Assignment (computer science)

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.

Navigation

  • Art and Cultures
    • Art (https://academickids.com/encyclopedia/index.php/Art)
    • Architecture (https://academickids.com/encyclopedia/index.php/Architecture)
    • Cultures (https://www.academickids.com/encyclopedia/index.php/Cultures)
    • Music (https://www.academickids.com/encyclopedia/index.php/Music)
    • Musical Instruments (http://academickids.com/encyclopedia/index.php/List_of_musical_instruments)
  • Biographies (http://www.academickids.com/encyclopedia/index.php/Biographies)
  • Clipart (http://www.academickids.com/encyclopedia/index.php/Clipart)
  • Geography (http://www.academickids.com/encyclopedia/index.php/Geography)
    • Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
    • Maps (http://www.academickids.com/encyclopedia/index.php/Maps)
    • Flags (http://www.academickids.com/encyclopedia/index.php/Flags)
    • Continents (http://www.academickids.com/encyclopedia/index.php/Continents)
  • History (http://www.academickids.com/encyclopedia/index.php/History)
    • Ancient Civilizations (http://www.academickids.com/encyclopedia/index.php/Ancient_Civilizations)
    • Industrial Revolution (http://www.academickids.com/encyclopedia/index.php/Industrial_Revolution)
    • Middle Ages (http://www.academickids.com/encyclopedia/index.php/Middle_Ages)
    • Prehistory (http://www.academickids.com/encyclopedia/index.php/Prehistory)
    • Renaissance (http://www.academickids.com/encyclopedia/index.php/Renaissance)
    • Timelines (http://www.academickids.com/encyclopedia/index.php/Timelines)
    • United States (http://www.academickids.com/encyclopedia/index.php/United_States)
    • Wars (http://www.academickids.com/encyclopedia/index.php/Wars)
    • World History (http://www.academickids.com/encyclopedia/index.php/History_of_the_world)
  • Human Body (http://www.academickids.com/encyclopedia/index.php/Human_Body)
  • Mathematics (http://www.academickids.com/encyclopedia/index.php/Mathematics)
  • Reference (http://www.academickids.com/encyclopedia/index.php/Reference)
  • Science (http://www.academickids.com/encyclopedia/index.php/Science)
    • Animals (http://www.academickids.com/encyclopedia/index.php/Animals)
    • Aviation (http://www.academickids.com/encyclopedia/index.php/Aviation)
    • Dinosaurs (http://www.academickids.com/encyclopedia/index.php/Dinosaurs)
    • Earth (http://www.academickids.com/encyclopedia/index.php/Earth)
    • Inventions (http://www.academickids.com/encyclopedia/index.php/Inventions)
    • Physical Science (http://www.academickids.com/encyclopedia/index.php/Physical_Science)
    • Plants (http://www.academickids.com/encyclopedia/index.php/Plants)
    • Scientists (http://www.academickids.com/encyclopedia/index.php/Scientists)
  • Social Studies (http://www.academickids.com/encyclopedia/index.php/Social_Studies)
    • Anthropology (http://www.academickids.com/encyclopedia/index.php/Anthropology)
    • Economics (http://www.academickids.com/encyclopedia/index.php/Economics)
    • Government (http://www.academickids.com/encyclopedia/index.php/Government)
    • Religion (http://www.academickids.com/encyclopedia/index.php/Religion)
    • Holidays (http://www.academickids.com/encyclopedia/index.php/Holidays)
  • Space and Astronomy
    • Solar System (http://www.academickids.com/encyclopedia/index.php/Solar_System)
    • Planets (http://www.academickids.com/encyclopedia/index.php/Planets)
  • Sports (http://www.academickids.com/encyclopedia/index.php/Sports)
  • Timelines (http://www.academickids.com/encyclopedia/index.php/Timelines)
  • Weather (http://www.academickids.com/encyclopedia/index.php/Weather)
  • US States (http://www.academickids.com/encyclopedia/index.php/US_States)

Information

  • Home Page (http://academickids.com/encyclopedia/index.php)
  • Contact Us (http://www.academickids.com/encyclopedia/index.php/Contactus)

  • Clip Art (http://classroomclipart.com)
Toolbox
Personal tools