CD Actual 15/CDACTUAL15.iso/cdactual/program/pascal/PAS_TUT.ZIP/CHAP03.TXT

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                                                        Chapter 3
                                                SIMPLE DATA TYPES


WHAT IS A DATA TYPE?
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A type in Pascal, and in several other popular programming
languages, defines a variable in such a way that it defines a range
of values which the variable is capable of storing, and it also
defines a set of operations that are permissible to be performed
on variables of that type.  TURBO Pascal has eight basic data types
which are predefined and can be used anywhere in a program provided
you use them properly.  This chapter is devoted to illustrating the
use of these eight data types by defining the allowable range of
values that can be assigned to them, and by illustrating the
operations that can be done to variables of these types.  The eight
types and a very brief description follows;

     integer          Whole numbers from -32768 to 32767
     byte             The integers from 0 to 255
     real             Floating point numbers from 1E-38 to 1E+38
     boolean          Can only have the value TRUE or FALSE
     char             Any character in the ASCII character set
     shortint         The integers from -128 to 127
     word             The integers from 0 to 65535
     longint          The integers from -2147483648 to 2147483647

Please note that four of these types of data (char, shortint, word,
and longint) are not a part of the standard Pascal definition but
are included as extensions to the TURBO Pascal compiler.

In addition to the above data types TURBO Pascal version 5.0 and
later have the following data types available;

     single           Real type with 7 significant digits
     double           Real type with 15 significant digits
     extended         Real type with 19 significant digits
     comp             The integers from about -10E18 to 10E18

TURBO Pascal version 5.0 and newer have these four types available
which use the 80X87 math coprocessor.  Because TURBO Pascal has a
software emulator for the floating point operations, an 80X87 math
coprocessor is not absolutely required to use these new types with
these versions.  Of course, your resulting program will run much
faster if you have the coprocessor available for use by the
program.

A complete definition of the available types for each compiler can
be found in the TURBO Pascal reference manual.  It would be good
to read these pages now for a good definition prior to learning how
to define and use them in a program.  Note that all of these will
be used in example programs in this chapter.


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                                    Chapter 3 - Simple Data Types

OUR FIRST VARIABLES
_________________________________________________________________

The integers are by far the easiest to             ==============
understand so we will start with a simple            INTVAR.PAS
program that uses some integers in a very simple   ==============
way.  Load INTVAR.PAS into your TURBO system and
let's take a look at it.

Immediately following the program statement is another reserved
word, var.  This reserved word is used to define a variable before
it can be used anywhere in the program.  There is an unbroken rule
of Pascal that states "Nothing can be used until it is defined."
The compiler will complain by indicating a compilation error if
you try to use a variable without properly defining it.  It seems
a bit bothersome to have to define every variable prior to its use,
but this rule will catch many spelling errors of variables before
they cause trouble.  Some other languages will simply define a new
variable with the new name and go merrily on its way producing some
well formatted garbage for you.

Notice that there is only one use of the reserved word var, but it
is used to define three different variables, Count, X, and Y.  Once
the word var is recognized, the compiler will continue to recognize
variable definitions line after line until it finds another
reserved word.  It would be permissible to put a var on the second
line also but it is not necessary.  It would also be permissible
to put all three variables on one line but your particular
programming style will dictate where you put the three variables.
Following the colon on each line is the word integer which is a
standard identifier, and is therefore different from a reserved
word.  A standard identifier is predefined like a reserved word,
but you can redefine it, thereby losing its original purpose and
meaning.  For now and for a long time, don't do that.


OUR FIRST ARITHMETIC
_________________________________________________________________

Now that we have three variables defined as integer type variables,
we are free to use them in a program in any way we desire as long
as we use them properly.  If we tried to assign a real value to X,
the compiler will generate an error, and prevent a garbage output.
Observe the start of the main body of the program.  There are three
statements assigning values to X, Y, and Count.  A fine point of
mathematics would state that Count is only equal to the value of
X+Y until one of them is modified, therefore the equal sign used
in so many other languages is not used here.  The sign := is used,
and can be read as "is replaced by the value of," when reading a
listing.  Another quicker way is to use the word "gets".  Thus X
:= X + 1 would be read, "X gets the value of X plus 1".  We will
see later that the simple equal sign is reserved for another use
in Pascal.

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                                    Chapter 3 - Simple Data Types

The first three statements give X the value of 12, Y the value of
13, and Count the value of 12 + 13 or 25.  If we have a requirement
to get those values out of the computer, we need another extension
to the Writeln statement.  The first part of the data within the
parentheses should be very familiar to you now, but the second part
is new.

Multiple outputs can be handled within one Writeln if the fields
are separated by a comma.  To output a variable, simply write the
variable's name in the output field.  The number following the
variable in each case is the number of output columns to be used
by the output data.  This number is optional and can be omitted
allowing the system to use as many columns as it needs.  For
purposes of illustration, they have all been assigned different
numbers of columns.  At this point, you can compile and run
INTVAR.PAS and examine its output.


To illustrate the various ways to output data,    ===============
load INTVAR2.PAS and observe that even though       INTVAR2.PAS
the output is identical, it is output in a        ===============
completely different manner.  The Writeln
statements are broken into pieces and the
individual pieces are output with Write and Writeln statements.
Observe especially that a Writeln all by itself simply moves the
cursor to the beginning of a new line on the video monitor.

Compile and run this program and observe its output after you are
certain that the two programs are actually identical.


NOW LET'S USE LOTS OF VARIABLES
_________________________________________________________________

Load ALLVAR.PAS to observe a short program using   ==============
five of the basic data types.  The variables are     ALLVAR.PAS
simply assigned values and the values are          ==============
printed.  A complete and detailed description of
the options available in the Write statement is
given in the TURBO Pascal reference manual.  Check the index to
find this information for the version you are using.  It would be
to your advantage to read this section at this time since very
little explanation will be given about Write statements from this
point on.  We will discuss the method by which we can write to disk
files or other output devices in a later chapter of this tutorial.

Back to the basic types.  Pascal does lots of cross checking for
obvious errors.  It is illegal to assign the value of any variable
with a value that is of the wrong type or outside the allowable
range of that variable.  There are routines to convert from one
system to another when that is necessary.  Suppose, for example,
that you wished to use the value of an integer in a calculation of
real numbers.  That is possible by first converting the integer
into a real number of the same value and using the new real type

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                                    Chapter 3 - Simple Data Types

variable in the desired calculations.  The new real type variable
must of course be defined in a var statement as a real type
variable before it can be used.  Details of how to do several
conversions of this kind will be given in the example program named
CONVERT.PAS later in this chapter.

Since we have some variables defined, it would   ================
be nice to use the properties of computers for     REALMATH.PAS
which they are famous, namely some arithmetic.   ================
Two programs are available for your observation
to illustrate the various kinds of math
available, REALMATH.PAS using real variables, and INTMATH.PAS using
integer variables.  You can edit, compile, and run these on your
own with no comment from me except the comments embedded into the
source files.  You should output some of the results using the
method of outputting illustrated in line 24 of the previous example
program.  Read the definition of how to do this in your TURBO
Pascal User's Guide.

The example program named INTMATH.PAS             ===============
illustrates some of the math capabilities of        INTMATH.PAS
Pascal when using integer class variables.  A     ===============
byte type variable is used just like an integer
variable but with a much smaller allowable
range.  Only one byte of computer memory is used for each variable
defined as a byte type variable, but 2 are used for each integer
type variable.


BOOLEAN VARIABLES
_________________________________________________________________

Let's take a look at a boolean variable, which is only allowed to
take on two different values, TRUE or FALSE.  This variable is used
for loop controls, end of file indicators or any other TRUE or
FALSE conditions in the program.  Variables can be compared to
determine a boolean value.  A complete list of the relational
operators available with Pascal is given in the following list.

     =         equal to
     <>        not equal to
     >         greater than
     <         less than
     >=        greater than or equal to
     <=        less than or equal to

These operators can be used to compare any of    ================
the simple types of data including integer,        BOOLMATH.PAS
char, byte, and real type variables or           ================
constants, and they can be used to compare
boolean variables.  An illustration is the best
way to learn about the boolean variable so load BOOLMATH.PAS and
observe it.


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                                    Chapter 3 - Simple Data Types

In BOOLMATH.PAS we define a few boolean variables and two integer
type variables for use in the program and begin by assigning values
to the two integer variables.  The expression Junk = Who in line
14 is actually a boolean operation that is not true since the value
of Junk is not equal to the value of Who,  The result is therefore
FALSE and that value is assigned to the boolean variable A.  The
boolean variable B is assigned the value of TRUE because the
boolean expression Junk = (Who - 1) is true.  The boolean variables
C and D are likewise assigned some values in a manner that should
not need any comment.  After assigning a value to the variable with
the big name, the values are all printed out.  Note that if either
A or B is TRUE, the result is TRUE in line 18.


WHERE DO WE USE THE BOOLEAN VARIABLES?
_________________________________________________________________

We will find many uses for the boolean type variable when we study
the loops and conditional statements soon, but until then we can
only learn what they are.  Often, in a conditional statement, you
will want to do something if both of two things are true, in which
case you will use the reserved word and with two boolean
expressions.  If both are true, the result will be true.  Line 29
is an example of this.  If the boolean variables B, C, and D, are
all true, then the result will be true and A will be assigned the
value of TRUE.  If any one of them is false, the result will be
false and A will be assigned the value of FALSE.

In Line 31, where the or operator is illustrated, if any of the
three boolean variables is true, the result will be true, and if
all three are false, the result will be false.  Another boolean
operator is the not which is illustrated in line 30.  Examine line
33 which says the result is true only if the variable Junk is one
less than Who, or if Junk is equal to Who.  This should indicate
the level of flexibility available with a boolean variable.

Compile and run this program, then add some additional printout to
see if the boolean variables change in the manner you think they
should in the last few statements.


SHORT CIRCUIT OR COMPLETE EVALUATION?
_________________________________________________________________

Suppose you have several boolean expressions "and"ed together, and
when evaluation starts, the first expression results in a FALSE.
Since the first expression is FALSE, it is impossible for the
following expressions to ever allow the final result to be TRUE
because the first FALSE will force the answer to be FALSE.  It
seems like a waste of execution time to continue evaluating terms
if the final result is already known, but that is exactly what
standard Pascal will do because of the language definition.  This
is known as complete evaluation of a boolean expression.  If the
system is smart enough to realize that the final result is known,

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                                    Chapter 3 - Simple Data Types

it could stop evaluation as soon as the final result is known.
This is known as short circuit evaluation of a boolean expression,
and could also be applied if a term of an "or"ed boolean expression
resulted in a TRUE, since the result would always be TRUE.

TURBO Pascal versions 5.0 and later, allows you to choose between
complete evaluation or short circuit evaluation.  The default for
these compilers is the short circuit form but it can be changed
through the Options menu when you are using the integrated
environment, or through use of a compiler directive.


LET'S LOOK AT THE CHAR TYPE VARIABLE
_________________________________________________________________

A char type variable is a very useful variable,  ================
but usually not when used alone.  It is very       CHARDEMO.PAS
powerful when used in an array or some other     ================
user defined data structure which is beyond the
scope of this chapter.  A very simple program,
CHARDEMO.PAS is included to give you an idea of how a char type
variable can be used.  Study, then compile and run CHARDEMO.PAS for
a very brief idea of what the char type variable is used for.

Examine the sample program CONVERT.PAS for        ===============
several examples of converting data from one        CONVERT.PAS
simple variable to another.  The comments make    ===============
the program self explanatory.


EXTENDED INTEGER TYPES
_________________________________________________________________

Display the program EXTINT.PAS for an example of   ==============
using the extended integer types available with      EXTINT.PAS
the Pascal compiler.  Four variables are defined   ==============
and values assigned to each, then the results
are displayed.  When you compile and run the
program, you will see that the variable Big_int can indeed handle
a rather large number.

It must be pointed out that the calculation in lines 13 and 21
result in a different answer even though they appear to be
calculating the same thing.  An explanation is in order.  The
quantity named MaxInt used in lines 10 and 13 is a constant built
into the system that represents the largest value that an integer
type variable can store.  On the first page of this chapter we
defined that as 32767 and when running the program you will find
that Index displays that value as it should.  The constant MaxInt
has a type that is of a universal_integer type as do all of the
numeric constants in line 13.  The result then is calculated to the
number of significant digits dictated by the left hand side of the
assignment statement which is of type longint resulting in a very
large number.

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                                    Chapter 3 - Simple Data Types


When we get to line 21, however, the variable Index is of type
integer so the calculations are done as though the constants were
of type integer also which causes some of the more significant
digits to be truncated.  The truncated result is converted to type
longint and assigned to the variable Big_int and the truncated
value is displayed by line 22.

After that discussion it should be apparent to you that it is
important what types you use for your variables.  It must be
emphasized that it would not be wise to use all large type
variables because they use more storage space and slow down
calculations.  Experience will dictate the proper data types to use
for each application.


EXTENDED REAL TYPES
_________________________________________________________________

Display the program EXTREAL.PAS for an example    ===============
using the new "real" types available with the       EXTREAL.PAS
newer versions of TURBO Pascal.                   ===============

If you are using a version of TURBO Pascal which
is 5.0 or newer, you can use the 80X87 math coprocessor.  If you
do not have a math coprocessor, TURBO Pascal version 5.0 and newer
has an emulator mode which can be used as instructed in the User's
Guide.  Keep in mind that, even though the emulator will allow you
to use these newer data types, the resulting program will execute
much slower due to the extra calculations required.

This program should be self explanatory so nothing will be said
except that when you run it, you can observe the relative accuracy
of each of the variable types.  Once again, you should keep in mind
that use of the larger "real" types costs you extra storage space
and reduced run-time speed, but gives you more accuracy.


PROGRAMMING EXERCISE
_________________________________________________________________

1.   Write a program containing several variable definitions and
     do some math on them, printing out the results.












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