HPAVC/HPAVC CD-ROM.iso/PASCTXT.ZIP/CHAP10.TXT

Text File | 1988-01-15 | 14KB | 323 lines

                    CHAPTER 10 - Standard Input/Output



             During  the course of this tutorial we have been  using
        the Write and Writeln procedures to display data, and it  is
        now time to discuss them fully.  Actually there is little to
        be said that has not already been said, but in order to  get
        all of the data in one place they will be redefined here.

             As  mentioned  earlier,  Write  and  Writeln  are   not
        actually  reserved words but are procedure calls.  They  are
        therefore merely identifiers that could be changed but there
        should never be a reason to do so.  Lets get on to our first
        example program WRITELNX which has lots of output.

                          MANY OUTPUT STATEMENTS

             Pascal has two output statements that are only slightly
        different  in  the  way they work.   The  Writeln  statement
        outputs  all of the data specified within it,  then  returns
        the  cursor  to the beginning of the next line.   The  Write
        statement outputs all of the data specified within it,  then
        leaves  the  cursor at the next character  where  additional
        data  can be output.  The Write statement can  therefore  be
        used  to  output a line in bits and pieces  if  desired  for
        programming convenience.  The first example program for this
        chapter,  WRITELNX,  has  many output  statements  for  your
        observation.   All outputs are repeated so you  can  observe
        where the present field ends and the next starts.

             Observe the integer output statements beginning in line
        13.   The  first simply directs the system to  output  Index
        twice,  and it outputs the value with no separating  blanks.
        The second statement says to output Index twice also, but it
        instructs  the  system  to put each output  in  a  field  15
        characters wide with the data right justified in the  field.
        This  makes the output look much better.   This  illustrates
        that  you have complete control over the appearance of  your
        output data.

             The  real output statements are similar to the  integer
        except that when the data is put into a field 15  characters
        wide, it is still displayed in scientific format.  Adding  a
        second field descriptor tells the system how many digits you
        want displayed after the decimal point.  Lines 21 through 23
        illustrate the second field and its use.

             The  boolean, char, and string examples should be  self
        explanatory.   Notice that when the string is  output,  even
        though  the  string  has been defined as  a  maximum  of  10
        characters,  it  has  been  assigned  a  string  of  only  8
        characters,  so only 8 characters are output.



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             Compile and run this program and observe the results.

             If  you are using TURBO Pascal version 4.0,  the  added
        data  types  described  in chapter 3 of  this  tutorial  are
        output  in  the  same manner as  those  illustrated  in  the
        program WRITELNX.

                   NOW FOR SOME INPUT FROM THE KEYBOARD

             The  example file READINT will illustrate reading  some
        integer data from the keyboard.  A message is output in line
        8  with  an  interesting fact that should  be  pointed  out.
        Anyplace where Pascal uses a string variable or constant, it
        uses  the apostrophe for a delimiter.   Therefore,  anyplace
        where  an  apostrophe is used in a string, it will  end  the
        string.   Two  apostrophes in a row will be construed  as  a
        single  apostrophe within the string and will not  terminate
        the  string.   The  term  'Read'  within  the  string   will
        therefore be displayed as shown earlier in this sentence.

             The variable Index is used to loop five times through a
        sequence  of statements with one Read statement in it.   The
        three  integer  values  are  read in  and  stored  in  their
        respective  variables with the one statement.  If less  than
        three are entered at the keyboard, only as many as are  read
        in  will be defined, the rest will be unchanged.   Following
        completion  of  the first loop, there is a  second  loop  in
        lines 19 through 25 that will be executed 5 times with  only
        one  minor  change, the Read statement is  replaced  by  the
        Readln  statement.  At this point it would be best run  this
        program trying several variations with input data.

             When  you run READINT, it will request three  integers.
        Reply with three small integers of your choice with as  many
        blank  spaces  between  each as you desire,  followed  by  a
        carriage  return.  The system will echo your  three  numbers
        back  out,  and request three more.  Respond with  only  one
        number  this time, different from each of the  first  three,
        and  a  carriage  return.   You will  get  your  new  number
        followed by your previous second and third number indicating
        that  you did not re-enter the last two  integer  variables.
        Enter  three  more numbers, this time including  a  negative
        number and observe the echo once again.

             Continue entering numbers until the system outputs  the
        message indicating that it will now be using the Readln  for
        reading data.  At this point enter the same numbers that you
        did in the previous section and notice the difference, which
        is  only  very slight.  Each time you hit the enter  key  to
        cause  the computer to process the data you have just  given
        it, it will echo the carriage return to the display, and the


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                    CHAPTER 10 - Standard Input/Output


        "Thank  you" message will be on a new line.   When  entering
        data  from  the keyboard, the only difference  in  Read  and
        Readln  is whether or not the carriage return is  echoed  to
        the display following the data read operation.

             It should not be a surprise to you that after you enter
        the  data, the data is stored within the program and can  be
        used anywhere that integer data is legal for use.  Thus, you
        could read in an integer, and use the integer to control the
        number of times through a loop, as a case selector, etc.

                        TIME TO CRASH THE COMPUTER

             Crashing the computer will not hurt a thing.  Rerun the
        above  program and instead of entering integer  data,  enter
        some  real data with decimal points, or even some  character
        data.   The  computer should display some  kind  of  message
        indicating that you have caused an I/O error (Input/Output),
        and TURBO Pascal will abort operation (that simply means  to
        stop  the  program  and  return  control  to  the  operating
        system).   No harm has been done, simply start it  again  to
        enter more numbers or errors.

                           READING REAL NUMBERS

             The  example  program READREAL will illustrate  how  to
        read  real  numbers  into the computer.   It  will  read  an
        integer  and three real numbers each time through the  loop.
        It  is perfectly fine to give the system a number without  a
        decimal  point for a real number.  The computer will  simply
        read  it  as a decimal number with zeros after  the  decimal
        point  and consider it as a real number internally.  As  you
        found  out in the last example program, however, it  is  not
        permissible  to include a decimal point in the data  if  the
        computer  is looking for an integer variable.  Include  some
        character  data  for a real number and crash the  system  in
        this program too.

                          READING CHARACTER DATA

             The  next example program, READCHAR, will read  in  one
        character each time through the loop and display it for you.
        Try  entering more than one character and you will see  that
        the  extra  characters will simply be ignored.   It  is  not
        possible  to  crash this program because any  character  you
        enter will be valid.

             The  next example, READARRY, will read in a  string  of
        characters  and display them for you if you are using  TURBO
        Pascal 3.0.  TURBO Pascal 4.0 does not allow reading into an
        array but does allow reading into the individual elements of


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        the array one element at a time.  This program does not work
        with TURBO Pascal 4.0 so you should go directly to the  next
        program, READSTRG, if you are using that version.

             Continuing  our  discussion  of  READARRY,  up  to   10
        characters  will be read, and if less than 10 are read,  the
        rest will be blank filled.  Try entering 10 characters, then
        4,  to  see that the residual 6 characters are  blanked  out
        before  storing and printing.  Since the array is  fixed  at
        ten  characters,  ten  characters are  always  printed  out,
        including trailing blanks.

             Finally  READSTRG will also read up to  10  characters,
        but  since  a string is a dynamic length variable,  it  will
        only print out the characters you input each time, up to the
        maximum  of 10 as defined in the var declaration.   It  will
        display  trailing blanks if you type them in because  blanks
        are valid characters.

                         BULLET PROOF PROGRAMMING

             It can be frustrating to be running a program and  have
        it  declare  an  I/O error and  terminate  operation  simply
        because  you  have  entered  an  incorrect  character.   The
        integer and real data inputs defined earlier in this chapter
        are   fine  for  quick  little  programs  to   do   specific
        calculations,  but if you are writing a  large  applications
        program  it is better to use another technique.   Since  the
        character  and string inputs cannot abort operation  of  the
        program,  it is best to use them to input the variable  data
        and  check  the  data  internally  under  your  own  program
        control.  An error message can then be given to the operator
        and  another opportunity granted to input the correct  data.
        All  well  written  large  application  programs  use   this
        technique.

                 HOW DO I PRINT SOMETHING ON THE PRINTER

             With  all of the Pascal knowledge you now have,  it  is
        the simplest thing in the world to get data to the  printer.
        The  example file PRINTOUT will show you graphically how  to
        do it.  Every Write or Writeln statement is required to have
        a  device  identifier prior to the first output  field.   If
        there is none, it is automatically defaulted to the standard
        output device, the display monitor.  The example program has
        a  few  outputs  to the monitor in lines 9 and 10  with  the
        device  identifier included, namely "Output".  This is  only
        done  to show you the general form of the Write  statements.
        There  are  also many statements in this  program  with  the
        display identifier "Lst", which is the standard name for the
        list device or the printer.


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                    CHAPTER 10 - Standard Input/Output



             Compile  and run this program with your printer  turned
        on  for some printer output.  If you are using TURBO  Pascal
        3.0,  you will have to comment out line 4 since it will  not
        be understood by your compiler.  It is required with version
        4.0 to tell the system where to find the output device  name
        Lst.

             Just  to supply you with a bit more information,  every
        Read and Readln statement is also required to have a  device
        identifier  prior  to  the first input field.   As  you  may
        suspect, it is also defaulted to Input if none is specified,
        and the standard input device is the keyboard.

                           PROGRAMMING EXERCISE

        1.  Write a program containing a loop to read in a character
            string  up to 60 characters long,  then print the string
            on your printer. When you run the program, you will have
            the  simplest word processing program in the  world.  Be
            sure  to  include a test to end the loop,  such as  when
            "END" is typed in.































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