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CHAP3.TXT
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CHAPTER 3 - The simple Pascal data types
Pascal has only 5 basic data types which are predefined
and can be used anywhere you desire provided you use them
properly. The five types and a very brief description
follows;
Integer The integers 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
Please note that the Byte type of data is not a part of
the standard Pascal definition but is included as an
extension to the TURBO Pascal compiler.
A complete definition of these can be found on pages 41
and 42 of the TURBO Pascal reference manual (version 3.0).
It would be good to read those two pages now for a good
definition prior to our learning how to define and use them
in a program. The integers are by far the easiest to
understand so we will start with a simple program that uses
some integers in a very simple way. Load INTVAR into your
TURBO system and lets take a look at it.
OUR FIRST VARIABLES
Immediately following the PROGRAM statement is another
reserved word, VAR. VAR 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 at you 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 VAR, but it is used to
define three different variables, "count", "x", and "y".
Once a 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 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 which is
different from a reserved word. An identifier is predefined
like a reserved word but you can redefine it thereby losing
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CHAPTER 3 - The simple Pascal data types
its original purpose and meaning. For now and for a long
time, don't do that. Page 38 contains a list of identifiers.
OUR FIRST ARITHMETIC
Now that we have three variables defined as integer, 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, once
again preventing 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 either was 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 to preserve the
mathematical purity of Pascal. 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 use in a different manner.
The first three statements give "x" the value of 12, "y"
the value of 13, and "count" the value of 12+13 or 25. We
need to get those values out of the computer, so we need
another extension to the WRITELN statement. The first part
of the data within the parentheses should be 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 to see its output.
To illustrate the various ways to output data, load
INTVAR2 and observe that even though the output is
identical, it is output in a completely different manner.
Observe especially that a WRITELN all by itself simply
returns to the beginning of a new line on the video monitor.
Compile and run this program also to observe its output.
NOW LET'S USE LOTS OF VARIABLES
Load ALLVAR to observe a short program using all 5 of
the basic data types. The variables are simply assigned
values and the values are printed. A complete and detailed
description of the options available in the WRITE statement
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CHAPTER 3 - The simple Pascal data types
is given in the TURBO reference manual on pages 111 through
113. 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 when the time comes.
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 variable in
the desired calculations. The new real variable must of
course be defined in a VAR as a real before it can be used.
Details of how to do the conversion will be given later.
Since we have some variables defined, it would be nice
to use the properties of computers for which they are
famous, namely some mathematics. Two programs are available
for your observation to illustrate the various kinds of math
available, REALMATH using real variables, and INTMATH 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. Chapter 6 on pages 51 to 54
of your TURBO reference manual completely defines the simple
mathematics available.
Byte is used just like integers but with a much smaller
value. Only one byte of computer memory is used for each
variable defined as byte but 2 are used for integer types.
BOOLEAN VARIABLES
Lets take a look at the 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. An illustration is the best way to learn about the
boolean variable so load BOOLMATH, observe, compile, and run
it.
Char is a very useful variable, but not all by itself.
It is very 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 is included to
give you an idea of how a char variable can be used.
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CHAPTER 3 - The simple Pascal data types
Observe and run CHARDEMO for a very brief idea of what the
char variable is all about.
Examine the sample program CONVERT for several examples
of converting data from one simple variable to another. The
program is self explanatory.
PROGRAMMING EXERCISE
1. Write a program containing several variable definitions
and do some math on them, printing out the results.
Page 14