🔷 Introduction to the Pascal Programming Language

1 What is Pascal?

Pascal is a procedural programming language designed by Niklaus Wirth and first published in 1970. It was created as a small, efficient language intended to encourage good programming practices through structured programming and data structuring. Pascal was named in honor of the French mathematician and philosopher Blaise Pascal.

Pascal was originally developed for teaching programming as a systematic discipline, but it quickly became popular for professional software development due to its clarity and strong typing. Key features include:

• Strong typing – all variables must be declared with specific types
• Structured programming – encourages use of procedures and functions
• Clear, readable syntax – uses English-like keywords
• Block structure – uses begin and end to group statements
• Rich data types – supports records, sets, enumerated types, and pointers

Pascal source files typically use the .pas or .pp extension. We will use the Free Pascal Compiler (FPC), which is a mature, open-source compiler supporting multiple platforms.

2 Your First Pascal Program

Create a file called hello.pas with the following content:

program Hello;

begin
    writeln('Hello, World!');
end.

Compiling and Running

Open a terminal and navigate to the directory containing your file. Compile and run with:

fpc hello.pas
./hello

You should see Hello, World! printed to the console.

Understanding the Code

3 Variable Types

Pascal is a statically typed language, meaning you must declare all variables and their types before using them. Variables are declared in a var section before the begin keyword.

Integer Types

Unsigned types are also available: byte, word, longword, qword.

Floating-Point Types

Other Fundamental Types

Variable Declaration Example

program Variables;

var
    { Each variable must be declared with its type }
    age: integer;
    gpa: real;
    name: string;
    enrolled: boolean;
    grade: char;

begin
    { Assign values to variables }
    age := 25;
    gpa := 3.85;
    name := 'Alice';
    enrolled := true;
    grade := 'A';

    { Output the values }
    writeln('Name: ', name);
    writeln('Age: ', age);
    writeln('GPA: ', gpa:0:2);
    writeln('Enrolled: ', enrolled);
    writeln('Grade: ', grade);
end.

4 Console Output

Pascal provides two primary output procedures:

program OutputDemo;

var
    x: integer;
    price: real;
    item: string;

begin
    x := 42;
    price := 19.99;
    item := 'widget';

    { Basic output }
    writeln('The answer is ', x);

    { Multiple values }
    writeln('Item: ', item, ', Price: , price:0:2);

    { Using write (no newline) }
    write('Processing');
    write('...');
    writeln(' Done!');

    { Formatted numeric output }
    writeln('The ', item, ' costs , price:0:2);
    writeln('Padded integer: ', x:8);
    writeln('Padded real: ', price:10:2);
end.

Formatting Output

Pascal uses field width specifiers to format output:

5 Console Input

Pascal provides two primary input procedures:

program InputDemo;

var
    name: string;
    age: integer;
    gpa: real;

begin
    { Reading a string }
    write('Enter your name: ');
    readln(name);

    { Reading an integer }
    write('Enter your age: ');
    readln(age);

    { Reading a real number }
    write('Enter your GPA: ');
    readln(gpa);

    { Display the results }
    writeln;
    writeln('Hello, ', name, '!');
    writeln('Age: ', age);
    writeln('GPA: ', gpa:0:2);
end.

Reading Multiple Values

You can read multiple values on a single line:

program MultipleInputs;

var
    x, y, z: integer;

begin
    write('Enter three integers separated by spaces: ');
    readln(x, y, z);
    writeln('Sum: ', x + y + z);
end.

6 Assignment Statements and Operators

Basic Assignment

The assignment operator := stores a value in a variable:

var
    x, y: integer;
    
begin
    x := 10;      { assign 10 to x }
    x := 20;      { assign new value }
    y := x;       { copy value from x to y }
end.

Arithmetic Operators

Increment and Decrement

Free Pascal provides Inc and Dec procedures:

var
    count: integer;

begin
    count := 5;
    Inc(count);      { count is now 6 }
    Inc(count, 3);   { count is now 9 (add 3) }
    Dec(count);      { count is now 8 }
    Dec(count, 2);   { count is now 6 (subtract 2) }
end.

Integer vs. Real Division

program DivisionDemo;

var
    a, b: integer;

begin
    a := 7;
    b := 2;

    writeln('7 / 2   = ', 7 / 2:0:4);     { prints 3.5000 (real division) }
    writeln('7 div 2 = ', 7 div 2);        { prints 3 (integer division) }
    writeln('7 mod 2 = ', 7 mod 2);        { prints 1 (remainder) }
    writeln('a / b   = ', a / b:0:4);     { prints 3.5000 }
    writeln('a div b = ', a div b);        { prints 3 }
end.

7 Control Structures

The if Statement

Execute code conditionally based on a boolean expression:

program GradeCalculator;

var
    score: integer;

begin
    write('Enter your score (0-100): ');
    readln(score);

    if score >= 90 then
        writeln('Grade: A')
    else if score >= 80 then
        writeln('Grade: B')
    else if score >= 70 then
        writeln('Grade: C')
    else if score >= 60 then
        writeln('Grade: D')
    else
        writeln('Grade: F');
end.

Using begin-end for Multiple Statements

if score >= 90 then
begin
    writeln('Grade: A');
    writeln('Excellent work!');
end
else
begin
    writeln('Grade: Below A');
    writeln('Keep studying!');
end;

Comparison Operators

Logical Operators

program DrivingCheck;

var
    age: integer;
    response: string;
    hasLicense: boolean;

begin
    write('Enter your age: ');
    readln(age);

    write('Do you have a license? (yes/no): ');
    readln(response);

    hasLicense := (response = 'yes');

    if (age >= 16) and hasLicense then
        writeln('You can drive.')
    else if (age >= 16) and (not hasLicense) then
        writeln('You''re old enough but need a license.')
    else
        writeln('You''re too young to drive.');
end.

The case Statement

Use case when comparing a single value against multiple options:

program DayOfWeek;

var
    day: integer;

begin
    write('Enter a day number (1-7): ');
    readln(day);

    case day of
        1: writeln('Monday');
        2: writeln('Tuesday');
        3: writeln('Wednesday');
        4: writeln('Thursday');
        5: writeln('Friday');
        6, 7: writeln('Weekend!');
    else
        writeln('Invalid day number');
    end;
end.

8 Looping Structures

The while Loop

Repeats a block of code as long as a condition is true. The condition is checked before each iteration:

program WhileDemo;

var
    i: integer;

begin
    { Count from 1 to 5 }
    i := 1;
    while i <= 5 do
    begin
        writeln('Count: ', i);
        Inc(i);
    end;
end.

The repeat-until Loop

Similar to while, but the condition is checked after each iteration. The loop body always executes at least once:

program GuessingGame;

var
    guess: integer;
    secretNumber: integer;

begin
    secretNumber := 7;

    repeat
        write('Guess a number (1-10): ');
        readln(guess);

        if guess < secretNumber then
            writeln('Too low!')
        else if guess > secretNumber then
            writeln('Too high!');

    until guess = secretNumber;

    writeln('Correct!');
end.

The for Loop

Best for counting a specific number of iterations:

program ForDemo;

var
    i: integer;

begin
    { Basic counting up }
    for i := 1 to 5 do
        writeln('Iteration ', i);

    writeln;

    { Counting down }
    writeln('Countdown:');
    for i := 10 downto 0 do
        writeln(i);
end.

Nested Loops

program MultiplicationTable;

var
    row, col: integer;

begin
    writeln('Multiplication Table (1-5):');
    writeln;

    { Print header row }
    write('     ');
    for col := 1 to 5 do
        write(col:4);
    writeln;
    writeln('   --------------------');

    { Print table rows }
    for row := 1 to 5 do
    begin
        write(row:2, ' |');
        for col := 1 to 5 do
            write((row * col):4);
        writeln;
    end;
end.

Loop Control: break and continue

program LoopControl;

var
    i: integer;

begin
    { break - exit the loop early }
    writeln('Finding first multiple of 7:');
    for i := 1 to 100 do
    begin
        if i mod 7 = 0 then
        begin
            writeln('Found: ', i);
            break;  { exit loop immediately }
        end;
    end;

    writeln;

    { continue - skip to next iteration }
    writeln('Numbers 1-10, skipping multiples of 3:');
    for i := 1 to 10 do
    begin
        if i mod 3 = 0 then
            continue;  { skip this iteration }
        writeln(i);
    end;
end.

9 Complete Example: Temperature Converter

This program demonstrates input, output, control structures, and loops working together:

program TemperatureConverter;

var
    choice: integer;
    celsius, fahrenheit: real;
    running: boolean;

begin
    writeln('=== Temperature Converter ===');
    writeln('1. Celsius to Fahrenheit');
    writeln('2. Fahrenheit to Celsius');
    writeln('3. Quit');

    running := true;

    while running do
    begin
        writeln;
        write('Enter choice (1-3): ');
        readln(choice);

        case choice of
            1:
            begin
                write('Enter temperature in Celsius: ');
                readln(celsius);
                fahrenheit := (celsius * 9.0 / 5.0) + 32.0;
                writeln(celsius:0:2, '°C = ', fahrenheit:0:2, '°F');
            end;

            2:
            begin
                write('Enter temperature in Fahrenheit: ');
                readln(fahrenheit);
                celsius := (fahrenheit - 32.0) * 5.0 / 9.0;
                writeln(fahrenheit:0:2, '°F = ', celsius:0:2, '°C');
            end;

            3:
            begin
                writeln('Goodbye!');
                running := false;
            end;
        else
            writeln('Invalid choice. Please enter 1, 2, or 3.');
        end;
    end;
end.