Encyclopedia of Computer Science and Technology

VBScript 491The value of the memory location labeled “Total” isobtained.The two values are added together.The result is stored in the location labeled “Total,” replacingits former value.Scope of VariablesIn early programming languages variables were generallyglobal, meaning that they could be accessed and changedfrom any part of the program. While this practice is convenient,it became riskier as programs became larger and morecomplex. One part of a program might be using a variablecalled Total or Subtotal to keep track of some quantity. Later,another part is written to deal with some other calculation,and uses the same names. The programmer may think of thesecond Total and Subtotal as being quite separate from thefirst, but in reality they refer to the same memory locationsand any change affects both of them. Thus, it’s easy to createunwanted “side effects” when using global variables.Starting in the 1960s and more systematically duringthe 1970s, there was great interest in designing computerlanguages that could better manage the structure and complexityof large programs (see structured programming).One way to do this is to break programs up into more manageablemodules that each deal with some specific task (seeprocedures and functions). Unless explicitly declared tobe global, variables within a procedure or function are localto that unit of code. This means that if two procedures bothhave a variable called Total, changes to one Total do notaffect the other.Generally, in block-structured languages such as Pascala variable is by default local to the block of code in whichit is defined. This means it can be accessed only within thatblock. (Its visibility is said to be limited to that block.) Thevariable will also be accessible to any block that is nestedwithin the defining block, unless another variable with thesame name is declared in the inner block. In that case theinner variable supersedes the outer one, which will not bevisible in the inner block.Some languages such as APL and early versions of LISPdefine scope differently. Since these languages are not blockstructured, scope is determined not by the relationship ofblocks of code but by the sequence in which functions arecalled. At run time each variable’s definition is searchedfor first in the code where it is first invoked, then in whateverfunction called that code, then in the function thatcalled that function, and so on. As with dynamic binding,dynamic scooping offers flexibility but at a considerableprice. In this case, the price is that the program’s effects onvariables will be hard to understand, and the search mechanismslows down program execution. Dynamic scoping isthus not often used today, even in LISP.Global variables were convenient because they allowedinformation generated by one part of a program to beaccessed by any other. However, such accessibility can beprovided in a safer, more controlled form by explicitly passingvariables or their values to a procedure or functionwhen it is called (see procedures and functions).Object-oriented languages provide another way to controlor encapsulate information. Variables describing dataused within a class are generally declared to be private(accessible only within the functions used by the class).Public (i.e., global) variables are used sparingly. The ideais that if another part of the program wants data belongingto a class, it will call a member function of the class, whichwill provide the data without giving unnecessary access tothe class’s internal variables.A final concept that is important for understanding variablesis that of lifetime, that is, how long the definition of avariable remains valid. For efficiency, the runtime environmentmust deallocate memory for variables when they canno longer be used by the program (that is go “out of scope”).Generally, a variable exists (and can be accessed) only whilethe block of code in which it was defined is being executed(including any procedures or functions called from thatblock). In the case of a variable declared in the main program,this will be until the program as a whole reaches itsend statement. For variables within procedures or functions,however, the lifetime lasts only until the procedure or functionends and control is returned to the calling statement.However, languages such as C allow the special keywordstatic to be used for a variable that is to remain in existenceas long as the program is running. This can be useful whena procedure needs to “remember” some information betweenone call and the next, such as an accumulating total.Further ReadingKernighan, Brian W., and Dennis Ritchie. The C Programming Language.2nd ed. Englewood Cliffs, N.J.: Prentice Hall, 1988.Sebesta, Robert W. Concepts of Programming Languages. 8th ed.Boston: Addison-Wesley, 2007.Stroustrup, Bjarne. The C++ Programming Language. special ed.Reading, Mass.: Addison-Wesley, 2000.VBScriptDating back to the mid-1990s, VBScript is a scripting languagedeveloped by Microsoft and based on its popularVisual Basic programming language (see basic and scriptinglanguage). It is also part of the evolution of whatMicrosoft called “active scripting,” based on componentsthat allow outside access to the capabilities of applications.The host environment in which scripts run is providedthrough Windows (as with Windows Script Host) or withinMicrosoft’s Internet Explorer browser.For client-side processing, VBScript can be used to writescripts embedded in HTML pages, which interact with thestandard Document Object Model (see dom) in a way similarto other Web scripting languages (in particular, seeJavaScript). However, VBScript is not supported by popularnon-Microsoft browsers such as Firefox and Opera, sodevelopers generally must use the widely compatible Java-Script instead. VBScript can also be used for processing onthe Web server, particularly in connection with Microsoft’sWeb servers (see active server pages).Because versions of Windows starting with Windows98 include Windows Script Host, VBScripts can also bewritten to run directly under Windows. 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