IFTHENELSE

Ifthenelse, commonly referred to as if-then-else, is a control flow construct used to select between two alternatives based on a boolean condition. It can be implemented as a statement, directing which block of code to execute, or as an expression, yielding a value.

Syntax and form vary by language. In C-like languages the conditional operator yields a value with the form condition ? thenExpr : elseExpr. In Python the structure is if condition: then_block else: else_block. In functional languages such as Haskell, if cond then a else b is an expression that evaluates to a value. In SQL, similar behavior is achieved with CASE WHEN condition THEN thenExpr ELSE elseExpr END. Some languages also provide a higher-order function or combinator named ifThenElse that takes three arguments: a condition, a then-branch, and an else-branch.

Evaluation and side effects: Many languages evaluate only the selected branch (short-circuiting or lazy evaluation), while others may assess both branches in certain contexts. The presence of side effects in the branches makes the exact evaluation order important for program behavior.

Usage and design considerations: Ifthenelse is used to select values, route behavior, or implement branching without duplicating code. However, excessive nesting can reduce readability. When appropriate, alternatives such as guard clauses, pattern matching, or early returns may improve clarity.

History and notes: The concept is a long-standing element of programming languages. The ternary operator popularized concise conditional expressions in imperative languages, while many languages implement if-then-else as a core, widely supported feature present in both imperative and functional paradigms.