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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. The first simply
directs the system to output "index" twice, and it does 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. The next few lines
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
NOW FOR SOME INPUT FROM THE KEYBOARD
The example file READINT will illustrate reading some
integer data from the keyboard. A message is output 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. Thus, 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 variables are read in and stored with the one
statement. If less than three are input in the statement,
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 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.
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 all three again, 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 "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
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CHAPTER 10 - Standard Input/Output
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 most implementations of Pascal will probably 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. 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.
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CHAPTER 10 - Standard Input/Output
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 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. Compile and run this program with your printer
turned on for some printer output.
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.
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CHAPTER 10 - Standard Input/Output
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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