In most imperative computer programming languages, the assignment operation is one of the basic operations. 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.

It can take on the form, for example,

variable := expression

or often an equals sign is used:

variable = expression

The semantics of this operation is that the current state of the executing program is modified:

• 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.

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 lvalue).

A common error regarding the assignment operation is when programmers confuse it with the equivalence expression . In most languages, the assignment operator is a single equal sign (=) while the equivalence operator is a pair of equal signs (==). Often, the programmer neglects to use two equal signs when he or she wishes to make a comparison, especially when he is proficient in using languages (e.g., BASIC) where the two operators are the same: a single equal sign.

A consequence of this error is that the assignment is made, and the operation then returns the value assigned, which usually is a TRUE value. If the supposed equivalence expression were used as the condition in an if statement, what is often executed is the then clause, which is a program logic error. Most compilers and interpreters are able to flag these types of errors, warning the programmer that a possible assignment operation was unintended in an if statement.

Languages such as Perl do not require variables to be declared before an assignment is made.