Programmer 7500/MAX_PROGRAMMERS.iso/PASCAL/PASCTUT1.ZIP/CHAP3.TXT

Text File | 1986-07-14 | 10.1 KB | 257 lines

                 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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        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.









































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