INTRODUCTION TO PASCAL by cVeXWP

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									INTRODUCTION TO PASCAL
PASCAL PROGRAMMING




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BRIEF HISTORY
   First published in 1970.
   High level programming language named
    after the famous French mathematician
    Blaise Pascal.
   Designed to teach students structured
    programming concepts.




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WRITING PROGRAMS IN PASCAL
 All Pascal program have the form:
  Program header
  Declaration of constants and variables
  Main body of the program




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PROGRAM HEADER
   This states what the program does.
   It normally takes one of two forms:
     PROGRAM sumofnumbers;
     PROGRAM sumofnumbers (input, output);

     The second one is one that we will use.
     The name that you use in your program
      header cannot be used anywhere else in the
      program. In this example, sumofnumbers is
      the name of the program.

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    CONSTANT DECLARATION
    SECTION
   You would not need constants in every
    program, but when you do, you must place
    them in the Constant Declaration
    Section.
   This section starts with the keyword Const
    (short for ‘constant’).
   You can use as many as you want but
    ONLY Constants – things that will not
    change e.g pi = 3.142; days_in_week = 7;
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VARIABLE DECLARATION
SECTION
   Var indicates the start of the variables
    declaration section – the place where you
    must declare any variables that you intend
    to use in the program.
   Only variables can be declared in this
    section – nothing more.
   CONST AND VAR can only appear
    once.


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MAIN BODY OF THE PROGRAM
This section:
 is enclosed between the words Begin and
  End. (with the full stop)
 tells the computer what to do.
 When the computer encounters the full
  stop, it knows that the program is
  finished.
   Even though the program may contain END several
    times only the one at the end will have the full stop.

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Terminating Statements
   All program statements and lines are
    terminated with a semi-colon, except
    the Begin and End keywords. Program
    statements immediately preceding an End
    statement do not require a semi-colon.




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VARIABLES
 Declaring a variable lets the computer
  know ahead of time what type of data it is
  allowed to hold.
 The computer therefore will make sure
  that the user will only enter the valid
  information.




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DATA TYPES USED IN PASCAL
INTEGER
 Positive and negative numbers which do
  NOT have a decimal point e.g -50, 79,0.

REAL
 Positive and negative numbers that may have
  a decimal point, e.g 56.876, -67.8




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 DATA TYPES USED IN PASCAL
CHAR
• A single character (letter, digit or symbol)
•E.g a, Z, 0, #, &, )

STRING
•A group of up to 255 characters

BOOLEAN
•Either true or false (and nothing else)


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DECLARING VARIABLES
   Variables must be declared using the
    following format:
   Variable name: variable type;
   E.g
   Var
    ◦ gender: char;
    ◦ num2, num3: integer;
    ◦ results: real;
      If 2 or more variables have the same type, you can
       declare them on the same line (separated with commas)
       in order to save space.

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VARIABLE NAMES
   The Rules below must be followed:
    ◦ The name must not contain a space.
    ◦ A variable name must only contain letters,
      digits and underscores.
    ◦ A variable name must begin with a letter.
    ◦ You cannot use a reserved word as a variable
      name i.e (a word that has a special meaning to
      Pascal e.g Program, If, While, Begin, End.
    ◦ Pascal does not care whether you use
      lowercase or uppercase variable names.
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CONSTANTS
 A value that cannot and will not change
  throughout the execution of the program.
 Const
    ◦ Pi = 3.142;
    ◦ VAT = 0.15;
    ◦ With constants you:
    1. can (and must) specify a value.
    2. don’t have to specify a type. The computer
        can look at the value and figure out the
        type.
    3. use ‘=‘ instead of ‘:’.
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COMMENTS
 When inserting comments in your
  program, you place them in curly brackets
  { }.
 Circumference:= 2 * radius * pi; {this
  calculates the circumference}
 The computer completely ignores
  comments so you can put anything you
  want in them – even if they are not
  necessarily true.
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COMMENTS
 Comments are helpful to explain how the
  program works to other persons reading
  or viewing it.
 Comments may span several lines. {This
  program asks the user to enter a radius
  of a circle and then uses that radius to
  calculate the diameter and circumference
  of the circle.}


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DISPLAYING MESSAGES & THE CONTENT OF
VARIABLES ON THE SCREEN
 If you want the computer to display a
  message, you must put it in single
  quotes(‘).
 Writeln(‘Good morning’);
 The computer will display the message on
  the screen word-for-word but without
  the quotes.
 To display the value of a variable, do NOT
  enclose using single quotes.
 Writeln(‘The average is: ’, avg);
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DIFFERENCE BETWEEN WRITELN
AND WRITE
 Writeln command displays a message on
  the screen and then moves down to the
  next line.
 Write command does not move to the
  next line.




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FORMATTING REAL NUMBERS
 When a calculation is done and a number
  like 1.0200000000E+01 displays – this is
  in scientific notation.
 To fix this so that it looks like a number
  you recognise, in your program you use
  the following:
 Writeln (result:8:2); this means to display
  to 2 decimal places and that the result
  should only take up exactly 8 characters.
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READ/READLN
 Read and Readln is just like Write and
  Writeln.
 Readln(entirename);
 When you run the program and the
  computer gets to this line, it will wait
  patiently for the user to type in some
  information.
 When the user presses the Enter key, the
  computer takes the information and
  stores it in the variable called entirename.
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IMPORTANT NOTE
 Any variables that you read MUST have
  been declared in the Var section.
 When reading more than one variable
  you must separate them with a comma.
 When typing in the values, you have to
  separate them by using spaces.




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OTHER USES OF READLN
 At sometime or another you may want
  your program to pause until the user
  presses the Enter key. You can do this by
  using a Readln command without any
  brackets. E.g
 Readln




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ASSIGNMENT STATEMENTS
   These allow you to change the value of a
    variable. In pascal it is written like this:
    ◦ Variable:= expression that you want to assign
      to the variable;
    ◦ E.gs
    ◦ Sex:= ‘M’; {Assigning a character constant}
    ◦ Cont:= count+1; {Increasing the value of a
      variable}



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ASSIGNMENT RULES
 You can’t assign something to a constant.
 Any variable on the right-hand side
  should have already been assigned initial
  values.
 The expression on the right of the :=
  must be compatible with the type of
  variable you have on the left.



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BOOLEAN VARIABLES
 These only have two values – true/false.
 An assignment statement would look like
  – found:= false;
 IsGreater:= (A>B);
    ◦ The computer
      Tests whether A is larger than B. Depending on the
       values of A and B, it may be true or false.
      Then it stores the result of the test in the variable –
       IsGreater.


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IF STATEMENTS
   An IF statement in Pascal is done like this:
    ◦ IF condition(s) THEN
         single or compound statement to be executed only
       if the condition is true;
     E.G If a person is more than 65, then call that person
       old.
     IF age > 65 THEN
       Writeln(‘You are really old’);




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 E.g A discount is given of 15% if the
  customer spends $150.
IF total >= 150 Then
Begin
  discount:= 15/100 *total;
  total:= total – discount;
  Writeln(‘Customer received a discount
   of ‘, discount);
End;

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IF-THEN-ELSE
IF (Boolean expression) THEN
  BEGIN
    Action 1
  END
ELSE
  BEGIN
    Action 2
  END;

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E.g
IF (mark>40)THEN
       WRITELN (‘Pass’)
ELSE
       WRITELN (‘Fail’);
  OR
IF (mark>40) THEN
   BEGIN
       WRITELN (‘Pass’);
   END
ELSE
   BEGIN
       WRITELN (‘Fail’);
   END;

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FOR
FOR <integer variable> := <integer> TO
         <integer> DO
 BEGIN

 END;




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WHILE
WHILE <condition> DO
     single or compound statement to be
  repeated;
E.G
Readln(num);
  While num <>999 DO
BEGIN
  Square:= num*num;
  Writeln(Square);
  Readln(num);
END;
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REAPEAT …UNTIL …
   Is used to loop 1 or more times. It is similar
    to the While loop except that it tests the
    condition at the bottom of the loop.
   In Pascal this is:
     REPEAT
     …..
     UNTIL (Boolean expression)
    REPEAT
     READLN (password);
     UNTIL(password:=‘abc’);

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ARRAYS
 A collection of data items of the same
  type and referred to by a single name.
  The location or position of each item is
  uniquely designated by an integer
  subscript.
 It allows a number of variables of the
  same type to be grouped and manipulated
  together.


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   A visual representation of an array with 5 items.

         Subscript          Numbers
         1                  102
         2                  95
         3                  113
         4                  109
         5                  106


 The Numbers column shows the items entered.
 Subscript refers to the location or position of the
  items as previously mentioned.

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Accessing Data in an Array
 Each data item in the array is accessed by
  using the array name and a subscript – for
  example
 STUDENTS[2] is the second position in
  the array called STUDENTS and contains
  95. STUDENTS[3] is the third position
  and contains 113, etc.



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Declaring Arrays
   Arrays are declared in almost the same
    way as normal variables except that you
    have to say how many items you want in
    the array.
 VAR name of array: ARRAY [1..Maximum index]
  of data type;
 E.g.
  VAR Students: ARRAY [1..5] of integer;



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Initialising an Array
 Initialising and array gives ALL locations in
  the array a starting value, usually zero for
  an integer array or a space for character
  arrays.
 We use For loops to assign values.
 E.g.
  for i := 1 to 5 do
      Students [i] := 0;


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Assigning Values to the Array
 E.g
  for i := 1 to 5 do
  begin
      write (‘Enter the number of students
      in each year group’, i);
      readln (students[i]);
 End;




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Displaying the Contents of an Array
 E.g
  for i := 1 to 5 do
  begin
      write (‘Number of students in year’, i);
      writeln (students[i]);
      readln;
 End;




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Array Output
 Number of students in year 1: 102
 Number of students in year 2: 95
 Number of students in year 3: 113
 Number of students in year 4: 109
 Number of students in year 5: 106




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DOCUMENTING YOUR
PROGRAM
Why document your program?
 To help other users learn how to use
  your program properly.
 It helps programmers to:
 ◦ understand (or remember) what each section
   of code does
 ◦ see any assumptions that were made when
   writing
 ◦ be made aware of any peculiarities the
   program may have.

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Internal Documentation
   Documentation that is inside the source
    code of the program.
    ◦ Comments
      To facilitate internal documentation.
      It explains sections of the program – explaining
       each step along the way.
      Explain what a particular variable stores.

      NB – you don’t have to comment on everything.
      Use self explanatory variable names.

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Indentation
   This is done to make it easy for
    programmers to read the code.
   It helps to identify quickly which code is
    inside a particular construct and which isn’t.
   It is convention to indent:
    ◦   Code between a Begin … End block.
    ◦   Declarations in the Var and Const sections.
    ◦   The code in the IF and Else parts of IF statement.
    ◦   The code inside loops.

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White Space
   These are things like blank lines and
    spaces. These can be used to visually
    divide your code into sections, making it
    easier for programmers to identify which
    pieces of code are related to a particular
    task.




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External Documentation
 Documentation that is outside the
  program’s source code.
 Two main types are:
    ◦ User manuals
    ◦ Technical manuals




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User Manuals
   These tell the user how to use the
    program. They include:
    ◦ The hardware and software requirements of
      the program
    ◦ Instructions on how to install and start the
      program
    ◦ Explanations on what certain menu and
      toolbar items do
    ◦ Instructions on how to use certain features
    ◦ Troubleshooting guides.

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Technical Manuals
   These documents are meant for
    programmers and they explain how the
    program was written so that future
    modifications can be made. They may
    include:
    ◦   the algorithm
    ◦    a program listing
    ◦   Test cases and expected results
    ◦   Information about data file structures
    ◦   Known problems and workarounds.

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INFORMATION TAKEN FROM
 Information Technology for CSEC –
  Kelvin Skeete/Kyle Skeete
 Information Technology for CSEC
  Examinations – Howard Campbell and
  Alan Wood




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