Introduction to Modula-2

Practicum I
25
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
:=
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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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..
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Not for real valued intervals!!
Practicum I 24
Subrange
MODULE demo; VAR
a, b, c : [2..15];
BEGIN
a := 2; b := 8; c := a * b;
END demo.
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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
((13 DIV 4) = 3) = TRUE)
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Characters
Built-in type: CHAR Values: all the entries of the ASCII table Character literals have to be surrounded by single quotes :
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 =>
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Variables
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. Practicum I
Johan@info.vub.ac.be 4K216
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Practicum I
2
Modula 2 : why?
Why not Java, C, C++, C# ?! Learning
Looks like English Very structured Safe (easy?) Has all the modern features
RECORD
day : [1..31]; month : [1..12]; year : [1900..3000];
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Conversions
Numbers can not be mixed! CARDINAL ≠ REAL ≠ INTEGER Conversion is needed
TRUNC(real) => CARDINAL FLOAT(cardinal/integer) => REAL
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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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Practicum I
27
Record
Delcaration VAR name: RECORD field1 : type1;
. .
fieldN : typeN; END;
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Record
Declaration VAR
date :
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Program structure
Program is called a MODULE MODULE name;
FROM …. IMPORT …; CONST pi = 3.1459; TYPE euro = REAL;
BEGIN …. (* comment *) END name.
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Practicum I
22
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
Practicum I
8
Variables
Need to be declared:
Name Type
:
:=
VAR
X : CARDINAL;
Change the value (assignment)
X := 451;
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Variables
Single types:
Modulo using MOD
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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.
Working with numbers
INTEGER, CARDINAL and REAL Operators:
Sum using + Substract using – Multiply using *
Divide using
DIV for CARDINAL and INTEGER / for REAL
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Variables
Convenient way to:
Store values Manipulate values
Closely related to the variable concept used in mathematics!
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Practicum I
16
BOOLEAN algebra
MODULE demo; VAR a, b : INTEGER; c, d : BOOLEAN; BEGIN a := -3; b := 33; c := a > b; d := NOT c; END demo.
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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
All programming languages are equal!!
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Programming
Way to control a computer
Write down the program (editor) Translate using special program (compiler) Execution by the computer
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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Practicum I