History and development

In early programming languages, especially some dialects of BASIC in the 1980s home computers, an if–then statement could only contain GOTO statements (equivalent to a 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, which allows (virtually) arbitrary statements to be put in statement blocks inside an if 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 and Modula-2 influenced modern BASIC variants for many years. While it is possible while using only GOTO statements in if–then 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 if–then–else 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, Java, JavaScript and Visual Basic .

if a then if b then s else s2

if a then (if b then s) else s2

if a then (if b then s else s2)

Else if

By using else if, 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 elseif statement, in the Ada language for example, is simply syntactic sugar for else followed by if. In Ada, the difference is that only one end if is needed, if one uses elseif instead of else followed by if. PHP uses the elseif keyword^[1] both for its curly brackets or colon syntaxes. Perl provides the keyword elsif to avoid the large number of braces that would be required by multiple if and else statements. Python uses the special keyword elif because structure is denoted by indentation rather than braces, so a repeated use of else and if would require increased indentation after every condition. Some implementations of BASIC, such as Visual Basic,^[2] use ElseIf too. Similarly, the earlier UNIX shells (later gathered up to the POSIX shell syntax^[3]) 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, BCPL and C, this special syntax for the else if construct is not present, nor is it present in the many syntactical derivatives of C, such as Java, ECMAScript, and so on. This works because in these languages, any single statement (in this case if cond...) can follow a conditional without being enclosed in a block.

This design choice has a slight "cost". Each else if 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 else if chains recursively.

If all terms in the sequence of conditionals are testing the value of a single expression (e.g., if x=0 ... else if x=1 ... else if x=2...), an alternative is the 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 else if 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).

  myvariable := if x > 20 then 1 else 2

;; 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

if' :: Bool -> a -> a -> a
if' True x _ = x
if' False _ y = y

condition ? evaluated-when-true : evaluated-when-false

my_variable = x > 10 ? "foo" : "bar";  // In C-like languages

if (x > 10)
    my_variable = "foo";
else
    my_variable = "bar";

x = TextWindow.ReadNumber()
If (x > 10) Then
    TextWindow.WriteLine("My variable is named 'foo'.")
Else
    TextWindow.WriteLine("My variable is named 'bar'.")
EndIf

if {$x > 10} {
   puts "Foo!"
}

// 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

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 GOTO 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 IF 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 but defined in the class Boolean as an abstract method that takes two parameters, both closures. Boolean has two subclasses, True and False, which both define the method, True executing the first closure only, False executing the second closure only.^[12]

var = condition 
    ifTrue: [ 'foo' ]
    ifFalse: [ 'bar' ]

JavaScript

JavaScript uses if-else statements similar to those in C languages. 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 Math.random() < 0.5 which outputs true if a random float value between 0 and 1 is greater than 0.5. The statement uses it to randomly choose between outputting You got Heads! or You got Tails! 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:

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

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))

1. true takes up to two arguments and once both are provided (see currying), it returns the first argument given.
2. false takes up to two arguments and once both are provided(see currying), it returns the second argument given.
3. 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:

 ((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 as follows:

 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.

The 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.