Encyclopedia of Computer Science and Technology

362 PascalLevine, John R., Tony Mason, and Doug Brown. lex & yacc. Sebastapol,Calif.: O’Reilly, 1995.Louden, Kenneth C. Compiler Construction: Principles and Practice.Boston: PWS Publishing, 1997.Metsker, Steven John. Building Parsers with Java. Boston: Pearson/Addison-Wesley, 2001.PascalBy the early 1960s, computer scientists had become increasinglyconcerned with finding ways to better organize orstructure programs. Indeed, one language (see Algol) hadalready been developed in part to demonstrate and encouragesound programming practices, including the proper useof control structures (see loop and structured programming).However, Algol lacked a full range of data typesand other features needed for practical programming, whilearguably being too complex and inconsistent to serve as agood teaching language.Niklaus Wirth at ETH (the Swiss Federal Institute ofTechnology) worked during the mid-1960s with a committeethat was trying to overcome the problems with Algoland make the language more practical and attractive tocomputer manufacturers and users. However, Wirth graduallybecame disillusioned with the committee’s unwieldyresults, and proceeded to develop a new language, Pascal,announcing its specifications in 1970.Pascal both streamlined Algol and extended it. Besidesproviding support for character, Boolean, and set data types,Pascal allows users to define new data types by combiningthe built-in types. This feature is particularly useful fordefining a “record” type that, for example, might combinean employee’s name and job title (characters), ID number (along integer), and salary (a floating-point number). The rigoroususe of data types also extends to the way proceduresare called and defined (see procedures and functions).Pascal attracted much interest among computer scientistsand educators by providing a well-defined languagein which algorithms could be expressed succinctly. Theacceptance of Pascal was also aided by its innovative compilerdesign. Unlike the machine-specific compilers of thetime, the Pascal compiler did not directly create machinecode. Rather, its output was “P-code,” a sort of abstractmachine language (see also pseudocode). A run-time systemwritten for each computer interprets the P-Code andexecutes the appropriate machine instructions. This meantthat Pascal compilers could be “ported” to a particularmodel of computer simply by writing a P-Code Interpreterfor that machine. This strategy would be used more thantwo decades later by the creators of a popular language forWeb applications (see Java).Structure of a Pascal ProgramThe following simple program illustrates the basic structureof a program in Pascal. (The words in bold type are keywordsused to structure the program.) The program beginswith a Type section that declares user-defined data types.These can include arrays, sets, and records (composite typesthat can include several different basic types of data). Herean array of up to 10 integers (whole numbers) is defined as atype called IntList.The Var (variable) section then declares specific variablesto be used by the program. Variables can be definedusing either the language’s built-in types (such as integer)or types previously defined in the Type section. An importantcharacteristic of Pascal is that user-defined types mustbe defined before they can be used in variable declarations,and variables in turn must be declared before they can beused in the program. Some programmers found this strictnessto be confining, but it guards against, for example, atypographical error introducing an undefined variable inplace of the one intended. Today most languages enforcethe declaration of variables before use.The word begin introduces the executable part of theprogram. The variables needed for the loop are first initializedby assigning them a value of zero. Note that in Pascal :=(colon and equals sign) is used to assign values. The outer ifstatement is used to ensure that the user does not input aninvalid number of items. The for loop then reads each inputvalue, assigns it to its place in the array, and keeps a runningtotal. That total is then used to compute the average,which is output by the writeln (write line) statement.program FindAvg (input, output);type IntList = array [1 . . 10] of integer;varInts: IntList;Items, Count, Total, Average: integer;beginAverage := 0;Total := 0;Readln (Items);If ((Items > 0) and (Items