High-level Programming Language (Pascal)

Introduction

Pascal is the high-level programming language that use English-like statements to instruct the computer how to complete the specific task. Programming is the task of preparing the instructions and data required to enable a computer to undertake a particular job.

General Structure of a Pascal Program

• consist of three parts
  • program heading
  • declaration part
  • program body (action part)
• three parts appear in a fixed order
  

(I) Program heading

program ProgramName ;

• consisting of the reserved word program followed by a user-defined ProgramName
• input and output indicate the standard input device (keyboard) and output device (monitor) used to connect to the computer system
• semicolon (;) at the end is used as a separator between statements
• the syntax diagram of <program heading>

  

(II) Declaration part

• is used to tell the compiler the name of memory cells that are needed in the program
• all the data items used in the program must be defined in the declarations beforehand
• consists of constant section (const), type section (type), variable section (var) and subprogram section (procedure and function) which appear in fixed order
• the syntax diagram of <declaration part>

  

  
  constant declaration

• is used to associate an user-defined identifier with a constant value
• once declared, the identifier can be used throughout the program
• changing the value in the definition will automatically change the value used everywhere the identifier is used
• it is not possible to assign a new value to a constant identifier during execution
• the type of a constant is restricted to one of real, integer, char, string and boolean
• for example :

const 　Pi = 3.14159 ; 　　{real constant}
　　　　Year = 2000 ; 　　 {integer constant}
　　　　Male = 'M' ; 　　　 {char constant}
　　　　MyName = 'Peter'; {string constant}
　　　　Found = true ; 　　{boolean constant}

• the syntax diagram of <constant declaration>

  

  variable declaration

• a variable is a named memory cell that holds a certain type of value
• defines an identifier for a variable of the given data type
• a variable may be thought of as a "box" which is used to store a value
• tells the compiler the name of the box (variable name) and the type of the value to be stored in it
• just after declaration, a variable has an undefined value, the variable must be initialized before it is used in the program
• all variables used in the program must be declared before they are used
• at any time, variable store the latest value only
• the type of each variable determines the type of value that may be stored in that variable
• use meaningful variable names that suggest what they represent, since this makes the program more readable and easier to understand
• for example :

  var 　Year : integer ; 　　　　 {integer variable}
  　　　Salary, Price : real ; 　　{real variables}
  　　　MyName : string[30] ;　 {string variable}
  　　　Address : string;　 　　　{string variable}
  　　　Found : boolean ; 　　　 {boolean variable}

• the syntax diagram of <variable declaration>

  

(III) Program body

• consists of a sequence of statements (i.e. a sequence of actions) enclosed by the reserved words begin and end and followed by a period (full-stop)
• tells the computer what to do
• begin and end indicate the beginning and ending of a program body
• for example :

  begin
  　writeln ('Hello !') ;
  　writeln ('How are you ?')
  end.

• the syntax diagram of <program body>