Software Vault: The Gold Collection/Software Vault - The Gold Collection (American Databankers) (1993).ISO/cdr48/pasclern.zip/CHAP7.TXT

Text File | 1993-04-01 | 6KB | 125 lines

                CHAPTER 7 - Strings and string procedures


                              PASCAL STRINGS

             According to the Pascal definition,  a string is simply
        an  array  of  2 of more characters of  type  CHAR,  and  is
        contained  in  an array defined in a VAR  declaration  as  a
        fixed length.  Look at the example program STRARRAY.  Notice
        that  the  strings are defined in the TYPE declaration  even
        though  they could have been defined in the VAR part of  the
        declaration. This is to begin getting you used to seeing the
        TYPE declaration.  The strings defined here are nothing more
        than arrays with CHAR type variables.

            The interesting part is the program.   Notice that  when
        the  variable  "first_name" is assigned a value,  the  value
        assigned  to  it must contain exactly 10 characters  or  the
        compiler  will generate an error.   Try editing out a  blank
        and you will get an invalid type error.   Pascal is neat  in
        allowing  you  to write out the values in the  string  array
        without specifically writing each character in a loop as can
        be  seen in the "WRITELN" statement.   To combine the  data,
        called  concatenation,   requires  the  use  of  the  rather
        extensive  looping and subscripting seen in the last part of
        the  program.   It  would  be even messier  if  we  were  to
        consider  variable length fields which is nearly always  the
        case in a real program.

            Two  things should be noticed in this  program.   First,
        notice  the fact that the string operations are truly  array
        operations  and  will  follow  all  of  the  characteristics
        discussed in the last chapter.  Secondly, it is very obvious
        that  Pascal is rather weak when it comes to its handling of
        text type data.   Keep in mind that Pascal will handle  text
        data,  even  though it may be difficult.   This concerns the
        standard description of Pascal,  we will see next that TURBO
        Pascal really shines here.

                       THE TURBO PASCAL STRING TYPE

            Look  at the example program STRINGS.   You will  see  a
        much  neater program that actually does more.   TURBO Pascal
        has,  as an extension to standard Pascal, the STRING type of
        variable.  It is used as shown, and the number in the square
        brackets in the VAR declaration is the maximum length of the
        string.   In actual use in the program,  the variable can be
        used  as any length from zero characters up to  the  maximum
        given  in the declaration.   The variable "first_name",  for
        example, actually has 11 locations stored for its data.  The
        current length is stored in "first_name[0]" and the data  is
        stored  in  "first_name[1]" through  "first_name[10]".   All
        data is stored as byte variables,  including the  size,  and



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                CHAPTER 7 - Strings and string procedures


        the  length  is  therefore  limited  to  a  maximum  of  255
        characters.

            Now  look  at  the  program  itself.   Even  though  the
        variable  "first_name" is defined as 10 characters long,  it
        is perfectly legal to assign it a 4 character constant, with
        "first_name[0]"  automatically set to four and the last  six
        characters undefined and unneeded.   When the program is run
        the  three variables are printed out all  squeezed  together
        indicating  that the variables are indeed shorter than their
        full  size  as defined in the VAR  declaration.   Using  the
        STRING  type  is  even easier when  you  desire  to  combine
        several  fields into one as can be seen in the assignment to
        "full_name".   Notice that there are even two blanks, in the
        form  of  constant fields,  inserted between  the  component
        parts of the full name.   When it is written out,  the  full
        name is formatted neatly and is easy to read.

                    WHAT IS IN A STRING TYPE VARIABLE?

            The next example program named WHATSTRG,  is intended to
        show you exactly what is in a string variable.  This program
        is  identical  to  the last program except  for  some  added
        statements  at the end.   Notice the assignment to  "total".
        The  function "length" is available in TURBO Pascal to  find
        out  what is the current length of any STRING type variable,
        it  returns a byte type variable with the value of  the  [0]
        position  of  the  variable.   We print out  the  number  of
        characters  in the string at this point,  and then print out
        each  character on a line by itself to illustrate  that  the
        TURBO  Pascal  STRING  type  variable  is  simply  an  array
        variable.

            The TURBO Pascal reference manual has a full description
        of  several more procedures and functions available in TURBO
        Pascal   only.    Refer  to  your  manual  for  a   complete
        description given in chapter 9,  beginning on page 67.   The
        use  of these should be clear after you grasp  the  material
        covered here.

                           PROGRAMMING EXERCISES

        1.  Write a program in which you store your  first,  middle,
            and last names as variables,  then display them one to a
            line. Concatenate the names with blanks between them and
            display your full name as a single variable.







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