''then''' ('''if''' b '''then''' s '''else''' s2)
depending on whether the <code>else</code> is associated with the first <code>if</code> or second <code>if</code>. This is known as the [[Dangling else|dangling else]] problem, and is resolved in various ways, depending on the language (commonly via the <code>end if</code> statement or <code>{...}</code> brackets).

=== Else if===

By using <code>else if</code>, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped.

 '''if''' condition '''then'''
    ''-- statements''
 '''elseif''' condition '''then'''
    ''-- more statements''
 '''elseif''' condition '''then'''
    ''-- more statements;''
 ...
 '''else'''
    ''-- other statements;''
 '''end if''';

For example, for a shop offering as much as a 30% discount for an item:

 '''if''' discount < 11% '''then'''
    print (you have to pay $30)
 '''elseif''' discount<21% '''then'''
    print (you have to pay $20)
 '''elseif''' discount<31% '''then'''
    print (you have to pay $10)
 '''end if''';

In the example above, if the discount is 10%, then the first if statement will be evaluated as true and "you have to pay $30" will be printed out. All other statements below that first if statement will be skipped.

The <code>elseif</code> statement, in the [[Ada (programming language)|Ada]] language for example, is simply [[Syntactic sugar|syntactic sugar]] for <code>else</code> followed by <code>if</code>. In Ada, the difference is that only one <code>end if</code> is needed, if one uses <code>elseif</code> instead of <code>else</code> followed by <code>if</code>. [[Organization:PHP|PHP]] uses the <code>elseif</code> keyword<ref name="php_elseif">''[http://php.net/manual/control-structures.elseif.php PHP elseif syntax]''</ref> both for its curly brackets or colon syntaxes. [[Perl]] provides the keyword <code>elsif</code> to avoid the large number of braces that would be required by multiple <code>if</code> and <code>else</code> statements. [[Python (programming language)|Python]] uses the special keyword <code>elif</code> because structure is denoted by indentation rather than braces, so a repeated use of <code>else</code> and <code>if</code> would require increased indentation after every condition. Some implementations of [[BASIC]], such as [[Visual Basic]],<ref name="vb_elseif">''[https://docs.microsoft.com/dotnet/visual-basic/language-reference/statements/if-then-else-statement Visual Basic ElseIf syntax]''</ref> use <code>ElseIf</code> too. Similarly, the earlier UNIX shells (later gathered up to the POSIX shell syntax<ref name="posixshell">''[http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html POSIX standard shell syntax]''</ref>) use elif too, but giving the choice of delimiting with spaces, line breaks, or both.

However, in many languages more directly descended from Algol, such as Simula, [[Pascal (programming language)|Pascal]], BCPL and [[C (programming language)|C]], this special syntax for the <code>else if</code> construct is not present, nor is it present in the many syntactical derivatives of C, such as [[Java (programming language)|Java]], [[ECMAScript]], and so on. This works because in these languages, any ''single'' statement (in this case <code>if ''cond''</code>...) can follow a conditional without being enclosed in a block.

This design choice has a slight "cost". Each <code>else if</code> branch effectively adds an extra nesting level. This complicates the job for the compiler (or the people who write the compiler), because the compiler must analyse and implement arbitrarily long <code>else if</code> chains recursively.

If all terms in the sequence of conditionals are testing the value of a single expression (e.g., <code>if x=0</code> ... <code>else if x=1</code> ... <code>else if x=2</code>...), an alternative is the [[Switch statement|switch statement]], also called case-statement or select-statement. Conversely, in languages that do not have a switch statement, these can be produced by a sequence of <code>else if</code> statements.

===If–then–else expressions===
Many languages support ''if expressions'', which are similar to if statements, but return a value as a result. Thus, they are true expressions (which evaluate to a value), not statements (which may not be permitted in the context of a value).

====Algol family====
[[ALGOL 60]] and some other members of the [[ALGOL]] family allow <code>if–then–else</code> as an expression:
<pre>
  myvariable := if x > 20 then 1 else 2
</pre> <!-- Don't place a semicolon at the end of the above ALGOL statement. It is NOT C. -->

====Lisp dialects====
In dialects of [[Lisp (programming language)|Lisp]]{{nbsp}}– [[Scheme (programming language)|Scheme]], [[Racket (programming language)|Racket]] and [[Common Lisp]]{{nbsp}}– the first of which was inspired to a great extent by ALGOL:
<source lang="scheme">
;; Scheme
(define myvariable (if (> x 12) 1 2))   ; Assigns 'myvariable' to 1 or 2, depending on the value of 'x'
</source>

<source lang="lisp">
;; Common Lisp
(let ((x 10))
  (setq myvariable (if (> x 12) 2 4)))  ; Assigns 'myvariable' to 2
</source>

====Haskell====
{{Main|Haskell (programming language)}}

In [[Haskell (programming language)|Haskell]] 98, there is only an ''if expression'', no ''if statement'', and the <code>else</code> part is compulsory, as every expression must have some value.<ref name="haskell98report">''[http://www.haskell.org/onlinereport/ Haskell 98 Language and Libraries: The Revised Report]''</ref> Logic that would be expressed with conditionals in other languages is usually expressed with [[Pattern matching|pattern matching]] in recursive functions.

Because Haskell is [[Lazy evaluation|lazy]], it is possible to write control structures, such as ''if'', as ordinary expressions; the lazy evaluation means that an ''if function'' can evaluate only the condition and proper branch (where a strict language would evaluate all three). It can be written like this:<ref name="haskell-ifthenelse-proposal">"[http://haskell.org/haskellwiki/If-then-else If-then-else Proposal on HaskellWiki]"</ref>
<source lang="haskell">
if' :: Bool -> a -> a -> a
if' True x _ = x
if' False _ y = y
</source>

====C-like languages====
[[C (programming language)|C]] and C-like languages have a special ternary operator ([[?:|?:]]) for conditional expressions with a function that may be described by a template like this:
 <code>condition ? evaluated-when-true : evaluated-when-false</code>
This means that it can be inlined into expressions, unlike if-statements, in C-like languages:
<source lang="c">
my_variable = x > 10 ? "foo" : "bar";  // In C-like languages
</source>
which can be compared to the Algol-family if–then–else ''expressions'' (in contrast to a ''statement'') (and similar in Ruby and Scala, among others).

To accomplish the same using an if-statement, this would take more than one line of code (under typical layout conventions), and require mentioning "my_variable" twice:
<source lang="c">
if (x > 10)
    my_variable = "foo";
else
    my_variable = "bar";
</source>

Some argue that the explicit if/then statement is easier to read and that it may compile to more efficient code than the ternary operator,<ref>{{cite web|url=http://embeddedgurus.com/stack-overflow/2009/02/efficient-c-tips-6-dont-use-the-ternary-operator/ |title=Efficient C Tips #6 – Don’t use the ternary operator « Stack Overflow |publisher=Embeddedgurus.com |date=2009-02-18 |access-date=2012-09-07}}</ref> while others argue that concise expressions are easier to read than statements spread over several lines containing repetition.

====Small Basic====
<source lang="vbnet">
x = TextWindow.ReadNumber()
If (x > 10) Then
    TextWindow.WriteLine("My variable is named 'foo'.")
Else
    TextWindow.WriteLine("My variable is named 'bar'.")
EndIf
</source>

First, when the user runs the program, a cursor appears waiting for the reader to type a number. If that number is greater than 10, the text "My variable is named 'foo'." is displayed on the screen. If the number is smaller than 10, then the message "My variable is named 'bar'." is printed on the screen.

====Visual Basic====
{{Main|Visual Basic}}
In [[Visual Basic]] and some other languages, a function called <code>[[IIf]]</code> is provided, which can be used as a conditional expression. However, it does not behave like a true conditional expression, because both the true and false branches are always evaluated; it is just that the result of one of them is thrown away, while the result of the other is returned by the IIf function.

====Tcl====
{{Main|Tcl}}
In [[Tcl]] <code>if</code> is not a keyword but a function (in Tcl known as command or <code>proc</code>). For example
<source lang="tcl">
if {$x > 10} {
   puts "Foo!"
}
</source>
invokes a function named <code>if</code> passing 2 arguments: The first one being the condition and the second one being the true branch. Both arguments are passed as strings (in Tcl everything within curly brackets is a string).

In the above example the condition is not evaluated before calling the function. Instead, the implementation of the <code>if</code> function receives the condition as a string value and is responsible to evaluate this string as an expression in the callers scope.<ref>{{cite web|title=New Control Structures|url=https://wiki.tcl-lang.org/page/New+Control+Structures|publisher=Tcler's wiki|access-date=August 21, 2020}}</ref>

Such a behavior is possible by using <code>uplevel</code> and <code>expr</code> commands:
:Uplevel makes it possible to implement new control constructs as Tcl procedures (for example, uplevel could be used to implement the while construct as a Tcl procedure).<ref>{{cite web|title=uplevel manual page|url=https://www.tcl.tk/man/tcl8.6/TclCmd/uplevel.htm|publisher=www.tcl.tk|access-date=August 21, 2020}}</ref>

Because <code>if</code> is actually a function it also returns a value:
:The return value from the command is the result of the body script that was executed, or an empty string if none of the expressions was non-zero and there was no bodyN.<ref>{{cite web|title=if manual page|url=http://www.tcl.tk/man/tcl8.6/TclCmd/if.htm|publisher=www.tcl.tk|access-date=August 21, 2020}}</ref>

====Rust====
{{Main|Rust (programming language)}}

In [[Rust (programming language)|Rust]], <code>if</code> is always an expression. It evaluates to the value of whichever branch is executed, or to the unit type <code>()</code> if no branch is executed. If a branch does not provide a return value, it evaluates to <code>()</code> by default. To ensure the <code>if</code> expression's type is known at compile time, each branch must evaluate to a value of the same type. For this reason, an <code>else</code> branch is effectively compulsory unless the other branches evaluate to <code>()</code>, because an <code>if</code> without an <code>else</code> can always evaluate to <code>()</code> by default.<ref>{{cite web|title=If and if let expressions|url=https://doc.rust-lang.org/reference/expressions/if-expr.html|access-date=November 1, 2020}}</ref>

<source lang="rust">
// Assign my_variable some value, depending on the value of x
let my_variable = if x > 20 {
    1
} else {
    2
};

// This variant will not compile because 1 and () have different types
let my_variable = if x > 20 {
    1
};

// Values can be omitted when not needed
if x > 20 {
    println!("x is greater than 20");
}
</source>

=== Arithmetic if ===
Up to Fortran 77, the language Fortran has an "arithmetic if" statement which is halfway between a computed IF and a case statement, based on the [[Trichotomy (mathematics)|trichotomy]] {{nowrap|''x'' < 0,}} {{nowrap|1=''x'' = 0,}} {{nowrap|''x'' > 0.}} This was the earliest conditional statement in Fortran:<ref name="fortran77">{{cite web |url = http://www.fortran.com/fortran/F77_std/rjcnf0001-sh-11.html#sh-11.4 |title = American National Standard Programming Language FORTRAN |access-date = 2007-09-09 |date = 1978-04-03 |archive-url = https://web.archive.org/web/20071011055359/http://www.fortran.com/fortran/F77_std/rjcnf0001-sh-11.html#sh-11.4 |archive-date = 2007-10-11 |url-status = dead }}</ref>

<source lang="fortran">
IF (e) label1, label2, label3
</source>

Where e is any numeric expression (not necessarily an integer); this is equivalent to

<source lang="fortran">
IF (e .LT. 0) GOTO label1
IF (e .EQ. 0) GOTO label2
GOTO label3
</source>

Because this [[Arithmetic IF|arithmetic IF]] is equivalent to multiple <code>GOTO</code> statements that could jump to anywhere, it is considered to be an unstructured control statement, and should not be used if more structured statements can be used. In practice it has been observed that most arithmetic <code>IF</code> statements referenced the following statement with one or two of the labels.

This was the only conditional control statement in the original implementation of Fortran on the IBM 704 computer. On that computer the test-and-branch op-code had three addresses for those three states. Other computers would have "flag" registers such as positive, zero, negative, even, overflow, carry, associated with the last arithmetic operations and would use instructions such as 'Branch if accumulator negative' then 'Branch if accumulator zero' or similar. Note that the expression is evaluated ''once only'', and in cases such as integer arithmetic where overflow may occur, the overflow or carry flags would be considered also.

=== Object-oriented implementation in Smalltalk ===

In contrast to other languages, in [[Smalltalk]] the conditional statement is not a [[Language construct|language construct]] but defined in the class <code>Boolean</code> as an abstract method that takes two parameters, both closures. <code>Boolean</code> has two subclasses, <code>True</code> and <code>False</code>, which both define the method, <code>True</code> executing the first closure only, <code>False</code> executing the second closure only.<ref name="Smalltalk conditionals">{{cite web |url = http://wiki.cs.uiuc.edu/VisualWorks/Conditional+Processing |title = VisualWorks: Conditional Processing |date = 2006-12-16 |access-date = 2007-09-09 |archive-url = https://web.archive.org/web/20071022201949/http://wiki.cs.uiuc.edu/VisualWorks/Conditional+Processing |archive-date = 2007-10-22 |url-status = dead }}</ref>

<source lang="smalltalk">
var = condition 
    ifTrue: [ 'foo' ]
    ifFalse: [ 'bar' ]
</source>

=== JavaScript ===
[[JavaScript]] uses if-else statements similar to those in [[C syntax|C]] languages. A Boolean value is accepted within parentheses between the reserved ''if'' keyword and a left curly bracket.
<source lang="javascript">
if (Math.random() < 0.5) {
  console.log("You got Heads!");
} else {
  console.log("You got Tails!");
}
</source>
The above example takes the conditional of <code>Math.random() < 0.5</code> which outputs <code>true</code> if a random float value between 0 and 1 is greater than 0.5. The statement uses it to randomly choose between outputting <code>You got Heads!</code> or <code>You got Tails!</code> to the console. Else and else-if statements can also be chained after the curly bracket of the statement preceding it as many times as necessary, as shown below:
<source lang="javascript">
var x = Math.random();
if (x < 1/3) {
  console.log("One person won!");
} else if (x < 2/3) {
  console.log("Two people won!");
} else {
  console.log("It's a three-way tie!");
}
</source>

=== Lambda calculus ===
In [[Lambda calculus]], the concept of an if-then-else conditional can be expressed using the following expressions:
 true = λx. λy. x
 false = λx. λy. y
 ifThenElse = (λc. λx. λy. (c x y))
# true takes up to two arguments and once both are provided (see [[Currying|currying]]), it returns the first argument given. 
# false takes up to two arguments and once both are provided(see [[Currying|currying]]), it returns the second argument given.
# ifThenElse takes up to three arguments and once all are provided, it passes both second and third argument to the first argument(which is a function that given two arguments, and produces a result). We expect ifThenElse to only take true or false as an argument, both of which project the given two arguments to their preferred single argument, which is then returned. 
'''note''': if ifThenElse is passed two functions as the left and right conditionals; it is '''necessary''' to also pass an empty tuple () to the result of ifThenElse in order to actually call the chosen function, otherwise ifThenElse will just return the function object without getting called.

In a system where numbers can be used without definition (like Lisp, Traditional paper math, so on), the above can be expressed as a single closure below:
<source lang="lisp">
 ((λtrue. λfalse. λifThenElse.
     (ifThenElse true 2 3)
 )(λx. λy. x)(λx. λy. y)(λc. λl. λr. c l r))
</source>
Here, true, false, and ifThenElse are bound to their respective definitions which are passed to their scope at the end of their block.

A working JavaScript analogy(using only functions of single variable for rigor) to this is as follows:
<source lang="JavaScript">
 var computationResult = ((_true => _false => _ifThenElse => 
     _ifThenElse(_true)(2)(3) 
 )(x => y => x)(x => y => y)(c => x => y => c(x)(y)));
</source>
The code above with multivariable functions looks like this:
<source lang="JavaScript">
 var computationResult = ((_true, _false, _ifThenElse) =>
     _ifThenElse(_true, 2, 3)
 )((x, y) => x, (x, y) => y, (c, x, y) => c(x, y));
</source>
Another version of the earlier example without a system where numbers are assumed is below.

The first example shows the first branch being taken, while second example shows the second branch being taken.
<source lang="lisp">
 ((λtrue. λfalse. λifThenElse.
     (ifThenElse true (λFirstBranch. FirstBranch) (λSecondBranch. SecondBranch))
 )(λx. λy. x)(λx. λy. y)(λc. λl. λr. c l r))

 ((λtrue. λfalse. λifThenElse.
     (ifThenElse false (λFirstBranch. FirstBranch) (λSecondBranch. SecondBranch))
 )(λx. λy. x)(λx. λy. y)(λc. λl. λr. c l r))
</source>
Smalltalk uses a similar idea for its  true and false representations, with True and False being singleton objects that respond to messages ifTrue/ifFalse differently.

Haskell used to use this exact model for its Boolean type, but at the time of writing, most Haskell programs use syntactic sugar "if a then b else c" construct which unlike ifThenElse does not compose unless
either wrapped in another function or re-implemented as shown in The Haskell section of this page.

== Case and switch statements ==

{{Main|Switch statement}}

[[Switch statement]]s (in some languages, ''case statements'' or multiway branches) compare a given value with specified constants and take action according to the first constant to match. There is usually a provision for a default action ('else','otherwise') to be taken if no match succeeds. Switch statements can allow [[Optimizing compiler|compiler optimizations]], such as [[Lookup table|lookup table]]s. In dynamic languages, the cases may not be limited to constant expressions, and might extend to [[Pattern matching|pattern matching]], as in the [[Shell script|shell script]] example on the right, where the '*)' implements the default case as a [[Regular expression|regular expression]] matching any string.

{| class="wikitable"
|-
! [[Pascal (programming language)|Pascal]]:
! [[C (programming language)|C]]:
! [[Shell script]]:
|-
|<source lang="pascal">
case someChar of
  'a': actionOnA;
  'x': actionOnX;
  'y','z':actionOnYandZ;
  else actionOnNoMatch;
end;
</source>
|<source lang="c">
switch (someChar) {
  case 'a': actionOnA; break;
  case 'x': actionOnX; break;
  case 'y':
  case 'z': actionOnYandZ; break;
  default: actionOnNoMatch;
}
</source>
|<source lang="bash">
case $someChar in 
   a)    actionOnA; ;;
   x)    actionOnX; ;;
   [yz]) actionOnYandZ; ;;
  *)     actionOnNoMatch  ;;
esac
</source>
|}

== Pattern matching ==

{{Main|Pattern matching}}

[[Pattern matching]] may be seen as an alternative to both ''if–then–else'', and ''case'' statements. It is available in many programming languages with functional programming features, such as [[Wolfram Language]], [[ML (programming language)|ML]] and many others. Here is a simple example written in the [[OCaml]] language:
<source lang="ocaml">
match fruit with
| "apple" -> cook pie
| "coconut" -> cook dango_mochi
| "banana" -> mix;;
</source>
The power of pattern matching is the ability to ''concisely'' match not only actions but also ''values'' to patterns of data. Here is an example written in [[Haskell (programming language)|Haskell]] which illustrates both of these features:
<source lang="haskell">
map _ []      = []
map f (h : t) = f h : map f t
</source>
This code defines a function ''map'', which applies the first argument (a function) to each of the elements of the second argument (a list), and returns the resulting list. The two lines are the two definitions of the function for the two kinds of arguments possible in this case – one where the list is empty (just return an empty list) and the other case where the list is not empty.

Pattern matching is not strictly speaking ''always'' a choice construct, because it is possible in Haskell to write only one alternative, which is guaranteed to always be matched – in this situation, it is not being used as a choice construct, but simply as a way to bind names to values. However, it is frequently used as a choice construct in the languages in which it is available.

== Hash-based conditionals ==
In programming languages that have [[Associative array|associative array]]s or comparable data structures, such as [[Python (programming language)|Python]], [[Perl]], [[Organization:PHP|PHP]] or [[Objective-C]], it is idiomatic to use them to implement conditional assignment.<ref>{{cite web|url=http://codingstyleguide.com/style/180/python-pythonic-way-to-implement-switchcase-statements|title=Pythonic way to implement switch/case statements|access-date=2015-01-19|archive-url=https://web.archive.org/web/20150120120833/http://codingstyleguide.com/style/180/python-pythonic-way-to-implement-switchcase-statements|archive-date=2015-01-20|url-status=dead}}</ref>

<source lang="python">
pet = input("Enter the type of pet you want to name: ")
known_pets = {
    "Dog": "Fido",
    "Cat": "Meowsles",
    "Bird": "Tweety",
}
my_name = known_pets[pet]
</source>

In languages that have [[Anonymous function|anonymous function]]s or that allow a programmer to assign a named function to a variable reference, conditional flow can be implemented by using a hash as a dispatch table.

==Predication==
An alternative to conditional branch instructions is predication. Predication is an [[Instruction set architecture|architectural]] feature that enables instructions to be conditionally executed instead of modifying the [[Control flow|control flow]].

== Choice system cross reference ==

This table refers to the most recent language specification of each language. For languages that do not have a specification, the latest officially released implementation is referred to.

{| class="wikitable"
|-
! rowspan=2 | [[Programming language]]
! colspan=3 | Structured if
! rowspan=2 | [[Switch statement|switch]]–select–case
! rowspan=2 | [[Arithmetic IF|Arithmetic if]]
! rowspan=2 | [[Pattern matching]]{{ref label|pattern matching|A|none}}
|-
! then
! else
! else–if
|-
|{{rh}}| [[Ada (programming language)|Ada]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[APL (programming language)|APL]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Software:Bash (Unix shell)|Bash shell]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
|- 
|{{rh}}| [[C (programming language)|C]], [[C++]]
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|preprocessor elif|B|1}}{{ref label|choice else if|C|6}}
| {{yes2|Fall-through}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[C Sharp (programming language)|C#]]
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|preprocessor elif|B|2}}{{ref label|choice else if|C|2}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[COBOL]]
| {{yes}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice else if|C|2}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[Eiffel (programming language)|Eiffel]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes|[[Switch statement#Eiffel|Yes]]}} 
| {{no}}
| {{no}}
|- 
|{{rh}}| [[F Sharp (programming language)|F#]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice const|D|1}}
| {{no}}
| {{yes}}
|- 
|{{rh}}| Fortran 90
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}{{ref label|fortran|G|none}}
| {{no}}
|- 
|{{rh}}| [[Go (programming language)|Go]]
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice else if|C|1}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[Haskell (programming language)|Haskell]]
| {{yes}}
| {{yes2|Needed}}
| {{no2|Unneeded}}{{ref label|choice else if|C|3}}
| {{yes-no|Yes, but unneeded}}{{ref label|choice const|D|2}}
| {{no}}
| {{yes}}
|- 
|{{rh}}| [[Java (programming language)|Java]] 
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice else if|C|4}}
| {{yes2|Fall-through}}<ref name="JLS">[http://java.sun.com/docs/books/jls/download/langspec-3.0.pdf Java.sun.com], Java Language Specification, 3rd Edition.</ref>
| {{no}}
| {{no}}
|- 
|{{rh}}| [[ECMAScript]] ([[JavaScript]])
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice else if|C|5}}
| {{yes2|Fall-through}}<ref name="ECMA-262">[http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf Ecma-international.org] {{Webarchive|url=https://web.archive.org/web/20150412040502/http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf |date=2015-04-12 }} ECMAScript Language Specification, 5th Edition.</ref>
| {{no}}
| {{no}}
|-
|{{rh}}| [[Software:Mathematica|Mathematica]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[Oberon (programming language)|Oberon]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[Perl]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| [[Organization:PHP|PHP]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes2|Fall-through}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Pascal (programming language)|Pascal]], [[Object Pascal]] ([[Delphi (programming language)|Delphi]])
| {{yes}}
| {{yes}}
| {{no2|Unneeded}}
| {{yes}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Python (programming language)|Python]]
| {{no}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[Software:QuickBASIC|QuickBASIC]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Ruby (programming language)|Ruby]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}{{ref label|ruby|H|none}}
|-
|{{rh}}| [[Rust (programming language)|Rust]]
| {{no}}
| {{yes}}
| {{yes}}
| {{no2|Unneeded}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[Scala (programming language)|Scala]]
| {{no}}
| {{yes}}
| {{no2|Unneeded}}{{ref label|choice else if|C|4}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[SQL]]
| {{yes}}{{ref label|SQL|F|1}}
| {{yes}}
| {{yes}}
| {{yes}}{{ref label|SQL|F|2}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Swift (programming language)|Swift]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[Tcl]]
| {{no}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
|-
|{{rh}}| [[Visual Basic]], classic
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|-
|{{rh}}| [[Visual Basic .NET]]
| {{yes}}
| {{yes}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
|- 
|{{rh}}| Windows PowerShell 
| {{no}}
| {{yes}}
| {{yes}}
| {{yes2|Fall-through}}
| {{no}}
| {{no}}
|}
{{ordered list|type=upper-alpha
|{{note label|pattern matching|A|none}} This refers to pattern matching as a distinct conditional construct in the programming language – as opposed to mere string pattern matching support, such as [[Regular expression|regular expression]] support.
|{{note label|preprocessor elif|B|1}}{{note label|preprocessor elif|B|2}} An #ELIF directive is used in the [[Preprocessor|preprocessor]] sub-language that is used to modify the code before compilation; and to [[Include directive|include]] other files.
|{{note label|choice else if|C|1}} {{note label|choice else if|C|2}} {{note label|choice else if|C|3}} {{note label|choice else if|C|4}} {{note label|choice else if|C|5}} {{note label|choice else if|C|6}}The often-encountered <code>else if</code> in the C family of languages, and in COBOL and Haskell, is not a language feature but a set of nested and independent ''if then else'' statements combined with a particular source code layout. However, this also means that a distinct else–if construct is not really needed in these languages.
|{{note label|choice const|D|1}} {{note label|choice const|D|2}} In Haskell and F#, a separate constant choice construct is unneeded, because the same task can be done with pattern matching.
|{{note label|case regexp|E|none}} In a Ruby <code>case</code> construct, [[Regular expression|regular expression]] matching is among the conditional flow-control alternatives available. For an example, see [https://stackoverflow.com/q/6803647 this] Stack Overflow question.
|{{note label|SQL|F|1}}{{note label|SQL|F|2}} SQL has two similar constructs that fulfill both roles, both introduced in [[SQL-92]]. A "searched <code>CASE</code>" expression {{code|2=sql|CASE WHEN cond1 THEN expr1 WHEN cond2 THEN expr2 [...] ELSE exprDflt END}} works like <code>if ... else if ... else</code>, whereas a "simple <code>CASE</code>" expression: {{code|2=sql|CASE expr WHEN val1 THEN expr1 [...] ELSE exprDflt END}} works like a switch statement. For details and examples see Case (SQL).
|{{note label|fortran|G|none}} Arithmetic <code>if</code> is obsolescent in Fortran 90.
|{{note label|ruby|H|none}} Pattern matching was added in Ruby 3.0.<ref>{{cite web |title=Pattern Matching |url=https://docs.ruby-lang.org/en/3.0.0/doc/syntax/pattern_matching_rdoc.html |website=Documentation for Ruby 3.0}}</ref> Some pattern matching constructs are still experimental.
}}

== See also ==
* [[Branch (computer science)]]
* [[Conditional compilation]]
* [[Dynamic dispatch]] for another way to make execution choices
* [[McCarthy Formalism]] for history and historical references
* [[Named condition]]
* [[Relational operator]]
* [[Software:Test (Unix)|Test (Unix)]]
* [[Yoda conditions]]
* Conditional move

== References ==
{{Reflist}}

==External links==
* [http://everythingfla.com/courses/video/8/69/ IF NOT (ActionScript 3.0)] video

{{DEFAULTSORT:Conditional (programming)}}
[[Category:Conditional constructs| ]]

{{Sourceattribution|Conditional (computer programming)}}