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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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CHAPTER 10 - Standard Input/Output
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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CHAPTER 10 - Standard Input/Output
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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