1. Introduction to Modula-2
Johan Parent
20/11/2018
Practicum I

2. Introduction
3 hours of total immersion into the programming language Modula-2
We start from the very basics
Johan Parent
4K216
20/11/2018
Practicum I

3. Modula 2 : why? Why not Java, C, C++, C# ?! Learning
Looks like English
Very structured
Safe (easy?)
Has all the modern features
All programming languages are equal!!
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4. Programming Way to control a computer Compiler is clever
Write down the program (editor)
Translate using special program (compiler)
Execution by the computer
Compiler is clever
Translates in machine language
Reports mistakes
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5. Modula 2 Has keywords: words from the language
Always in CAPITAL letters!
DO, FOR, IF, WHILE, END, THEN, UNTIL, LOOP, ELSE, RECORD, ARRAY, OF, TYPE, CONST, IMPORT, FROM, SET, VAR, WITH, CASE, MODULE, REAL, INTEGER…
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6. Program structure Program is called a MODULE MODULE name; BEGIN
FROM …. IMPORT …;
CONST pi = ;
TYPE euro = REAL;
BEGIN
…. (* comment *)
END name.
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7. Syntax Keywords completely in CAPITAL letters Spaces do not matter
White lines do not matter
Signal the end of a command with semi colon ;
End of program with a period .
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8. Variables Convenient way to:
Store values
Manipulate values
Closely related to the variable concept used in mathematics!
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9. : := Variables := Need to be declared: VAR
Name
Type
VAR
X : CARDINAL;
Change the value (assignment)
X := 451; : := :=
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10. Variables Single types: Why the name « single types »?
Characters (‘a’) : CHAR
Integers (-40) : INTEGER
Reals (2.7) : REAL
Pos number (3) : CARDINAL
Logical (TRUE/FALSE) : BOOLEAN
Why the name « single types »?
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11. := Variables Keyword Declarations Assignments MODULE demo; VAR
a : CHAR;
b : INTEGER;
c : REAL;
d : CARDINAL;
e : BOOLEAN:
BEGIN
a := ‘z’;
b := -345;
c := 0.35E-3;
d := 345;
e := FALSE;
END demo.
Declarations
Assignments :=
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12. Variables vs. Literals
Variable can have several values (not at one time)
Literals are the values themselves: -3
123
‘z’
4.342
TRUE
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13. Working with numbers INTEGER, CARDINAL and REAL Operators:
Sum using +
Substract using –
Multiply using *
Divide using
DIV for CARDINAL and INTEGER
/ for REAL
Modulo using MOD
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14. Working with numbers MODULE demo; VAR a, b : INTEGER; d : REAL;
f : CARDINAL;
BEGIN
a := -3;
b := a * 2;
d := 5.0 / 3.0;
f := 5 DIV 3;
END demo.
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15. Conversions Numbers can not be mixed! CARDINAL  REAL  INTEGER
Conversion is needed
TRUNC(real) => CARDINAL
FLOAT(cardinal/integer) => REAL
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16. BOOLEAN algebra Built-in type: BOOLEAN 2 values: TRUE, FALSE
OPERATORS:
Logical : AND, OR, NOT
Comparison : <, >, =, <=, >=, <>, #
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17. BOOLEAN algebra MODULE demo; VAR a, b : INTEGER; c, d : BOOLEAN; BEGIN
c := a > b;
d := NOT c;
END demo.
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18. Precedence rules Define which operator is evaluated first.
Multiplication, division, sum, …
13 DIV 4 = 3 = TRUE
Is this correct?
What is the value then?
Use brackets to ensure the order of evalution =>
((13 DIV 4) = 3) = TRUE)
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19. Characters Built-in type: CHAR
Values: all the entries of the ASCII table
Character literals have to be surrounded by single quotes :
‘z’
How can we represent text… later
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20. = Constant Keyword CONST Usage identical to variable
Assignment is impossible
MODULE demo;
CONST
pi = ;
code = 9342; =
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21. Constant What is the type of the constant?
Can I use constant together with variables and literals?
MODULE demo;
CONST
Pi = ;
VAR
x, r : REAL;
BEGIN
r := 5.0;
x := 2.0 * Pi * r;
END demo.
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22. Enumeration First flexible type: defined by you
Explicit definition of the possible values of a variable during the declaration
VAR
x : ( value1, value2, …, valueN);
Variable x can only have the values listed between brackets
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23. Enumeration MODULE demo; VAR
continent : ( europe, asia, africa, antartica, america);
BEGIN
continent := antartica;
END demo.
Danger: values of enumeration can not be used as name for other constants or variable
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24. .. Subrange Equivalent to interval in mathematics x  [0, 10]
Again defined by you during the declaration
VAR
x : [ ];
c : [’a’ .. ‘d’];
Not for real valued intervals!! ..
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25. Subrange MODULE demo; VAR BEGIN END demo. a, b, c : [2..15]; a := 2;
c := a * b;
END demo.
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26. Structured types Most types are built-in: Self defined types:
CHAR, REAL, BOOLEAN, CARDINAL, INTEGER
Self defined types:
Enumeration
Subrange
Until now simple types i.e. one variable = one value at a time
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27. Structured types Store more than one value per variable
RECORD: possible to combine different types in one variable
ARRAY: represent several values of the same type in one variable
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28. Record Delcaration VAR name: RECORD field1 : type1; fieldN : typeN;.
fieldN : typeN;
END;
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29. Record Declaration VAR date : RECORD END; day : [1..31];
month : [1..12];
year : [ ];
END;
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30. Record
Since a record contains several values we use the field name to access the value:
record.field_name
date.day := 8;
date.month := 10;
date.year := 2002; .
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31. Record Possible? VAR point1, point2 : RECORD x, y : REAL; END; BEGIN
point1.x := 5.4;
point1.y := 2.3;
point2 := point1;
Possible?
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32. Array Declaration VAR name : ARRAY index_type OF type;
Array contains one or more values depending on the index_type
x : ARRAY [1..10] OF REAL;
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33. Array
As with RECORDs with need more work to take one value out of an array => indexing
array[position]
x[8] := 4.3;
x[2] := x[8] + 1.2;
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34. Array VAR price : ARRAY (junior, normal, maxi) OF REAL; BEGIN
price[normal] := 2.25;
price[maxi] := 3.00;
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35. Array VAR taste : ARRAY (bread, rice) OF (bad, normal, good);
class : ARRAY(john, alan) OF RECORD
age : [0..200];
weight : REAL;
END;
BEGIN
taste[bread] := good;
taste[rice] := good;
class[john].age := 25;
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36. Array Respect the index type Arrays can not be copied in 1 step
VAR x : ARRAY [3..10] OF BOOLEAN;
BEGIN
x[1] := TRUE;
Arrays can not be copied in 1 step
VAR x, y : ARRAY [3..10] OF BOOLEAN;
x := y;
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37. Strings Character can be represented using the built-in CHAR type
One character at a time
There is no seperate type in Modula 2 for strings of characters!
VAR
text : ARRAY [1..30] OF CHAR;
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38. Writing on screen
Writing on the computer screen can be done using the following procedures
String : WrStr(x);
INTEGER: WrInt(x, 0);
REAL: WrReal(x, 7, 0);
CARDINAL: WrCard(x, 0);
BOOLEAN: WrBool(x);
CHAR: WrChar(x);
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39. N-dimensional array
Modula 2 arrays can have more than one index => more than one dimension
VAR
x : ARRAY [1..5] OF
ARRAY [1..3] OF REAL;
BEGIN
x[5][2] := 5.0;
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40. N-dimensional array Declaration short hand for n-dim arrays:
x : ARRAY type1 OF ARRAY type2 OF ARRAY type3 OF type 
x : ARRAY type1,type2,type3 OF type
Indexing short hand for n-dim arrays:
x[3][3][4]
x[3,3,4]
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41. = TYPE Modula 2 comes with built-in types
You can also define new types
MODULE demo;
TYPE
Mp3 = RECORD
name : ARRAY [1..40] OF
CHAR;
length : [ ];
END; =
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42. TYPE MODULE demo; TYPE VAR song : Mp3; BEGIN
Mp3 = RECORD
name : ARRAY [1..40] OF CHAR;
length : [ ];
END;
VAR
song : Mp3;
BEGIN
song.name := "4 seasons – winter";
END demo.
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43. Control instruction Normal program execution
Top down
Line by line
Statements that influence the execution of the program
3 groups:
Branches
Loops
Procedures
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44. IF
Choose an alternative based on result of the boolean expression
IF boolean_express1 THEN
statements…
ELIF boolean_express2 THEN
ELSE
END;
Optional
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45. IF MODULE demo; FROM IO IMPORT WrStr; VAR x : CARDINAL; BEGIN x := 3;
IF (x < 5) THEN
WrStr(" x is smaller than 5");
END;
END demo.
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46. IF MODULE demo; FROM IO IMPORT WrStr; VAR x : CARDINAL; BEGIN x := 3;
IF (x < 5) THEN
WrStr(" x is smaller than 5");
ELSE
WrStr(" x is >= 5");
END;
END demo.
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47. IF MODULE demo; FROM IO IMPORT WrStr; VAR x : CARDINAL; BEGIN x := 3;
IF (x < 5) THEN
WrStr(" x is smaller than 5");
ELIF (x > 5) THEN
WrStr(" x is bigger than 5");
ELSE
WrStr("x equals 5");
END;
END demo.
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48. CASE Optional Choose one alternative using a label, no test!!
CASE expression OF
label1, label2 : statements… |
label3 : staments… |
ELSE
statements…
END;
Optional
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49. CASE MODULE demo; VAR menu : (junior, normal, maxi); price : REAL;
BEGIN menu := junior;
CASE menu OF
junior : price := 1.25; |
normal : price := 2.25; |
maxi : price := 3.00; |
ELSE
price := ;
END;
END demo.
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50. FOR Loop a fixed number of times using a counter variable
FOR counter := start TO stop BY step DO
statements…
END;
BY step is optional
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51. FOR The counter will be changed automatically by the FOR loop
If BY step is omitted a default step of 1 is used
start < stop if step is > 0
FOR counter := 10 TO 1 BY –1 DO
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52. FOR MODULE demo; VAR i : CARDINAL; sum : CARDINAL; BEGIN sum := 0;
FOR i := 1 TO 20 DO
sum := sum + i;
END;
END demo.
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53. WHILE Loop as long as the condition return TRUE
WHILE boolean_express DO
statements…
END;
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54. WHILE MODULE demo; VAR i : CARDINAL; sum : CARDINAL; BEGIN sum := 0;
WHILE (i <= 20) DO
sum := sum + i;
INC(i);
END;
END demo.
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55. REPEAT ! Loop until condition becomes TRUE REPEAT statements…
UNTIL boolean_express;
Always at least 1 loop!! !
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56. REPEAT MODULE demo; VAR i : CARDINAL; sum : CARDINAL; BEGIN sum := 0;
INC(sum, i);
INC(i);
UNTIL (i > 20);
END demo.
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57. LOOP The most basic loop LOOP statements… END;
No counter
No stop condition
LOOP
statements…
END;
Use EXIT to escape the loop
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58. LOOP MODULE demo; VAR i : CARDINAL; sum : CARDINAL; BEGIN sum := 0;
INC(sum, i);
INC(i);
IF (i >20) THEN
EXIT;
END;
END demo.
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59. PROCEDURE
Frequently used statement can be seperated and put into a procedure => code reuse
In Modula 2 a procedure a is little program on its own
Difference with program:
Has a name
Possibly some input values
Possibly some output values
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60. PROCEDURE
2 aspect:
Definition: which computation are being performed, which inputs and outputs
Procedure call: correct use of the procedure in the program
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61. PROCEDURE Definition Name Number of parameters Type of each parameter
Does the procedure return a value:
Type of the value return value
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62. PROCEDURE 2 types of procedures
Procedures that compute a value and return it
Function procedure
Procedures that have a side effect (write to disk, reset a value, …)
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63. PROCEDURE PROCEDURE name (arg1 : TYPE1; arg2 : TYPE2; …);
(* Declarations just like for a MODULE *)
CONST ….
TYPE ….
VAR ….
BEGIN
(* The code comes here *) ….
END name;
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64. PROCEDURE PROCEDURE name (arg1 : TYPE1; arg2 : TYPE2; …) : TYPE6;
(* Declarations just like for a MODULE *)
CONST ….
TYPE ….
VAR ….
BEGIN
(* The code comes here *) ….
END name;
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65. PROCEDURE Keyword RETURN Used to: Jump out of the procedure
Return a value and jump out of the procedure
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66. PROCEDURE MODULE demo; PROCEDURE add ( a , b :CARDINAL) : CARDINAL;
BEGIN
RETURN a+b;
END add;
VAR
sum : CARDINAL;
sum := add( 4, 2);
END demo.
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67. PROCEDURE MODULE demo; FROM IO IMPORT WrStr; PROCEDURE odd( x : REAL);
BEGIN
IF (x < 4.5) THEN
RETURN;
END;
WrStr("x is not < 4.5");
END odd;
odd(3.);
odd(5.5);
END demo.
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68. SCOPING Procedure are little programs on their own:
Own declarations : types, constants, variables and even other procedures
Which variables, constants and procedure can one use inside procedure?
 Scoping rules
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69. SCOPING Global vs. Local delcarations Top down Modules
Global ~ defined in the main part of the module
Local ~ defined in procedures
Top down
Modules
Hiding phenomenon
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70. SCOPING MODULE demo; VAR v : CHAR; CONST c = 0; PROCEDURE p; z : CHAR;
…(* v, c, z, p *)
END p;
PROCEDURE q;
v = 6;
…(* v, c, p, q *)
END q;
BEGIN
END demo.
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71. VAL/VAR MODULE demo; PROCEDURE reset(c : REAL); BEGIN c := 0.0;
END zero;
VAR
r : REAL;
r := 99.99;
reset(r);
END demo.
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72. VAL/VAR The previous program does not work, why?
Parameter passing mechanism:
Makes copy of the arguments (value parameter VAL)
Execute the procedure code
VAR parameters are not copied!
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73. VAL/VAR MODULE demo; PROCEDURE zero (VAR c : REAL); BEGIN c := 0.0;
END zero;
VAR x : REAL
BEGIN reset(x);
END demo.
Works as was expected!
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74. VAL/VAR
VAR parameters are not copied but a reference to the original variable is passed instead
VAR parameter required variables during the procedure call, why?
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75. OPEN ARRAY Passing arrays as parameters is not easy?!
PROCEDURE p(a : ARRAY [1..10] OF REAL);
To make it work:
Declare a new type first
Use type in declaration
TYPE Arr = ARRAY [1..10] OF REAL;
PROCEDURE p(a :Arr);
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76. OPEN ARRAY Not convenient: Solution : open array parameters
Declaration
Not flexible in terms of the size arrays passed
Solution : open array parameters
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77. OPEN ARRAY Advantage: ARRAY OF type PROCEDURE p(a: ARRAY OF REAL);
No need to declare a type
Flexible in terms of size
ARRAY OF type
PROCEDURE p(a: ARRAY OF REAL);
Procedure p can accept arrays of any size
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78. OPEN ARRAY All arrays start at index 0
HIGH returns the index of the last position
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79. OPEN ARRAY PROCEDURE sum(a : ARRAY OF REAL): REAL; VAR i : CARDINAL;
tot : REAL;
BEGIN
tot := 0.0;
FOR i := 0 TO HIGH(a) DO
tot := tot + a[i];
END;
RETURN tot;
END sum;
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80. OPEN ARRAY VAR PROCEDURE myWrStr( str : ARRAY OF CHAR); BEGIN
firstAr : ARRAY [30..35] OF CHAR;
secondAr : ARRAY [1..10] OF CHAR;
PROCEDURE myWrStr( str : ARRAY OF CHAR);
i : CARDINAL;
BEGIN
FOR i := 0 TO HIGH(str) DO
WrChar(str[i]);
END;
END myWrStr;
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81. RECURSION f(x) = g(f(x)) Example: factorial
fac(n) := n * fac(n-1); (n>1)
fac(n) := 1; (n <=1)
PROCEDURE fac(n : CARDINAL): CARDINAL;
BEGIN
IF (n <= 1) THEN
RETURN n * fac(n-1);
ELSE
RETURN 1;
END;
END fac;
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82. POINTERS Pointers are variables containing memory addresses
Declaration
VAR
a : POINTER TO INTEGER;
a contains the address of a memory block which contains an INTEGER value.
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83. POINTER Adresses of memory locations are not interesting
The value stored at the memory location is => dereference
VAR
a : POINTER TO INTEGER;
BEGIN …
a^ := -3;
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84. POINTERS
Pointers are used for dynamic memory allocation! But pointers do not do the allocation.
There are special Modula2 procedures to work with memory:
IMPOR from the SYSTEM module
NEW used to ask memory
DISPOSE to release the memory you got
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85. Pointers: DEMO MODULE memory; FROM SYSTEM IMPORT NEW, DISPOSE;
FROM IO IMPORT WrStr, WrLn;
VAR
i_ptr : POINTER TO INTEGER;
i : INTEGER;
BEGIN
WrStr("Start"); WrLn;
(* We reserve some memory for 1 integer *)
NEW(i_ptr);
i_ptr^ := 9;
i := i_ptr^;
(* We tell the computer we will not use this memory anymore *)
DISPOSE(i_ptr);
WrStr("Stop"); WrLn;
END memory.
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86. POINTERS
Dynamic memory allocation is the main reason for using pointers.
Special structures are used to do this:
Linked lists
Trees
These are RECURSIVE DATA STRUCTURES
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87. POINTERS Linked list Tree data : REAL; next : LinkPtr; END;
TYPE
LinkPtr = POINTER TO LinkRc;
LinkRc = RECORD
data : REAL;
next : LinkPtr;
END;
Tree
NodePtr = POINTER TO NodeRc;
NodeRc = RECORD
left, right : NodePtr;
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88. POINTERS These structures can be contected to each other!
Thereby allowing us to build big structures where each element contains data!
In this way we can use as much memory as we need! And this without knowing in advance how much memory we would need.
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89. Pointers: memory allocation
We can thus ask memory explicitly using NEW but:
the computer may run out of memory!!!
We can know that when NEW returns NIL as value for our pointer
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90. Pointers: memory allocation (2)
In order to save memory we have to recycle the memory we do not use anymore
we have to deallocate the memory using DISPOSE
we have to be careful and avoid using memory we deallocated!!
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91. MODULES
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