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If-Then-Else flow diagram A nested "If–Then–Else" flow diagram In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs,) are 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, 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 is not usually classified as a conditional construct, it is another way to select between alternatives at runtime. Terminology[edit]In imperative programming languages, the term "conditional statement" is usually used, whereas in functional programming, the terms "conditional expression" or "conditional construct" are preferred, because these terms all have distinct meanings. If–then(–else)[edit]"if-then-else" redirects here. For the album, see If Then Else. The 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 When an
interpreter finds an After either branch has been executed, control returns to the point after the History and development[edit]In early programming languages, especially some dialects of BASIC in the 1980s
home computers, an The "dangling else" problem[edit]The 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 Else if[edit]By using 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 However, in many languages more directly descended from Algol, such as Simula,
Pascal, BCPL and C, this special syntax for the This design choice
has a slight "cost". Each If all terms in the sequence of conditionals are testing the value of a single expression (e.g., If–then–else expressions[edit]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[edit]ALGOL 60 and some other members of the ALGOL family allow myvariable := if x > 20 then 1 else 2 Lisp dialects[edit]In dialects of Lisp – Scheme, Racket and Common Lisp – the first of which was inspired to a great extent by ALGOL: ;; Scheme (define myvariable (if (> x 12) 1 2)) ; Assigns 'myvariable' to 1 or 2, depending on the value of 'x' ;; Common Lisp (let ((x 10)) (setq myvariable (if (> x 12) 2 4))) ; Assigns 'myvariable' to 2 Haskell[edit]In Haskell 98, there is only an if expression, no if statement, and the Because Haskell is 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:[5] if' :: Bool -> a -> a -> a if' True x _ = x if' False _ y = y C-like languages[edit]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:
This means that it can be inlined into expressions, unlike if-statements, in C-like languages: my_variable = x > 10 ? "foo" : "bar"; // In C-like languages 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: if (x > 10) my_variable = "foo"; else my_variable = "bar"; 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,[6] while others argue that concise expressions are easier to read than statements spread over several lines containing repetition. Small Basic[edit]x = TextWindow.ReadNumber() If (x > 10) Then TextWindow.WriteLine("My variable is named 'foo'.") Else TextWindow.WriteLine("My variable is named 'bar'.") EndIf 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[edit]In Visual Basic and some other languages, a function called Tcl[edit]In
Tcl if {$x > 10} { puts "Foo!" } invokes a function named In the above example the condition is not evaluated before
calling the function. Instead, the implementation of the Such a behavior is possible by using Because Rust[edit]In
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"); } Arithmetic if[edit]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 x < 0, x = 0, x > 0. This was the earliest conditional statement in Fortran:[11] IF (e) label1, label2, label3 Where e is any numeric expression (not necessarily an integer); this is equivalent to IF (e .LT. 0) GOTO label1 IF (e .EQ. 0) GOTO label2 GOTO label3 Because this arithmetic IF is equivalent to multiple 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[edit]In contrast to other languages, in Smalltalk the conditional statement is not a
language construct but defined in the class var = condition ifTrue: [ 'foo' ] ifFalse: [ 'bar' ] JavaScript[edit]JavaScript uses if-else statements similar to those in C languages or similar. A boolean value is accepted within parentheses between the reserved if keyword and a left curly bracket. if (Math.random() < 0.5) { console.log("You got Heads!"); } else { console.log("You got Tails!"); } The above example takes the conditional of 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!"); } Lambda calculus[edit]In Lambda calculus, the concept of an if-then-else conditional can be expressed using the expressions: true = λx. λy. x false = λx. λy. y ifThenElse = (λc. λx. λy. (c x y))
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: ((λtrue. λfalse. λifThenElse. (ifThenElse true 2 3) )(λx. λy. x)(λx. λy. y)(λc. λl. λr. c l r)) 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: var computationResult = ((_true => _false => _ifThenElse => _ifThenElse(_true)(2)(3) )(x => y => x)(x => y => y)(c => x => y => c(x)(y))); The code above with multivariable functions looks like this: var computationResult = ((_true, _false, _ifThenElse) => _ifThenElse(_true, 2, 3) )((x, y) => x, (x, y) => y, (c, x, y) => c(x, y)); another version of the earlier example without a system where numbers are assumed is below. First example shows the first branch being taken, while second example shows the second branch being taken. ((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)) 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[edit]Switch statements (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 compiler optimizations, such as lookup tables. In dynamic languages, the cases may not be limited to constant expressions, and might extend to pattern matching, as in the shell script example on the right, where the '*)' implements the default case as a regular expression matching any string.
Pattern matching[edit]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 and many others. Here is a simple example written in the OCaml language: match fruit with | "apple" -> cook pie | "coconut" -> cook dango_mochi | "banana" -> mix;; 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 which illustrates both of these features: map _ [] = [] map f (h : t) = f h : map f t 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[edit]In programming languages that have associative arrays or comparable data structures, such as Python, Perl, PHP or Objective-C, it is idiomatic to use them to implement conditional assignment.[13] pet = input("Enter the type of pet you want to name: ") known_pets = { "Dog": "Fido", "Cat": "Meowsles", "Bird": "Tweety", } my_name = known_pets[pet] In languages that have anonymous functions 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[edit]An alternative to conditional branch instructions is predication. Predication is an architectural feature that enables instructions to be conditionally executed instead of modifying the control flow. Choice system cross reference[edit]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.
See also[edit]
References[edit]
External links[edit]Look up then or else in Wiktionary, the free dictionary.
What is a decision making structures?Decision making structures require that the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.
What are the 4 types of control structures?The following are the different types of control structures:. Sequential control structure.. Selection control structure.. Iteration control structure.. What are two types of decision structures?There are two major types of decision structures: conditionals and loops. Code contained in a conditional block may or may not be executed; the decision is made at runtime based on a given condition.
What is decision control structure?Decision Control Structure. • A statement or set of statements that is executed when a particular condition is True and ignored when the condition is False is called Decision Control Structure. • The decision to execute a particular section is based on checking a condition.
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