ifthen

An if-then statement is a control-flow construct that evaluates a boolean condition and executes a block of statements when the condition is true. It is a basic mechanism for conditional execution and appears in many programming languages as well as in formal logic. If the condition is false, the program continues with the next statement, or follows an alternative path if an else branch is provided.

Origins and usage: Historically, forms of conditional branching appeared in early imperative languages such as Algol and Fortran, introducing the idea of a test followed by a then-clause. Different languages adopted varying syntax: Pascal uses if condition then and begin...end blocks; Ada uses if condition then ... elsif ... then ... else ... end if; C-style languages implement the same idea with if (condition) { ... } without the then keyword. In some functional languages the conditional is an expression that returns a value rather than a block of statements.

Variants: most languages support an optional else-part to specify an alternate block when the condition is false; many also provide else-if or elsif chains for multiple branches. Nested if-then constructs enable multi-level decision trees. In some languages conditional expressions enable concise forms such as If(condition, thenValue, elseValue) or the ternary operator condition ? thenValue : elseValue, though these are typically treated as expressions rather than statements.

Impact: The if-then construct is central to algorithm design, enabling branches, guards, and early exit patterns. Its clarity and readability depend on consistent style, proper indentation, and avoiding excessive nesting. See also: conditional expression, switch/case statement, pattern matching.