Editing Assignment (computer science) (section)

==Semantics==
An assignment operation is a process in [[imperative programming]] in which different values are associated with a particular variable name as time passes.<ref name="2CS24">{{cite web|url=http://www.csc.liv.ac.uk/~frans/OldLectures/2CS24/declarative.html|title=2cs24 Declarative|website=www.csc.liv.ac.uk|access-date=20 April 2018|archive-url=https://web.archive.org/web/20060424045449/http://www.csc.liv.ac.uk/~frans/OldLectures/2CS24/declarative.html|archive-date=24 April 2006|url-status=dead}}</ref> The program, in such model, operates by changing its state using successive assignment statements.<ref name="weisskop"/><ref name="Flaig2008">{{cite book |author=Ruediger-Marcus Flaig |title=Bioinformatics programming in Python: a practical course for beginners |url=https://books.google.com/books?id=kS7Dye-dv54C&pg=PA98|access-date=25 December 2010 |year=2008 |publisher=Wiley-VCH |isbn=978-3-527-32094-3 |pages=98–99}}</ref> Primitives of imperative programming languages rely on assignment to do [[iteration#Computing|iteration]].<ref name="Bruce Tate"/> At the lowest level, assignment is implemented using [[assembly language|machine operations]] such as <code>MOVE</code> or <code>STORE</code>.<ref name="weisskop"/><ref name="Bruce Tate">[http://www.ibm.com/developerworks/java/library/j-cb07186.html Crossing borders: Explore functional programming with Haskell] {{webarchive |url=https://web.archive.org/web/20101119190821/http://www.ibm.com/developerworks/java/library/j-cb07186.html |date=November 19, 2010 }}, by Bruce Tate</ref>

[[variable (programming)|Variables]] are containers for values. 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.<ref name="Flaig2008"/> Consequently, assignment is dependent on the concept of [[variable (programming)|variables]]. In an assignment:

* The <code>''expression''</code> is evaluated in the current state of the program.
* The <code>''variable''</code> is assigned the computed value, replacing the prior value of that variable.

Example: Assuming that <code>a</code> is a numeric variable, the assignment <code>a := 2*a</code> means that the content of the variable <code>a</code> is doubled after the execution of the statement.

An example segment of [[C (programming language)|C]] code:

<syntaxhighlight lang="c">
int x = 10; 
float y;
x = 23;
y = 32.4f;
</syntaxhighlight>

In this sample, the variable <code>x</code> is first declared as an int, and is then assigned the value of 10. Notice that the declaration and assignment occur in the same statement. In the second line, <code>y</code> is declared without an assignment. In the third line, <code>x</code> is reassigned the value of 23. Finally, <code>y</code> is assigned the value of 32.4.

For an assignment operation, it is necessary that the value of the <code>''expression''</code> is well-defined (it is a valid [[value (computer science)|rvalue]]) and that the <code>''variable''</code> represents a modifiable entity (it is a valid modifiable (non-[[const]]) [[Value (computer science)|lvalue]]). In some languages, typically [[Dynamic programming language|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.