Category : Modula II Source Code
Archive   : M2TUTOR.ZIP
Filename : CHAP10.TXT

 
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Chapter 10

SCALARS, SUBRANGES, & SETS



PREREQUISITES FOR THIS MATERIAL
______________________________________________________________

In order to understand the material in this chapter, you
should have a fairly good understanding of most of the
material in Part I of this tutorial.

A scalar, also called an enumerated type, ===============
is a list of values which a variable of ENTYPES.MOD
that type may assume. Examine the file ===============
named ENTYPES.MOD for an example of some
scalars. The first TYPE declaration defines Days as being a
type which, if used to define a variable, the variable can be
assigned any one of the seven values listed. Since, within
the VAR declaration, Day is assigned the type of Days, then
Day is a variable which can assume any one of seven different
values. Moreover, Day can be assigned the value mon, or tue,
etc., which makes the program easier to follow and understand.
Internally, the Modula-2 system does not actually assign the
value mon to the variable Day, but it uses an integer
representation for each of the names. This is important to
understand because you must realize that you cannot print out
mon, tue, etc., but can only use them for indexing control
statements.

Note that there is an upper limit of 16 values for an
enumerated type placed on you by most implementations of
Modula-2. This is actually a very low limit and it is most
unfortunate that such a small limit exists. You should check
your documentation because your particular compiler may allow
more.


ANOTHER ENUMERATED TYPE
______________________________________________________________

The second line of the type definition defines TimeOfDay as
another enumerated type. The variable Time can only be
assigned one of four values since it is defined as the type
TimeOfDay. It should be clear that even though it can be
assigned morning, it cannot be assigned morningtime or any
other variant spelling of morning, since each enumerated value
is simply another identifier which must have an exact spelling
to be understood by the compiler.
Several real variables are defined to allow us to demonstrate
the use of the enumerated variables. After writing a header
for neat formatting of the output, the real variables are
initialized to some values that are probably not real life
values, but will serve as a platform to illustrate use of the
enumerated variable.

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Chapter 10 - Scalars, Subranges and Sets

A BIG SCALAR VARIABLE LOOP
______________________________________________________________

The remainder of the program is one large loop being
controlled by the variable Day as it goes through all of its
values, one at a time. Note that the loop could have gone
from tue to sat or any portion of the range desired. It does
not have to go through all of the values of Day. Using Day
as the case variable, the name of one of the days of the week
is written out each time we go through the loop. Another loop
controlled by Time is executed four times, once for each value
of Time. The two CASE statements within the inner loop are
used to calculate the total pay rate for each time period and
each day. The data is formatted carefully to make a nice
looking table of pay rates as a function of Time and Day.

Take careful notice of the fact that the scalar variables
never entered into the calculations, and they were not printed
out. They were only used to control the flow of the logic.
It was much neater than trying to remember that mon is
represented by a 0, tue is represented by a 1, etc. In fact,
those numbers are used for the internal representation of the
scalars, but we can relax and let the Modula-2 system worry
about the internal representation of our scalars.



THE OUTPUT MAY NOT BE VERY NEAT
______________________________________________________________

Compile and run this program and observe the form of the
output data. The only format available with some compilers
is the E (exponential) notation which does not make for a very
well formatted or easily read output. Don't let this worry
you, when we get to Part III of this tutorial we will see how
we can write our own output routines to display or print
floating point numbers in any format we can think up.

One other thing should be pointed out in this module. If you
observe the case statements you will notice that the one that
starts in line 33 does not have an else clause. It is really
not needed because every possible value of the variable Day
is covered in the various branches. In the case statement
starting in line 51, there is an else clause, because only two
of the possible 7 values are acted on in the case body itself.
Without the else, the program would not know what to do with
a value of mon through fri, so the else is required here, but
not in the earlier one.





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Chapter 10 - Scalars, Subranges, and Sets

A COMMENT ON PROGRAM STYLE
______________________________________________________________

The enumerations in this program used a named type, but an
anonymous type can also be used. Good programming practice
would be to use a named type however.

The case statement in lines 51 through 55 could easily be
replaced by an if statement. The case statement was selected
here simply because the other constructs use the case
statement and it was felt that using the same construct would
lead to a clearer program.


LETS LOOK AT SOME SUBRANGES
______________________________________________________________

Examine the program SUBRANGE.MOD for an ================
example of subranges. It may be SUBRANGE.MOD
expedient, for a particular programming ================
problem, to define some variables that
only cover a part of the full range as defined in a scalar
type. Notice that Days is declared a scalar type as in the
last program, and Work is declared a type with an even more
restricted range, a subrange of Days. The variable named Rest
is declared as a different subrange of Days. In the var
declaration, Day is once again defined as the days of the week
and can be assigned any of the days by the program. The
variable Workday, however, is assigned the type Work, and can
only be assigned the days mon through fri because of the more
limited range of the type. If an attempt is made to assign
Workday the value sat, a runtime error will be generated
because sat is not a legal value to be assigned to Workday.
A carefully written program will never attempt to do so, and
it would therefore be an indication that something is wrong
with either the program or the data. This is one of the
advantages of Modula-2 over older less structured languages.

Further examination will reveal that Index is declared as
being capable of storing only the range of values from 1 to
12, another subrange. During execution of the program, if an
attempt is made to assign Index any value outside of that
range, a runtime error will be generated. Suppose the
variable named Index was intended to refer to your employees,
and you have only 12. If an attempt was made to refer to
employee number 27, or employee number -8, there is clearly
an error somewhere in the data and you would want to stop
running the payroll to fix the problem. Modula-2 would have
saved you a lot of grief.

The way Index is defined, it will be of type cardinal because
the entire range is composed of positive numbers. It is
therefore not possible to use the variable Index in
calculations involving integer type numbers. If the range

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Chapter 10 - Scalars, Subranges, and Sets

were defined with at least one end negative, it would be of
type integer. Index2 will be of type integer because we force
the compiler to use the integer type through use of the type
written prior to the range. Modula-2 gives you complete
control over the types used.


SOME STATEMENTS WITH ERRORS IN THEM
______________________________________________________________

In order to have a program that would compile without errors,
and yet include some examples of errors, the first part of the
program is not really a part of the program since it is within
a comment area. This is a trick to remember when you are
debugging a program, a troublesome part can be commented out
until you are ready to include it with the rest. The errors
should be self explanatory.

Going beyond the area commented out, there are seven
assignment statements in lines 31 through 37 given as examples
of subrange variable use. Notice that the variable Day can
always be assigned the value of either Workday or Weekend, but
the reverse is not true because Day can store values that
would be illegal for the other variables.


THREE VERY USEFUL FUNCTIONS
______________________________________________________________

Lines 39 through 45 of the example program demonstrate the use
of three very important functions when using scalars. The
first is the INC function which returns the value of the
scalar following the value of the scalar used as the actual
parameter. If the parameter is the last value in the list,
a runtime error is generated. The next function is the DEC
that returns the value of the scalar prior to the one used in
the actual parameter. All scalars have an internal
representation starting at 0 and increasing by one until the
end is reached. The third function is the ORD which simply
returns the numerical value of the scalar variable.

In our example program, ORD(Day) is 5 if Day has been assigned
sat, but ORD(Weekend) is 0 if Weekend has been assigned sat.
As you gain experience in programming with scalars and
subranges, you will realize the value of these three
functions.

A few more thoughts about subranges are in order before we go
on to another topic. A subrange is always defined by two
predefined constants, and is always defined in an ascending
order. A variable defined as a subrange type is actually a
variable defined with a restricted range, and should be used
as often as possible in order to prevent garbage data. There
are actually very few variables ever used in any computer

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Chapter 10 - Scalars, Subranges, and Sets

program that cannot be restricted by some amount. The limits
may give a hint at what the program is doing and can therefore
help in understanding the program operation.

Subrange types can only be constructed using the simple data
types, not including the real type, and any operation that can
be performed on a particular type can be performed on a
subrange of that type.


SETS
______________________________________________________________

Now for a new topic, sets. Examining the ================
example program SETS.MOD will reveal the SETS.MOD
use of some sets. A scalar type is ================
defined first, in this case the scalar
type named Goodies. A set type is then defined with the
reserved words SET OF followed by a predefined scalar type.
Most microcomputers have an upper limit of 16 elements that
can be used in a set. Your compiler documentation will tell
you if yours allows a larger range.

Several variables are defined as sets of Treat, after which
they can individually be assigned portions of the entire set.
Consider the variable IceCreamCone which has been defined as
a set of type Treat. This variable is composed of as many
elements of Goodies as we care to assign to it. In the
program, we define it as being composed of IceCream, and Cone.
The set IceCreamCone is therefore composed of two elements,
and it has no numerical or alphabetic value as most other
variables have. Continuing in the program, you will see 4
more delicious deserts defined as sets of their components.
Notice that the banana split is first defined as a range of
terms, then another term is added to the group. All five of
the defined sweets are combined in the set named Mixed, then
Mixed is subtracted from the entire set of values to form the
set of ingredients that are not used in any of the deserts.
Each ingredient is then checked to see if it is IN the set of
unused ingredients, and printed out if it is. Running the
program will reveal a list of the unused elements.

In this example, better programming practice would have
dictated defining a new variable, possibly called Remaining
for the ingredients that were unused. It was desirable to
illustrate that Mixed could be assigned a value based on
subtracting itself from the entire set, so the poor variable
name was used.

This example resulted in some nonsense results but hopefully
it led your thinking toward the fact that sets can be used for
inventory control, possibly a parts allocation scheme, or some
other useful system.

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Chapter 10 - Scalars, Subranges, and Sets

THE BITSET TYPE
______________________________________________________________

The only predefined set type is the BITSET. This is a set of
bits contained in one word of the host computer, and is
typically composed of 16 bits or elements. The type BITSET
acts as if it were defined as;

TYPE BITSET = SET OF [0..15];

Its use will be left to the students study. There is an
example of use in chapter 16 of this tutorial. It will be
found in the program named BITOPS.MOD and it would be a
profitable study to examine this program.


SET OPERATIONS
______________________________________________________________

A complete list of all operations available with sets is as
follows;

+ Union
- Difference
* Intersection
/ Symmetric difference


= Test for equality
# Test for inequality (Modern method)
<> Test for inequality (Becoming obsolete)
<= Inclusion
>= Inclusion

A complete discussion of set theory is beyond the scope of
this tutorial. The interested student will be referred to
academia and popular press.

















10-6



  3 Responses to “Category : Modula II Source Code
Archive   : M2TUTOR.ZIP
Filename : CHAP10.TXT

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