Ternary conditional operator

In computer programming, the ternary conditional operator is a conditional expression with three parts: the Boolean condition, the then-expression, and the else-expression. If the condition is true, the then-expression is evaluated; otherwise, the else-expression is evaluated; and the value is returned. Thus it is a non-strict operator, like other conditional expressions. It is also called a conditional operator, ternary if, immediate if, or inline if (iif).

Although a ternary operator in general is any operator with three arguments, the three-argument conditional operator is the only common one in programming, so it is loosely called the ternary operator.

C introduced the syntax , read as "if a then b otherwise c", which is widely used by other languages with C-like syntax. Some languages use functional syntax: in Visual Basic, it is written .

Many languages support conditional expressions with two, three, or more clauses. Languages following ALGOL-like syntax often support them with the same syntax as conditional statements, .

The construct first appeared in CPL in 1963, as <code>a → b, c</code>.

Patterns

Assignment

The value of the operator can be assigned to a variable. For a weakly typed language, the data type of the selected value may determine the type of the assigned value. For a strongly typed language, both value expressions must evaluate to a type that is compatible with the target variable.

The operator is similar to the way conditional expressions (if-then-else) work in functional programming languages, like Scheme, ML, Haskell, and XQuery, since if-then-else forms an expression instead of a statement in those languages.

The operator allows for initializing a variable via a single statement which otherwise might require multiple statements. Use in variable assignment reduces the probability of a bug from a faulty assignment as the assigned variable is stated only once.

For example, in Python:

x: str = 'foo' if b else 'bar'

</syntaxhighlight>

instead of:

x: str

if b:

x = 'foo'

else:

x = 'bar'

</syntaxhighlight>

In a language with block scope, a variable must be declared before the if-else statement. For example:

std::string s;

if (b) {

s = "foo";

} else {

s = "bar";

}

</syntaxhighlight>

Use of the conditional operator simplifies this:

std::string s = b ? "foo" : "bar";

</syntaxhighlight>

Furthermore, since initialization is now part of the declaration, rather than a separate statement, the identifier can be a constant. For example:

const std::string s = b ? "foo" : "bar";

</syntaxhighlight>

Case selector

The conditional operator can be used for case selectors. For example:

vehicle = arg == 'B' ? bus :

arg == 'A' ? airplane :

arg == 'T' ? train :

arg == 'C' ? car :

arg == 'H' ? horse :

feet;

</syntaxhighlight>

Variations

The syntax and semantics of the operator vary by language.

Major differences include whether the expressions can have side effects and whether the language provides short-circuit evaluation semantics, whereby only the selected expression is evaluated.

If a language supports expressions with side effects but does not specify short-circuit evaluation, then a further distinction exists about which expression evaluates first. If no order is guaranteed, a distinction exists about whether the result is then classified as indeterminate (the value obtained from some order) or undefined (any value at all at the whim of the compiler in the face of side effects, or even a crash).

If a language does not permit side-effects in expressions (common in functional languages), then the order of evaluation has no value semantics though it may yet bear on whether an infinite recursion terminates, or have other performance implications (in a functional language with match expressions, short-circuit evaluation is inherent, and natural uses for the ternary operator arise less often, so this point is of limited concern).

For these reasons, in some languages the statement form can have subtly different semantics than the block conditional form .

In almost all languages, the ternary operator is right associative, so that evaluates intuitively as . This means it can be chained similarly to an <code>if ... else if ... else if ... else</code> chain. The main exception is PHP, in which it was left-associative (such that the same expression evaluates to , which is rarely what the programmer expects) prior to version 8, and is non-associative thereafter.

Furthermore, in all C-family languages and many others, the ternary conditional operator has low operator precedence.

Examples

Ada

The 2012 edition of Ada introduced conditional expressions (using and ), as part of an enlarged set of expressions including quantified expressions and expression functions. The Rationale for Ada 2012 states motives for Ada not having had them before, as well as motives for now adding them, such as to support "contracts" (also new).

Pay_per_Hour := (if Day = Sunday then 12.50 else 10.00);

</syntaxhighlight>

When the value of an if_expression is itself of Boolean type, then the part may be omitted, the value being True. Multiple conditions may chained using .

ALGOL 60

ALGOL 60 introduced conditional expressions (ternary conditionals) to imperative programming languages.

This conditional statement:

integer opening_time;

if day = Sunday then

opening_time := 12;

else

opening_time := 9;

</syntaxhighlight>

Can be rewritten with the conditional operator:

integer opening_time;

opening_time := if day = Sunday then 12 else 9;

</syntaxhighlight>

ALGOL 68

Both ALGOL 68's choice clauses (if and case clauses) support the following:

; Single if choice clause: or a brief form: <code>( condition | statements | statements )</code>

; Chained if choice clause: or a brief form: <code>( condition1 | statements |: condition2 | statements | statements )</code>.

Bash

A true ternary operator exists for arithmetic expressions:

((result = condition ? value_if_true : value_if_false))

</syntaxhighlight>

For strings there are workarounds, like e.g.:

result=$(if condition ; then echo "value_if_true" ; else echo "value_if_false" ; fi)

</syntaxhighlight>

Where can be any bash command. When it exits with success, the first echo command is executed, otherwise the second one is executed.

C family

The following code in C assigns to the value of <code>x</code> if <code>a > b</code>, and otherwise to the value of <code>y</code>. This is the same syntax as in many related languages including C++, Java, JavaScript, and Dart.

result = a > b ? x : y;

</syntaxhighlight>

Only the selected expression is evaluated. In this example, <code>x</code> and <code>y</code> require no evaluation, but they can be expressions with side effects. Only the side-effect for the selected expression value will occur.

If <code>x</code> and <code>y</code> are of the same data type, the conditional expression generally has that type. Otherwise, the rules governing the resulting data type vary a little between languages:

In C++, the usual arithmetic type conversions are performed to convert <code>x</code> and <code>y</code> to a common type. If both are pointer or reference types, or one is a pointer type and the other is a constant expression evaluating to 0, pointer or reference conversions are performed to convert them to a common type.
In C#, if one expression is implicitly convertible to the type of the other, that type is used. Otherwise, a compile-time error occurs.
In dynamically typed languages, the evaluated expression has the type of the selected expression.

Furthermore, in C++, a conditional expression can be used as an lvalue, if both <code>x</code> and <code>y</code> are lvalues, though this is rarely used in practice:

(foo ? bar : baz) = frink;

</syntaxhighlight>

Common Lisp

Assignment using a conditional expression in Common Lisp:

(setq result (if (> a b) x y))

</syntaxhighlight>

Alternative form:

(if (> a b)

(setq result x)

(setq result y))

</syntaxhighlight>

dBASE

In dBase, the conditional function <code>iif(<expL>, <expT>, <expF>)</code> is called "Immediate IF". It uses shortcut evaluation (it only evaluates one of or ).

For example, to sort a list by the street name and then (in most cases) house number, one could type

to indexfile

at the dBASE III command prompt, and then copy or export the table.

Fortran

As part of the Fortran-90 Standard, the ternary operator was added to Fortran as the intrinsic function :

variable = merge(x,y,a>b)

</syntaxhighlight>

Note that both x and y are evaluated before the results of one or the other are returned from the function. Here, x is returned if the condition holds true and y otherwise.

Fortran-2023 added conditional expressions which evaluate one or the other of the expressions based on the conditional expression:

variable = ( a > b ? x : y )

</syntaxhighlight>

Kotlin

Kotlin does not include the traditional ternary operator, however, an can be used as an expression that can be assigned, achieving the same results.

val max = if (a > b) a else b

</syntaxhighlight>

Lua

Lua does not have a traditional conditional operator. However, the short-circuiting behavior of its and operators allows the emulation of this behaviour. The following is equivalent to: <code>var = cond ? a : b</code>.

var = cond and a or b

</syntaxhighlight>

This will succeed unless is logically false; in this case, the expression will always result in . This can result in some surprising behavior if ignored.

There are also other variants that can be used, but they're generally more verbose:

var = (

{

[true] = a,

[false] = b

}

)[not not cond]

</syntaxhighlight>

Luau, a dialect of Lua, has ternary expressions that look like if statements, but unlike them, they have no keyword, and the clause is required. One may optionally add clauses. It's designed to replace the idiom and is expected to work properly in all cases.

-- in Luau

var = if cond then a else b

-- with elseif clause

sign = if var < 0 then -1 elseif var == 0 then 0 else 1

</syntaxhighlight>

Object Pascal extensions

Pascal was both a simplification and extension of ALGOL 60, but it removed conditional expressions.

RemObjects Oxygene added a ternary operator to Object Pascal in approximately 2011, and in 2025 Delphi followed suit. Oxygene supports case/switch statements, essentially a repeated if, as expressions evaluating to a value as well.

Python

Python has had a ternary conditional expression since release 2.5 (September 2006). Python's conditional operator differs from the C-style operator in the order of its operands. The general form is:

result = x if a > b else y

</syntaxhighlight>

This form invites considering as the normal value and as an exceptional case. Unlike Python conditional statements, the <code>elif</code> clause is not supported, but nested conditional expressions are allowed.

Rust

As an expression-oriented programming language, all Rust conditionals are expressions, not statements, so the <code>if expr<sub>1</sub> else expr<sub>2</sub></code> syntax behaves like the C-style ternary operator. Earlier versions of the language had a operator but it was removed due to duplication with .

Note the lack of semi-colons in the code below compared to an imperative ... block, and the semi-colon at the end of the assignment to .

let x = 5;

let y = if x == 5 {

} else {

};

</syntaxhighlight>

This could also be written as:

let y = if x == 5 { 10 } else { 15 };

</syntaxhighlight>

Curly braces are mandatory in Rust conditional expressions.

You could also use a expression:

let y = match x {

5 => 10,

_ => 15,

};

</syntaxhighlight>

Smalltalk

Every expression (message send) has a value. Thus can be used:

|x y|

x := 5.

y := (x == 5) ifTrue:[10] ifFalse:[15].

</syntaxhighlight>

SQL

The SQL expression is a generalization of the ternary operator. Instead of one conditional and two results, n conditionals and n+1 results can be specified.

With one conditional it is equivalent (although more verbose) to the ternary operator:

SELECT (CASE WHEN a > b THEN x ELSE y END) AS CONDITIONAL_EXAMPLE

FROM tab;

</syntaxhighlight>

This can be expanded to several conditionals:

SELECT (CASE WHEN a > b THEN x WHEN a < b THEN y ELSE z END) AS CONDITIONAL_EXAMPLE

FROM tab;

</syntaxhighlight>

Visual Basic

Visual Basic provides a ternary conditional function, , as shown in the following code:

Dim opening_time As Integer = IIf((day = SUNDAY), 12, 9)

</syntaxhighlight>

As a function, the values of the three arguments are evaluated before the function is called. To avoid evaluating the expression that is not selected, the keyword was added (in Visual Basic .Net 9.0) as a true ternary conditional operator. This allows the following code to avoid an exception if it were implemented with instead:

Dim name As String = If(person Is Nothing, "", person.Name)

</syntaxhighlight>

References

External links

Description of If operator in Visual Basic
Description of Conditional Expression in Python (PEP 308)
Description in the Java Language Specification
Description in the PHP Language Documentation

de:Bedingte Anweisung und Verzweigung#Auswahloperator

Patterns

Assignment

Case selector

Variations

Examples

Ada

ALGOL 60

ALGOL 68

Bash

C family

Common Lisp

dBASE

Fortran

Kotlin

Lua

Object Pascal extensions

Python

Rust

Smalltalk

SQL

Visual Basic

See also

References

External links