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An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code. [1] In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently ...
Safely handling undefined values is important in optimistic concurrency control systems, which detect race conditions after the fact. For example, reading a shared variable protected by seqlock will produce an undefined value before determining that a race condition happened. It will then discard the undefined data and retry the operation.
For example, the imaginary number is undefined within the set of real numbers. So it is meaningless to reason about the value, solely within the discourse of real numbers. However, defining the imaginary number i {\displaystyle i} to be equal to − 1 {\displaystyle {\sqrt {-1}}} , allows there to be a consistent set of mathematics referred to ...
The final value of k is undefined. The answer that it must be 10 assumes that it started at zero, which may or may not be true. Note that in the example, the variable i is initialized to zero by the first clause of the for statement. Another example can be when dealing with structs.
Note: There is no built-in language literal for undefined. Thus (x === undefined) is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write var undefined = "I'm defined now";. A more robust approach is to compare using (typeof x === 'undefined').
Similarly, when dealing with complex variables, a critical point is a point in the function's domain where its derivative is equal to zero (or the function is not holomorphic). [3] [4] Likewise, for a function of several real variables, a critical point is a value in its domain where the gradient norm is equal to zero (or undefined). [5]
Another benefit from allowing signed integer overflow to be undefined is that it makes it possible to store and manipulate a variable's value in a processor register that is larger than the size of the variable in the source code. For example, if the type of a variable as specified in the source code is narrower than the native register width ...
This is an accepted version of this page This is the latest accepted revision, reviewed on 9 January 2025. Undefined may refer to: Mathematics Undefined (mathematics), with several related meanings Indeterminate form, in calculus Computing Undefined behavior, computer code whose behavior is not specified under certain conditions Undefined value, a condition where an expression does not have a ...