Encyclopedia of Computer Science and Technology

VvariableVirtually all computer programs must keep track of a varietyof items of information that can change as a result ofprocessing. Such values might include totals or subtotals,screen coordinates, the current record in a database, or anynumber of other things. A variable is a name given to sucha changeable quantity, and it actually represents the area ofcomputer memory that holds the relevant data.Consider the following statement in the C language:int Total = 0;Variables have several attributes. First, every variablehas a name—Total in this case. Although this name actuallyrefers to an address in memory, in most cases the programmercan use the much more readable name instead ofthe actual address.It is possible to have more than one name for the samevariable by having another variable point to the first variable’scontents, or by declaring a “reference” variable (seepointers and indirection).Each variable has a data type, which might be number,character, string, a collection (such as an array), a datarecord, or some special type defined by the programmer(see data types). With some exceptions (see scriptinglanguages) most modern programming languages requirethat the programmer declare each variable before it is used.The declaration specifies the variable’s type—in the currentexample, the type is int (integer, or whole number).A variable is usually given an initial value by using anassignment statement; in the example above the variableTotal is given an initial value of 0, and the assignment iscombined with the declaration. (Some languages automaticallyassign a default value such as 0 for a number or a nullcharacter for a string, but with other languages failure toassign a value results in the variable having as its valuewhatever happens to be currently stored in the memoryaddress associated with the variable. An explicit assignmentis thus always safer and more readable.)When exactly do variables get set up, and when do theyget their values? This varies with the programming language(see binding). With C and similar languages, a variablereceives its data type when the program is compiled(compile time). The type in turn determines the range ofvalues that the variable can hold (physically based on thenumber of bytes of memory allocated to it). The variable’svalue is actually stored in that location when the program isexecuted (run time).A few languages such as APL and LISP use dynamicbinding, meaning that a data type is not associated with avariable until run time. This makes for flexibility in programming,but at some cost in efficiency of storage andexecution speed.During processing, a variable’s value can changethrough the use of operators in expressions (see operatorsand expressions). Thus, the example value Total might bechanged by a statement such as:Total = Total + Subtotal;When this statement is executed, the following happens:The value of the memory location labeled “Subtotal” isobtained.490