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March 03, 2025 00:21
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Modula-2 Cheatsheet

A concise cheat sheet for the Modula-2 programming language, covering syntax, data types, control structures, and module structure.

Basic Syntax and Structure

Program Structure

Modula-2 programs are organized into modules. A main module is the entry point.

MODULE MainModule;
(* Declarations *)
BEGIN
(* Statements *)
END MainModule.

Comments are enclosed in (* and *).

(* This is a comment *)

Identifiers are case-sensitive and must start with a letter.

ValidIdentifier (* Correct *)
invalidIdentifier (* Incorrect - Case-sensitive *)

Data Types

INTEGER

Whole numbers (e.g., -1, 0, 100).

REAL

Floating-point numbers (e.g., 3.14, -0.5).

BOOLEAN

Logical values: TRUE or FALSE.

CHAR

Single characters (e.g., 'A', 'z', '9').

CARDINAL

Non-negative integers (e.g., 0, 1, 100).

STRING

Array of characters (e.g., 'Hello')

Variable Declarations

Variables must be declared before use. Use VAR keyword.

VAR
  age: INTEGER;
  name: ARRAY [0..31] OF CHAR;
  isReady: BOOLEAN;

Constants are declared using the CONST keyword.

CONST
  Pi = 3.14159;
  MaxSize = 100;

Control Structures

Conditional Statements 

IF condition THEN
  (* Statements *)
ELSIF anotherCondition THEN
  (* Statements *)
ELSE
  (* Statements *)
END;

Example:

IF age >= 18 THEN
  WriteLn("Adult");
ELSE
  WriteLn("Minor");
END;

Looping Statements

FOR Loop

 
FOR i := start TO end BY step DO
  (* Statements *)
END;

Example:

FOR i := 1 TO 10 DO
  WriteInt(i, 0);
END;

WHILE Loop

 
WHILE condition DO
  (* Statements *)
END;

Example:

WHILE age < 25 DO
  age := age + 1;
END;

REPEAT Loop

 
REPEAT
  (* Statements *)
UNTIL condition;

Example:

REPEAT
  ReadInt(input);
UNTIL input > 0;

CASE Statement 

CASE expression OF
  value1: (* Statements *)
| value2: (* Statements *)
ELSE
  (* Statements *)
END;

Example:

CASE grade OF
  'A': WriteLn("Excellent");
| 'B': WriteLn("Good");
ELSE
  WriteLn("Needs Improvement");
END;

Modules and Procedures

Module Structure

A module consists of a definition part and an implementation part.

Definition Module:

DEFINITION MODULE MyModule;
(* Exported declarations *)
END MyModule.

Implementation Module:

IMPLEMENTATION MODULE MyModule;
(* Implementation details *)
END MyModule.

Importing modules:

MODULE MainModule;
IMPORT MyModule;
BEGIN
  (* Use MyModule's exported procedures *)
END MainModule.

Procedure Declaration

Procedure declaration syntax:

 
PROCEDURE MyProcedure(param1: DataType; ...): ReturnType;
VAR
  (* Local variables *)
BEGIN
  (* Statements *)
  RETURN returnValue;
END MyProcedure;

Example:

 
PROCEDURE Add(a, b: INTEGER): INTEGER;
BEGIN
  RETURN a + b;
END Add;

Function Procedures

Procedures can also act as functions, returning a value.

PROCEDURE Square(x: INTEGER): INTEGER;
BEGIN
  RETURN x * x;
END Square;

Advanced Features

Pointers

Pointers are used to store the address of a variable.

TYPE
  IntPtr = POINTER TO INTEGER;
VAR
  ptr: IntPtr;
  num: INTEGER;

NEW(ptr);
ptr^ := 10;
num := ptr^;

NEW(ptr) allocates memory, and ptr^ dereferences the pointer.

Arrays

Array declaration:

 
VAR
  myArray: ARRAY [0..9] OF INTEGER;

Accessing array elements:

 
myArray[0] := 100;
WriteInt(myArray[0], 0);

Records

Records are used to group related data items.

TYPE
  Person = RECORD
    name: ARRAY [0..31] OF CHAR;
    age: INTEGER;
  END;
VAR
  person1: Person;

person1.name := "John";
person1.age := 30;