Conditional (computer programming)

In computer programming, a conditional statement directs program control flow based on the value of a condition; a Boolean expression. A conditional expression evaluates to a value without the side-effect of changing control flow.

Many programming languages (such as C) have distinct conditional statements and expressions. In pure functional programming, a conditional expression does not have side-effects, many functional programming languages with conditional expressions (such as Lisp) support side-effects.

Although the syntax of an if-then-else statement varies by language, the general syntax is shown as pseudocode below. The part represented by the condition placeholder is an expression that evaluates to either true or false. If true, control passes to consequent and when complete to after end if. If false, control passes to alternative and when complete to after end if. As the else clause is optional, the else alternative part can be omitted. Typically, both consequent and alternative can be either a single statement or a block of statements.

The following example, also in pseudocode, replaces placeholders with example logic.

In early programming languages, especially dialects of BASIC, an if–then-else statement could only contain goto statements but this tended to result in hard-to-read spaghetti code. As a result, structured programming, which supports control flow via code blocks, gained in popularity, until it became the norm in most BASIC variants and all languages. Such mechanisms and principles were based on the ALGOL family of languages, including Pascal and Modula-2. While it is possible to use goto in a structured way, structured programming makes this easier. A structured if–then–else statement is one of the key elements of structured programming, and it is present in most popular languages such as C, Java, JavaScript and Visual Basic.

The convention is that an else clause, like the then clause, responds to the nearest preceding if clause; however, the semantics in some early languages, such as ALGOL 60, of nested conditionals, were less than clear; that is to say, the syntax was inadequate to specify one-and-always-the-same predicate if clause. Thus, the parser might randomly pair the else with any one of the perhaps manifold if clauses in the intended nested hierarchy.

This is known as the dangling else problem. It is resolved in various ways, depending on the language (in some, by means of explicit block-ending syntax (such as end if ) or a block enclosure, such as curly brackets ( {⋯} ).

Chaining conditionals is often provided in a language via an else-if construct. Only the statements following the first condition that is true are executed. Other statements are skipped. In placeholder pseudocode: