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Semantics[ edit ] An assignment operation is a process in imperative programming in which different values are associated with a particular variable name as time passes.
It is possible to put a value into a variable and later replace it with a new one. An assignment operation modifies the current state of the executing program.
The expression is evaluated in the current state of the program. The variable is assigned the computed value, replacing the prior value of that variable. Assuming that a is a numeric variable, the assignment a: An example segment of C code: Notice that the declaration and assignment occur in the same statement.
In the second line, y is declared without an assignment. In the third line, x is reassigned the value of Finally, y is assigned the value of For an assignment operation, it is necessary that the value of the expression is well-defined it is a valid rvalue and that the variable represents a modifiable entity it is a valid modifiable non- const lvalue.
In some languages, typically dynamic ones, it is not necessary to declare a variable prior to assigning it a value. In such languages, a variable is automatically declared the first time it is assigned to, with the scope it is declared in varying by language.
Static single assignment form Any assignment that changes an existing value e. Single assignment is an example of name binding and differs from assignment as described in this article in that it can only be done once, usually when the variable is created; no subsequent reassignment is allowed.
An evaluation of expression does not have a side effect if it does not change an observable state of the machine,  and produces same values for same input. Single assignment is the only form of assignment available in purely functional languages, such as Haskellwhich do not have variables in the sense of imperative programming languages  but rather named constant values possibly of compound nature with their elements progressively defined on-demand.
Purely functional languages can provide an opportunity for computation to be performed in parallelavoiding the von Neumann bottleneck of sequential one step at time execution, since values are independent of each other.
For example, in Scheme, both single assignment with let and true assignment with set! Non- backtracking Prolog code can be considered explicit single-assignment, explicit in a sense that its named variables can be in explicitly unassigned state, or be set exactly once.
In Haskell, by contrast, there can be no unassigned variables, and every variable can be thought of as being implicitly set to its value or rather to a computational object that will produce its value on demand when it is created.
Value of an assignment[ edit ] In some programming languages, an assignment statement returns a value, while in others it does not. In other programming languages, Scheme for example, the return value of an assignment is undefined and such idioms are invalid.
In Haskell there is no variable assignment; but operations similar to assignment like assigning to a field of an array or a field of a mutable data structure usually evaluate to the unit typewhich is represented as.
This type has only one possible value, therefore containing no information. It is typically the type of an expression that is evaluated purely for its side effects.
Variant forms of assignment[ edit ] Certain use patterns are very common, and thus often have special syntax to support them. These are primarily syntactic sugar to reduce redundancy in the source code, but can also simplify compilation by clarifying the programmer's intent and easing analysis of the source code.
Chained assignments are equivalent to a sequence of assignments, but the evaluation strategy differs between languages. For simple chained assignments, like initializing multiple variables, the evaluation strategy does not matter, but if the targets l-values in the assignment are connected in some way, the evaluation strategy affects the result.
In some programming languages C for examplechained assignments are supported because assignments are expressions, and have values. In this case chain assignment can be implemented by having a right-associative assignmentand assignments happen right-to-left.
In Pythonassignment statements are not expressions and thus do not have a value. Instead, chained assignments are a series of statements with multiple targets for a single expression. This is most often known as parallel assignment; it was introduced in CPL inunder the name simultaneous assignment,  and is sometimes called multiple assignment, though this is confusing when used with "single assignment", as these are not opposites.Typing Assignments LLC, Cut Off, Louisiana.
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