Introduction to Modula-2

About This Presentation

Title:

Introduction to Modula-2

Description:

Pos number (3) : CARDINAL. Logical (TRUE/FALSE) : BOOLEAN. Why the name ' single types ' ... CHAR, REAL, BOOLEAN, CARDINAL, INTEGER. Self defined types: ... – PowerPoint PPT presentation

Number of Views:54

Avg rating:3.0/5.0

Slides: 92

Provided by: paralle

Category:

more less

Transcript and Presenter's Notes

Title: Introduction to Modula-2

1
Introduction to Modula-2

Johan Parent

2
Introduction

3 hours of total immersion into the programming
language Modula-2
We start from the very basics
Johan Parent
Johan_at_info.vub.ac.be
4K216

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!!

4
Programming

Way to control a computer
Write down the program (editor)
Translate using special program (compiler)
Execution by the computer
Compiler is clever
Translates in machine language
Reports mistakes

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

6
Program structure

Program is called a MODULE
MODULE name
FROM . IMPORT
CONST pi 3.1459
TYPE euro REAL
BEGIN
. ( comment )
END name.

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 .

8
Variables

Convenient way to
Store values
Manipulate values
Closely related to the variable concept used in
mathematics!

9
Variables

Need to be declared
Name
Type
VAR
X CARDINAL
Change the value (assignment)
X 451

10
Variables

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 ?

11
Variables
Keyword

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

12
Variables vs. Literals

Variable can have several values (not at one
time)
Literals are the values themselves
-3
123
z
4.342
TRUE

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

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.

15
Conversions

Numbers can not be mixed!
CARDINAL ? REAL ? INTEGER
Conversion is needed
TRUNC(real) gt CARDINAL
FLOAT(cardinal/integer) gt REAL

16
BOOLEAN algebra

Built-in type BOOLEAN
2 values TRUE, FALSE
OPERATORS
Logical AND, OR, NOT
Comparison lt, gt, , lt, gt, ltgt,

17
BOOLEAN algebra

MODULE demo
VAR
a, b INTEGER
c, d BOOLEAN
BEGIN
a -3
b 33
c a gt b
d NOT c
END demo.

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 gt
((13 DIV 4) 3) TRUE)

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

20
Constant

Keyword CONST
Usage identical to variable
Assignment is impossible
MODULE demo
CONST
pi 3.14159
code 9342

21
Constant

What is the type of the constant?
Can I use constant together with variables and
literals?
MODULE demo
CONST
Pi 3.14159
VAR
x, r REAL
BEGIN
r 5.0
x 2.0 Pi r
END demo.

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

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

24
Subrange

Equivalent to interval in mathematics x ? 0, 10
Again defined by you during the declaration
VAR
x 0 .. 10
c a .. d
Not for real valued intervals!!

..
25
Subrange

MODULE demo
VAR
a, b, c 2..15
BEGIN
a 2
b 8
c a b
END demo.

26
Structured types

Most types are built-in
CHAR, REAL, BOOLEAN, CARDINAL, INTEGER
Self defined types
Enumeration
Subrange
Until now simple types i.e. one variable one
value at a time

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

28
Record

Delcaration
VAR
name RECORD
field1 type1
.
.
fieldN typeN
END

29
Record

Declaration
VAR
date
RECORD
day 1..31
month 1..12
year 1900..3000
END

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

.
31
Record

VAR
point1, point2 RECORD
x, y REAL
END
BEGIN
point1.x 5.4
point1.y 2.3
point2 point1

Possible?
32
Array

Declaration
VAR
name ARRAY index_type OF type
Array contains one or more values depending on
the index_type
VAR
x ARRAY 1..10 OF REAL

33
Array

As with RECORDs with need more work to take one
value out of an array gt indexing
arrayposition
x8 4.3
x2 x8 1.2

34
Array

VAR
price ARRAY (junior, normal, maxi) OF REAL
BEGIN
pricenormal 2.25
pricemaxi 3.00

35
Array

VAR
taste ARRAY (bread, rice) OF (bad, normal,
good)
class ARRAY(john, alan) OF RECORD
age 0..200
weight REAL
END
BEGIN
tastebread good
tasterice good
classjohn.age 25

36
Array

Respect the index type
VAR x ARRAY 3..10 OF BOOLEAN
BEGIN
x1 TRUE
Arrays can not be copied in 1 step
VAR x, y ARRAY 3..10 OF BOOLEAN
BEGIN
x y

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

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)

39
N-dimensional array

Modula 2 arrays can have more than one index gt
more than one dimension
VAR
x ARRAY 1..5 OF
ARRAY 1..3 OF REAL
BEGIN
x52 5.0

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
x334
?
x3,3,4

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 1..3600
END

42
TYPE

MODULE demo
TYPE
Mp3 RECORD
name ARRAY 1..40 OF CHAR
length 1..3600
END
VAR
song Mp3
BEGIN
song.name "4 seasons winter"
END demo.

43
Control instruction

Normal program execution
Top down
Line by line
Statements that influence the execution of the
program
3 groups
Branches
Loops
Procedures

44
IF

Choose an alternative based on result of the
boolean expression
IF boolean_express1 THEN
statements
ELIF boolean_express2 THEN
statements
ELSE
statements
END

Optional
45
IF

MODULE demo
FROM IO IMPORT WrStr
VARx CARDINAL
BEGINx 3
IF (x lt 5) THEN
WrStr(" x is smaller than 5")
END
END demo.

46
IF

MODULE demo
FROM IO IMPORT WrStr
VARx CARDINAL
BEGINx 3
IF (x lt 5) THEN
WrStr(" x is smaller than 5")
ELSE
WrStr(" x is gt 5")
END
END demo.

47
IF

MODULE demo
FROM IO IMPORT WrStr
VARx CARDINAL
BEGINx 3
IF (x lt 5) THEN
WrStr(" x is smaller than 5")
ELIF (x gt 5) THEN
WrStr(" x is bigger than 5")
ELSE
WrStr("x equals 5")
END
END demo.

48
CASE

Choose one alternative using a label, no test!!
CASE expression OF
label1, label2 statements
label3 staments
ELSE
statements
END

Optional
49
CASE

MODULE demo
VARmenu (junior, normal, maxi)
price REAL
BEGINmenu junior
CASE menu OF
junior price 1.25
normal price 2.25
maxi price 3.00
ELSE
price -99.999
END
END demo.

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

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 lt stop if step is gt 0
FOR counter 10 TO 1 BY 1 DO

52
FOR

MODULE demo
VARi CARDINAL
sum CARDINAL
BEGINsum 0
FOR i 1 TO 20 DO
sum sum i
END
END demo.

53
WHILE

Loop as long as the condition return TRUE
WHILE boolean_express DO
statements
END

54
WHILE

MODULE demo
VARi CARDINAL
sum CARDINAL
BEGINsum 0
i 1
WHILE (i lt 20) DO
sum sum i
INC(i)
END
END demo.

55
REPEAT

Loop until condition becomes TRUE
REPEAT
statements
UNTIL boolean_express
Always at least 1 loop!!

!
56
REPEAT

MODULE demo
VARi CARDINAL
sum CARDINAL
BEGINsum 0
i 1
REPEAT
INC(sum, i)
INC(i)
UNTIL (i gt 20)
END demo.

57
LOOP

The most basic loop
No counter
No stop condition
LOOP
statements
END
Use EXIT to escape the loop

58
LOOP

MODULE demo
VARi CARDINAL
sum CARDINAL
BEGINsum 0
i 1
LOOP
INC(sum, i)
INC(i)
IF (i gt20) THEN
EXIT
END
END
END demo.

59
PROCEDURE

Frequently used statement can be seperated and
put into a procedure gt 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

60
PROCEDURE

2 aspect
Definition which computation are being
performed, which inputs and outputs
Procedure call correct use of the procedure in
the program

61
PROCEDURE

Definition
Name
Number of parameters
Type of each parameter
Does the procedure return a value
Type of the value return value

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, )

63
PROCEDURE

PROCEDURE name (arg1 TYPE1 arg2 TYPE2 )
( Declarations just like for a MODULE )
CONST .
TYPE .
VAR .
BEGIN
( The code comes here )
.
END name

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

65
PROCEDURE

Keyword RETURN
Used to
Jump out of the procedure
Return a value and jump out of the procedure

66
PROCEDURE

MODULE demo
PROCEDURE add ( a , b CARDINAL) CARDINAL
BEGIN
RETURN ab
END add
VAR
sum CARDINAL
BEGIN
sum add( 4, 2)
END demo.

67
PROCEDURE

MODULE demo
FROM IO IMPORT WrStr
PROCEDURE odd( x REAL)
BEGIN
IF (x lt 4.5) THEN
RETURN
END
WrStr("x is not lt 4.5")
END odd
BEGIN
odd(3.)
odd(5.5)
END demo.

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

69
SCOPING

Global vs. Local delcarations
Global defined in the main part of the module
Local defined in procedures
Top down
Modules
Hiding phenomenon

70
SCOPING

MODULE demo
VAR
v CHAR
CONST
c 0
PROCEDURE p
VAR
z CHAR
( v, c, z, p )
END p
PROCEDURE q
CONST
v 6
( v, c, p, q )
END q
BEGIN
END demo.

71
VAL/VAR

MODULE demo
PROCEDURE reset(c REAL)
BEGIN
c 0.0
END zero
VAR
r REAL
BEGIN
r 99.99
reset(r)
END demo.

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!

73
VAL/VAR

MODULE demo
PROCEDURE zero (VAR c REAL)
BEGIN
c 0.0
END zero
VARx REAL
BEGINreset(x)
END demo.
Works as was expected!

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?

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)

76
OPEN ARRAY

Not convenient
Declaration
Not flexible in terms of the size arrays passed
Solution open array parameters

77
OPEN ARRAY

Advantage
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

78
OPEN ARRAY

All arrays start at index 0
HIGH returns the index of the last position

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 ai
END
RETURN tot
END sum

80
OPEN ARRAY

VAR
firstAr ARRAY 30..35 OF CHAR
secondAr ARRAY 1..10 OF CHAR
PROCEDURE myWrStr( str ARRAY OF CHAR)
VAR
i CARDINAL
BEGIN
FOR i 0 TO HIGH(str) DO
WrChar(stri)
END
END myWrStr

81
RECURSION

f(x) g(f(x))
Example factorial
fac(n) n fac(n-1) (ngt1)
fac(n) 1 (n lt1)
PROCEDURE fac(n CARDINAL) CARDINAL
BEGIN
IF (n lt 1) THEN
RETURN n fac(n-1)
ELSE
RETURN 1
END
END fac

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.

83
POINTER

Adresses of memory locations are not interesting
The value stored at the memory location is gt
dereference
VAR
a POINTER TO INTEGER
BEGIN
a -3

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

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.

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

87
POINTERS

Linked list
TYPE
LinkPtr POINTER TO LinkRc
LinkRc RECORD
data REAL
next LinkPtr
END
Tree
TYPE
NodePtr POINTER TO NodeRc
NodeRc RECORD
data REAL
left, right NodePtr
END

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.

89
Pointers memory allocation