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CHAPTER 11 - Files
One of the most common operations when using a computer
is to either read from, or write to a file. You are already
somewhat experienced in file handling from the last chapter,
because in computer terminology, the keyboard, terminal, and
printer are all classified as files. A file is any serial
input or output device that the computer has access to.
Since it is serial, only one piece of information is
available to the computer at any instant of time. This is in
contrast to an array, for example, in which all elements of
the array are stored internally and are all available at any
time.
Several years ago computers were all large cumbersome
machines with large peripheral devices such as magnetic tape
drives, punch card readers, paper tape readers or punches,
etc. It was a simple task to assign the paper tape reader a
symbol and use that symbol whenever it was necessary to read
a paper tape. There was never more than one file on the
paper tape being read, so it was simply read sequentially,
and hopefully the data was the desired data. With the
advent of floppy disks, and hard disks too, it became
practical to put several files of data on one disk, none of
which necessarily had anything to do with any of the other
files on that disk. This led to the problem of reading the
proper file from the disk, not just reading the disk.
Pascal was originally released in 1971, before the
introduction of the compact floppy disk. The original
release of Pascal had no provision for selecting a certain
file from among the many included on the disk. Each
compiler writer had to overcome this deficiency and he did
so by defining an extension to the standard Pascal system.
Unfortunately, all of the extensions were not the same, and
there are now several ways to accomplish this operation.
There are primarily two ways, one using the ASSIGN
statement, and the other using the OPEN statement. They are
similar to each other and they accomplish the same end
result.
All of the above was described to let you know that we
will have a problem in this chapter, namely, how do we cover
all of the possible implementations of Pascal available?
The answer is, we can't. Most of what is covered in this
chapter will apply to all compilers, and all that is covered
will apply to the TURBO Pascal compiler. If your compiler
complains about some of the statements, it will be up to you
to dig out the details of how to do the intended operations.
If there is no way to do any of these operations, you should
seriously consider getting another compiler because all of
these operations are needed in a useful Pascal environment.
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CHAPTER 11 - Files
READING AND DISPLAYING A FILE
Examine the file READFILE for an example of a program
that can read a text file from the disk, in fact it will
read itself from the disk and display it on the video
monitor. The first statement in the program is the ASSIGN
statement. This is TURBO Pascal's way of selecting which
file on the disk will be either read from or written to. In
this case we will read from the disk. The first argument in
the ASSIGN statement is the device specifier similar to
"lst" used in the last chapter for the printer. We have
chosen to use "turkey", but could have used any valid
identifier. This identifier must be defined in a VAR
declaration as a TEXT type variable. The next argument is
the filename desired. The filename can be defined as a
string constant, as it is here, or as a string variable.
The TEXT type is a predefined type and is used to define
a file identifier. It is predefined as a "file of CHAR", so
it can only be used for a text file. We will see later that
there is another type of file, a binary file.
Now that we have a file identified, it is necessary to
prepare it for reading by executing a RESET statement. The
reset statement positions the read pointer at the beginning
of the file ready to read the first piece of information in
the file. Once we have done that, data is read from the
file in the same manner as it was when reading from the
keyboard. In this program, the input is controlled by the
WHILE loop which is executed until we exhaust the data in
the file.
WHAT ARE THE "EOF" AND "EOLN" FUNCTIONS?
The "eof" function is new and must be defined. When we
read data from the file, we move closer and closer to the
end, until finally we reach the end and there is no more
data to read. This is called "end of file" and is
abbreviated "eof". Pascal has this function which is false
until we reach the last line of the file, but when there is
no more data in the file to be read, the function "eof"
becomes true. To use the function, we merely give it our
file identifier as an argument. It should be clear that we
will loop until we read all of the data available in the
file.
The "eoln" function is not used in this program but is a
very useful function. If the input pointer is anywhere in
the text file except at the end of a line, the "eoln"
function is false, but at the end of a line, it becomes
true. This function can therefore be used to find the end
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CHAPTER 11 - Files
of a line of text for variable length text input. The
"eoln" function is not available, and in fact meaningless
when you are reading a binary file, to be defined later.
To actually read the data, we use the READLN procedure,
giving it our identifier "turkey" and the name of the
variable we want the data read into. In this case, we read
up to 80 characters into the string and if more are
available, ignore them. Remember this from the keyboard
input? It is the same here. Since we would like to do
something with the data, we simply output the line to the
default device, the video monitor. It should be clear to
you by now that the program will simply read the entire file
and display it on the monitor.
Finally, we CLOSE the file "turkey". It is not really
necessary to close the file because the system will close it
for you automatically at program termination, but it is a
good habit to get into. It must be carefully pointed out
here, that you did not do anything to the input file, you
only read it and left it intact. You could RESET it and
reread it again in this same program. Compile and run this
program to see if it does what you expect it to do.
A PROGRAM TO READ ANY FILE
Examine the next program READDISP for an improved file
reading program. This is very similar except that it asks
you for the name of the file that you desire to display, and
enters the name into a 12 character string named
"name_of_file_to_input". This is then used in the ASSIGN
statement to select the file to be read, and the file is
reset as before. A header is then displayed, and the
program is identical to the last one with some small
additions. In order to demonstrate the use of a function
within the WRITELN specification, the program calls for the
length of the input string and displays it before each line.
The lines are counted as they are read and displayed, and
the line count is then displayed at the end of the listing.
You should be able to see clearly how each of these
operations is accomplished. Compile and run this program,
entering any filename we have used so far (be sure to
include the .PAS). After a successful run, enter a
nonexistent filename and see the I/O error.
HOW TO COPY A FILE (SORT OF)
Examine the file READSTOR for an example of both reading
from a file and writing to another one. In this program we
request an operator input for the filename to read, after
which we ASSIGN the name to the file and RESET it. Next, we
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CHAPTER 11 - Files
request a different filename to write to, which is assigned
to a different identifier. The next statement is new to us,
the REWRITE statement. This name apparently comes from the
words REset for WRITEing because that is exactly what it
does. It clears the entire file of any prior data and
prepares to write into the very beginning of the file. Each
time you write into it, the file grows by the amount of the
data written.
Once the identifier has been defined, and the REWRITE
has been executed, writing to the file is identical to
writing to the display with the addition of the identifier
being specified before the first output field. With that in
mind, you should have no trouble comprehending the operation
of the program. It is similar to the last program, except
that it numbers the lines as the file is copied. After
running the program, look on your default disk for the
filename you input when it asked for the output filename.
Examine that file to see if it is truly a copy of the input
file with line numbers added. One word of caution, if you
used an existing filename for the output file, the file was
overwritten, and the original destroyed. In that case, it
was good that you followed instructions at the beginning of
this tutorial and made a working copy. You did do that,
didn't you?
HOW TO READ INTEGER DATA FROM A FILE
It is well and good to be able to read text from a file,
but now we come to the time to read data from a file. First
we will read data from a text file, then later from a binary
file. Examine the program READINTS for an example of
reading data from a text file. A text file is an ASCII file
that can be read by a text editor, printed, displayed, or in
some cases, compiled and executed. It is simply a file made
up of a long string of CHAR type data, and usually includes
linefeeds, carriage returns, and blanks for neat formatting.
Nearly every file on the Tutorial disk you received with
this package is a text file. The notable exception is the
file named LIST.COM, which is an executable program file.
The example program has nothing new, you have seen
everything in it before. We have an assignment, followed by
a reset of our file, followed by four read and write loops.
Each of the loops has a subtle difference to illustrate the
READ and READLN statements. Notice that the same file is
read in four times with a RESET prior to each, illustrating
the nondestructive read mentioned a few paragraphs ago.
The file we will be using is named INTDATA.TXT and is on
your disk. You could display it at this time using the
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program READDISP we covered recently. Notice that it is
simply composed of the integer values from 101 to 148
arranged four to a line with a couple of spaces between each
for separation and a neat appearance. The important thing
to remember is that there are four data points per line.
READ AND READLN ARE SLIGHTLY DIFFERENT
As variables are read in with either procedure, the
input file is scanned for the variables using blanks as
delimiters. If there are not enough data points on one line
to satisfy the arguments in the input list, the next line is
searched also, and the next, etc. Finally when all of the
arguments in the input list are satisfied, the READ is
complete, but the READLN is not. If it is a READ procedure,
the input pointer is left at that point in the file, but if
it is a READLN procedure, the input pointer is advanced to
the beginning of the next line. The next paragraph should
clear that up for you.
The input data file INTDATA.TXT has four data points per
line but the first loop in the program READINTS.PAS requests
only three each time through the loop. The first time
through, it reads the values 101, 102, and 103, and displays
those values, leaving the input pointer just prior to the
104, because it is a READ procedure. The next time through,
it reads the value 104, advances to the next line and reads
the values 105, and 106, leaving the pointer just prior to
the 107. This continues until the 5 passes through the loop
are completed.
The next loop contains a READLN procedure and also reads
the values 101, 102, and 103, but when the input parameter
list is satisfied, it moves the pointer to the beginning of
the next line, leaving it just before the 105. The values
are printed out and the next time we come to the READLN, we
read the 105, 106, and 107, and the pointer is moved to the
beginning of the next line. It would be good to run the
program now to see the difference in output data for the two
loops.
When you come back to the program again, notice the last
two loops, which operate much like the first two except that
there are now five requested integer variables, and the
input file still only has four per line. This is no
problem. Both input procedures will simply read the first
four in the first line, advance to the second line for its
required fifth input, and each will do its own operation
next. The READ procedure will leave the input pointer just
before the second data point of the second line, and the
READLN will advance the input pointer to the beginning of
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CHAPTER 11 - Files
the third line. Run this program and observe the four
output fields to see an illustration of these principles.
NOW TO READ SOME REAL VARIABLES FROM A FILE
By whatever method you desire, take a look at the file
named REALDATA.TXT supplied on your Pascal Tutorial disk.
You will see 8 lines of what appears to be scrambled data,
but it is good data that Pascal can read. Notice especially
line 4 which has some data missing, and line 6 which has
some extra data.
Examine the program file READDATA which will be used to
illustrate the method of reading REAL data. Everything
should be familiar to you, since there is nothing new here.
Notice the READLN statement. It is requesting one integer
variable, and three real variables, which is what most of
the input file contained. When we come to the fourth line,
there are not enough data points available, so the first two
data points of the next line are read to complete the fourth
pass. Since the pointer is advanced to the beginning of the
next line, we are automatically synchronized with the data
again. When we come to the sixth line, the last two data
points are simply ignored. Run the program to see if the
results are as you would predict.
If a READ were substituted for the READLN, the pointer
would not be advanced to the beginning of line 6, after the
fourth pass through the loop. The next attempt to read
would result in trying to read the .0006 as an INTEGER, and
a run time error would result. Modify the program and see
if this is not true.
That is all there is to reading and writing text files.
If you learn the necessities, you will not be stumbling
around in the area of input/output which is very
intimidating to many people. Remember to ASSIGN, then RESET
before reading, REWRITE before writing, and CLOSE before
quitting. It is of the utmost importance to close a file
you have been writing to before quitting to write the last
few buffers to the file, but it is not important to close
read files unless you are using a lot of them, as there is
an implementation dependent limit of how many files can be
open at once. It is possible to read from a file, close it,
reopen it, and write in it in one program. You can reuse a
file as often as you desire in a program, but you cannot
read from and write into a file at the same time.
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NOW FOR BINARY INPUT AND OUTPUT
Examine the file BINOUT for an example of writing data
to a file in binary form. First there is a record defined
in the type declaration part composed of three different
variable types. In the VAR part, "output_file" is defined
as a "FILE of dat_rec", the record defined earlier. The
variable "dog_food" is then defined as an array of the
record, and a simple variable is defined.
Any file assigned a type of TEXT, which is a "FILE of
CHAR", is a text file. A text file can be read and modified
with a text editor, printed out, displayed on the monitor,
etc. If a file is defined with any other definition, it will
be a binary file and will be in an internal format as
defined by the Pascal compiler. Attempting to display such
a file will result in very strange looking gibberish on the
monitor.
When we get to the program, the output file is assigned
a name, and a REWRITE is performed on it to reset the input
point to the beginning of the file, empty the file, and
prepare for writing data into it. The next loop simply
assigns nonsense data to all of the variables in the 20
records so we have something to work with.
We finally write a message to the display that we are
ready to start outputting data, and we output the data one
record at a time with the standard WRITE statement. A few
cautions are in order here. The output file can be defined
as any simple variable type, INTEGER, BYTE, REAL, or a
record, but cannot be mixed. The record however, can be any
combination of data including other records, if desired, but
any file can only have one type of record written to it.
Also, a WRITELN statement is illegal when writing to a
binary file because a binary file is not line oriented. A
WRITE statement is limited to one output field per
statement. It is a simple matter to put one WRITE statement
in the program for each variable you wish to write out to
the file. It is important to CLOSE the file when you are
finished writing to it.
WHY USE A BINARY FILE
A binary file written by a Pascal program cannot be read
by a word processor, a text editor, any application program
such as a database or spreadsheet, and it may not even be
readable by a Pascal program compiled by a different
companies compiler because the data is implementation
dependent. It can't even be read by a Pascal program using
the correct compiler unless the data structure is identical
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CHAPTER 11 - Files
to the one used to write the file. With all these rules, it
seems like a silly way to output data, but there are
advantages to using a binary output.
A binary file uses less file space than a corresponding
text file because the data is stored in a packed mode.
Since all significant digits of REAL data are stored, it is
more precise unless you are careful to output all
significant data to the corresponding TEXT file. Finally,
since the binary data does not require formatting into ASCII
characters, it will be considerably faster than outputting
it in TEXT format. When you run the example program, it
will create the file KIBBLES.BIT, and put 20 records in it.
Return to DOS and look for this file and verify its
existence. If you try to TYPE it, you will have a real
mess, but that might be a good exercise.
READING A BINARY FILE
BININ is another example program that will read in the
file we just created. Notice that the variables are named
differently, but the types are all identical to those used
to write the file. An additional line is found in the
program, the IF statement. We must check for the "end of
file" marker to stop reading when we find it or Pascal will
list an error and terminate operation. Three pieces of
information are written out to verify that we actually did
read the data file in.
Once again, a few rules are in order. A READLN is
illegal since there are no lines in a binary file, and only
one variable or record can be read in with a READ statement.
WHAT ABOUT FILE POINTERS, GET, AND PUT STATEMENTS?
File pointers and the GET and PUT procedures are a part
of standard Pascal, but since they are redundant, they are
not a part of TURBO Pascal. The standard READ and WRITE
procedures are more flexible, more efficient, and easier to
use. The use of GET and PUT will not be illustrated or
defined here. If you ever have any need for them, they will
be covered in detail in your Pascal reference manual for the
particular implementation you are using.
Pointers will be covered in detail in the next chapter
of this tutorial.
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PROGRAMMING EXERCISES
1. Write a program to read in any text file, and display it
on the monitor with line numbers and the number of
characters in each line. Finally display the number of
lines found in the file, and the total number of
characters in the entire file. Compare this number with
the filesize given by the DOS command DIR.
2. Write a silly program that will read two text files and
display them both on the monitor on alternating lines.
This is the same as "shuffling" the two files together.
Take care to allow them to end at different times,
inserting blank lines for the file that terminates
earlier.
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