Ifelse

Ifelse refers to a conditional statement used in programming to execute different code blocks depending on a boolean expression. It is a fundamental control-flow construct found in nearly all languages. An if-else statement generally evaluates a condition; if true, the program executes the then-branch; otherwise it executes the else-branch. Some languages support multiple branches via else if or elif.

Common syntax variations: In C-like languages: if (condition) { // then } else { // else } The condition is evaluated; nonzero or true is considered true, zero false. In Python: if condition: ... elif condition2: ... else: ... Indentation defines blocks; the elif is equivalent to else if. In languages with a ternary operator: condition ? expr1 : expr2; This yields a value and can be nested.

Special case: vectorized ifelse: In R, the function ifelse(test, yes, no) is vectorized, applying elementwise: if test[i] is TRUE, yields yes[i], else no[i]. This differs from the typical statement form by returning a value rather than controlling flow.

Notes and pitfalls: Be careful with accidental assignment in condition, rely on equality operators; ensure all possible cases are handled; optimize for readability over cleverness; in performance-critical code, branches may impact prediction and caching. See also conditional expression, ternary operator, switch statement.