Difference between revisions of "Pascal (programming language)"

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Prof. Niklaus Wirth.

Pascal is a computer programming language created by Prof. Niklaus Wirth, and published in 1970. It became widely popular with the release of Borland's Turbo Pascal, an integrated editor and compiler for Pascal on MS DOS, in 1981. Pascal was eventually made an ISO standard (ISO 7185).

Description

Pascal uses a series of statements, which, like BASIC, are English-like words. Statements can wrap onto multiple lines, and each statement is terminated by a semi-colon or the reserved word end.

Statements may be simple or compound. A simple assignment statement is:

a:=3;

A loop statement that appends characters to a string might look like the following:

for i:=1 to 6 do s:=s+' more';

The for statement executes the assignment statement each time through the loop. If the loop is to execute more than one statement, they are incorporated into a compound statement, bracketed by begin and end reserved words:

 for i:=1 to 6 do
 begin
   s:=s+' more';
   count:=count+1;
   y:=y-3;
 end;

Technical Summary

Pascal, as defined by Wirth, is a general-purpose programming language with the following reserved words: array, begin, case, const, do, downto, else, end, file, for, function, goto, if, label, nil, of, packed, procedure, program, record, repeat, set, then, to, type, until, var, while, with. The following reserved words are operators: and, or, not, div, mod, in. Built in simple data types: integer, char, boolean, real. In addition, arrays of data types, files of data types, and records (structures) can be defined, as well as enumerations (called scalars). Unique among major programming languages is the support of sets. Built-in functions: abs, arctan, chr, cos, eof, eoln, exp, ln, odd, ord, pred, round, sin, sqr, sqrt, succ, trunc. Build-in procedures: dispose, get, new, put, read, readln, reset, rewrite, write, writeln. Operators: + - * ( ) < > = <> <= >=

A major fault in the original definition of Pascal was the lack of string support. Namely, strings are packed arrays of chars, with minimal string-specific support. This shortcoming has been addressed by every modern Pascal compiler, which provide well-supported native string data types.

Sets

In Pascal, a set is a collection of ordinal values of the same type. Operators can test for the existence of a value in a set, and or or two sets together, compare sets, and do other operations. The following code shows an example of declaring and using a set, in this case of the days of the month that are Sundays.

type
  DaysOfMonthSet = set of 1..31;  {Declare type as a set representing the days of a month}


procedure Demo;
var  SundaysInMonth: DaysOfMonthSet;  {Declare variable of type DaysOfMonthSet}
     DaysWorked:     DaysOfMonthSet;  {And another}
     Day: Byte;  {Declare 8-bit variable to hold a day of the month}
begin
  SundaysInMonth:=[2,9,16,23,30];    {Allocate the 2nd, 9th, etc. of month to the set SundaysInMonth.}
  DaysWorked:=[10..14, 17..21];      {Allocate two ranges of dates to the set DaysWorked.}
  ...
  if Day in SundaysOfMonth           {'in' operator tests for a value in a set.}
    then writeln('Day is a Sunday')
    else writeln('Day is not a Sunday'); 
  ...
  if DaysWorked * SundaysInMonth = []    {'*' operator ands both sets together.  '[]' is an empty set.}
    then writeln('No Sunday shifts worked.');
end;

References

• Delphi Study Guide
• Jensen, K. and Wirth, N., Pascal User Manual and Report, Springer-Verlag, 1976.
• ISO 7185:1990