High Quality Content by WIKIPEDIA articles! In computing (particularly, in programming), undefined value is a condition where an expression has not a correct value, although it is syntactically correct. Undefined value may not be confused with empty string, boolean "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to undefined value may lead to exception or undefined behaviour, but in some programming languages undefined values can occur during a normal, predictable course of program execution. Dynamically typed languages usually treat undefined values explicitly (when possible), e.g. Perl has undef operator which can "assign" such value to a variable. In other type systems an undefined value can mean an unknown, unpredictable value, or merely a program failure on attempt of its evaluation. Nullable types offer an intermediate approach; see below.

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