Conditional (computer programming)

Although dynamic dispatch is not usually classified as a conditional construct, it is another way to select between alternatives at runtime.

Although the syntax varies from language to language, the basic structure (in pseudocode form) looks like this: For example: 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 True or False) but having no intrinsic meaning.

This is known as the dangling else problem, and is resolved in various ways, depending on the language (commonly via the end if statement or {...} brackets).

Perl provides the keyword elsif to avoid the large number of braces that would be required by multiple if and else statements.

This works because in these languages, any single statement (in this case if cond...) can follow a conditional without being enclosed in a block.

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.

Many languages support conditional expressions, which are similar to if statements, but return a value as a result.

The concept of conditional expressions was first developed by John McCarthy during his research into symbolic processing and LISP in the late 1950s.

The idea of including conditional expressions was suggested by John McCarthy, though the ALGOL committee decided to use English words rather than McCarthy's mathematical syntax: Conditional expressions have always been a fundamental part of Lisp .

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: 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.

First, when the user runs the program, a cursor appears waiting for the reader to type a number.

In Visual Basic and some other languages, a function called IIf is provided, which can be used as a conditional expression.

For example invokes a function named if passing 2 arguments: The first one being the condition and the second one being the true branch.

[7] Such a behavior is possible by using uplevel and expr commands: Because if is actually a function it also returns a value: In Rust, if is always an expression.

GCL is intended primarily for reasoning about programs, but similar notations have been implemented in Concurrent Pascal and occam.

This was the only conditional control statement in the original implementation of Fortran on the IBM 704 computer.

A Boolean value is accepted within parentheses between the reserved if keyword and a left curly bracket.

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.

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: In Lambda calculus, the concept of an if-then-else conditional can be expressed using the following expressions: 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: Here, true, false, and ifThenElse are bound to their respective definitions which are passed to their scope at the end of their block.

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.

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.

Predication is an architectural feature that enables instructions to be conditionally executed instead of modifying the control flow.