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In most imperative computer programming languages, the assignment statement is one of the basic statements. It sets or re-sets the value assigned to a variable. This means that the same variable name will possibly stand for different values at different times; the variables are not handled in the same way as the unknowns x, y, z ... of algebra, which stand always for the same value.
Notation
Common representations of the assignment operator include an equals sign ("="), ":=", or a left-arrow; this last is less common in modern languages as this character is not available on most standard keyboard layouts. e.g. variable = expression Fortran, C, ... variable := expression Algol, Pascal, Ada, ... variable <- expression S, R variable ← expression APL MOVE expression TO variable COBOL
Some expression-oriented languages uniformly use functional syntax for all statements, including assignment: (setq variable expression) LISP set variable expression Tcl
Operation
Semantically, an assignment operation modifies the current state of the executing program:
- The
expressionis evaluated in the current state of the program. - The
variableis assigned the computed value, replacing the prior value of that variable.
Example: Assuming that a is a numeric variable, the assignment a := 2*a means that the content of the variable a is doubled after the execution of the statement.
An example segment of C code:
int x=10; float y; x=23; y=32.4;
In this sample, the variable x 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, y is declared without an assignment. In the 3rd line, x is reassigned the value of 23. Finally, y is assigned the value of 32.4.
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, such as Perl, it is not necessary to declare a variable prior to assigning it a value.
Assignment versus equality
A common error regarding the assignment operation is when programmers confuse it with the equivalence expression, readily associated with the mathematical use of "=" to mean equality. Whereas assignment is an operation which alters the value of a variable, equivalence is an expression which tests whether two variables or expressions evaluate to the same value.
In many languages, the assignment operator is a single equals sign ("=") while the equivalence operator is a pair of equals signs ("=="), but this is far from universal; indeed some languages, such as BASIC, use a single equals sign for both, determining which is meant based on context.
This can lead to errors if the programmer forgets which form ("=", "==", ":=") is appropriate, particularly in languages such as C, where the assignment operator also returns the value assigned, and can be validly nested inside expressions (in the same way that a function returns a value). If the intention was to compare two values in an if statement, for instance, an assignment is quite likely to return a value interprettable as TRUE, in which case the then clause will be executed, leading the program to behave unexpectedly. While not necessarily a syntax error, most compilers and interpretters are able to detect such situations, and warn the programmer that an assignment operation may have been used where a comparison was intended.
Parallel assignment
Some programming languages, such as Python and Ruby, allow several variables to be assigned in parallel. In pseudocode:
a,b := 0,1
Simultaneously assigns 0 to a and 1 to b. More interestingly,
a,b := b,a
Swaps the values of a and b. In languages without parallel assignment, this would have to be written to use a temporary variable
var t := a a := b b := t
as the simpler construct a:=b ; b:=a would first assign the value of b to a, and then assign this same value back to b, thus ending with a and b both having the original value of b.
See also
Přiřazení | Zuweisung | Affectation | Присваивание (программирование)
"Assignment (computer science)"