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Chapter 10 - File Input/Output
OUTPUT TO A FILE
Load and display the file named FORMOUT.C for your
first example of writing data to a file. We begin as before
with the "include" statement for "stdio.h", then define some
variables for use in the example including a rather strange
looking new type.
The type "FILE" is used for a file variable and is
defined in the "stdio.h" file. It is used to define a file
pointer for use in file operations. The definition of C
contains the requirement for a pointer to a "FILE", and as
usual, the name can be any valid variable name.
OPENING A FILE
Before we can write to a file, we must open it. What
this really means is that we must tell the system that we
want to write to a file and what the filename is. We do
this with the "fopen" function illustrated in the first line
of the program. The file pointer, "fp" in our case, points
to the file and two arguments are required in the
parentheses, the filename first, followed by the file type.
The filename is any valid DOS filename, and can be expressed
in upper or lower case letters, or even mixed if you so
desire. It is enclosed in double quotes. For this example
we have chosen the name TENLINES.TXT. This file should not
exist on your disk at this time. If you have a file with
this name, you should change its name or move it because
when we execute this program, its contents will be erased.
If you don't have a file by this name, that is good because
we will create one and put some data into it.
READING ("r")
The second parameter is the file attribute and can be
any of three letters, "r", "w", or "a", and must be lower
case. When an "r" is used, the file is opened for reading,
a "w" is used to indicate a file to be used for writing, and
an "a" indicates that you desire to append additional data
to the data already in an existing file. Opening a file for
reading requires that the file already exist. If it does
not exist, the file pointer will be set to NULL and can be
checked by the program.
WRITING ("w")
When a file is opened for writing, it will be created
if it does not already exist and it will be reset if it does
resulting in deletion of any data already there.
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Chapter 10 - File Input/Output
APPENDING ("a")
When a file is opened for appending, it will be created
if it does not already exist and it will be initially empty.
If it does exist, the data input point will be the end of
the present data so that any new data will be added to any
data that already exists in the file.
OUTPUTTING TO THE FILE
The job of actually outputting to the file is nearly
identical to the outputting we have already done to the
standard output device. The only real differences are the
new function names and the addition of the file pointer as
one of the function arguments. In the example program,
"fprintf" replaces our familiar "printf" function name, and
the file pointer defined earlier is the first argument
within the parentheses. The remainder of the statement
looks like, and in fact is identical to, the "printf"
statement.
CLOSING A FILE
To close a file, you simply use the function "fclose"
with the file pointer in the parentheses. Actually, in this
simple program, it is not necessary to close the file
because the system will close all open files before
returning to DOS. It would be good programming practice for
you to get in the habit of closing all files in spite of the
fact that they will be closed automatically, because that
would act as a reminder to you of what files are open at the
end of each program.
You can open a file for writing, close it, and reopen
it for reading, then close it, and open it again for
appending, etc. Each time you open it, you could use the
same file pointer, or you could use a different one. The
file pointer is simply a tool that you use to point to a
file and you decide what file it will point to.
Compile and run this program. When you run it, you
will not get any output to the monitor because it doesn't
generate any. After running it, look at your directory for
a file named TENLINES.TXT and "type" it. That is where your
output will be. Compare the output with that specified in
the program. It should agree.
Do not erase the file named TENLINES.TXT yet. We will
use it in some of the other examples in this chapter.
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Chapter 10 - File Input/Output
OUTPUTTING A SINGLE CHARACTER AT A TIME
Load the next example file, CHAROUT.C, and display it
on your monitor. This program will illustrate how to output
a single character at a time.
The program begins with the "include" statement, then
defines some variables including a file pointer. We have
called the file pointer "point" this time, but we could have
used any other valid variable name. We then define a string
of characters to use in the output function using a "strcpy"
function. We are ready to open the file for appending and
we do so in the "fopen" function, except this time we use
the lower cases for the filename. This is done simply to
illustrate that DOS doesn't care about the case of the
filename. Notice that the file will be opened for appending
so we will add to the lines inserted during the last
program.
The program is actually two nested "for" loops. The
outer loop is simply a count to ten so that we will go
through the inner loop ten times. The inner loop calls the
function "putc" repeatedly until a character in "others" is
detected to be a zero.
THE "putc" FUNCTION
The part of the program we are interested in is the
"putc" function. It outputs one character at a time, the
character being the first argument in the parentheses and
the file pointer being the second and last argument. Why
the designer of C made the pointer first in the "fprintf"
function, and last in the "putc" function is a good question
for which there may be no answer. It seems like this would
have been a good place to have used some consistency.
When the textline "others" is exhausted, a newline is
needed because a newline was not included in the definition
above. A single "putc" is then executed which outputs the
"\n" character to return the carriage and do a linefeed.
When the outer loop has been executed ten times, the
program closes the file and terminates. Compile and run
this program but once again there will be no output to the
monitor.
Following execution of the program, "type" the file
named TENLINES.TXT and you will see that the 10 new lines
were added to the end of the 10 that already existed. If
you run it again, yet another 10 lines will be added. Once
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Chapter 10 - File Input/Output
again, do not erase this file because we are still not
finished with it.
READING A FILE
Load the file named READCHAR.C and display it on your
monitor. This is our first program to read a file.
This program begins with the familiar "include", some
data definitions, and the file opening statement which
should require no explanation except for the fact that an
"r" is used here because we want to read it. In this
program, we check to see that the file exists, and if it
does, we execute the main body of the program. If it
doesn't, we print a message and quit. If the file does not
exist, the system will set the pointer equal to NULL which
we can test.
The main body of the program is one "do while" loop in
which a single character is read from the file and output to
the monitor until an EOF (end of file) is detected from the
input file. The file is then closed and the program is
terminated.
CAUTION CAUTION CAUTION
At this point, we have the potential for one of the
most common and most perplexing problems of programming in
C. The variable returned from the "getc" function is a
character, so we could use a "char" variable for this
purpose. There is a problem with that however, because on
some, if not most, implementations of C, the EOF returns a
minus one which a "char" type variable is not capable of
containing. A "char" type variable can only have the values
of zero to 255, so it will return a 255 for a minus one on
those compilers that use a minus one for EOF. This is a
very frustrating problem to try to find because no
diagnostic is given. The program simply can never find the
EOF and will therefore never terminate the loop. This is
easy to prevent, always use an "int" type variable for use
in returning an EOF. You can tell what your compiler uses
for EOF by looking at the "stdio.h" file where EOF is
defined. That is the standard place to define such values.
There is another problem with this program but we will
worry about it when we get to the next program and solve it
with the one following that.
After you compile and run this program and are
satisfied with the results, it would be a good exercise to
change the name of "TENLINES.TXT" and run the program again
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Chapter 10 - File Input/Output
to see that the NULL test actually works as stated. Be sure
to change the name back because we are still not finished
with "TENLINES.TXT".
READING A WORD AT A TIME
Load and display the file named READTEXT.C for an
example of how to read a word at a time.
This program is nearly identical as the last except
that this program uses the "fscanf" function to read in a
string at a time. Because the "fscanf" function stops
reading when it finds a space or a newline character, it
will read a word at a time, and display the results one word
to a line. You will see this when you compile and run it,
but first we must examine a programming problem.
THIS IS A PROBLEM
Inspection of the program will reveal that when we read
data in and detect the EOF, we print out something before we
check for the EOF resulting in an extra line of printout.
What we usually print out is the same thing printed on the
prior pass through the loop because it is still in the
buffer "oneword". We therefore must check for EOF before we
execute the "printf" function. This has been done in
READGOOD.C, which you will shortly examine, compile, and
execute.
Compile and execute the original program we have been
studying, READTEXT.C and observe the output. If you haven't
changed TENLINES.TXT you will end up with "Additional" and
"lines." on two separate lines with an extra "lines."
displayed because of the "printf" before checking for EOF.
Compile and execute READGOOD.C and observe that the
extra "lines." does not get displayed because of the extra
check for the EOF in the middle of the loop. This was also
the problem referred to when we looked at READCHAR.C, but I
chose not to expound on it there because the error in the
output was not so obvious.
FINALLY, WE READ A FULL LINE
Load and display the file READLINE.C for an example of
reading a complete line. This program is very similar to
those we have been studying except for the addition of a new
quantity, the NULL.
We are using "fgets" which reads in an entire line,
including the newline character into a buffer. The buffer
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Chapter 10 - File Input/Output
to be read into is the first argument in the function call,
and the maximum number of characters to read is the second
argument, followed by the file pointer. This function will
read characters into the input buffer until it either finds
a newline character, or it reads the maximum number of
characters allowed minus one. It leaves one character for
the end of string NULL character. In addition, if it finds
an EOF, it will return a value of NULL. In our example,
when the EOF is found, the pointer "c" will be assigned the
value of NULL. NULL is defined as zero in your "stdio.h"
file.
When we find that "c" has been assigned the value of
NULL, we can stop processing data, but we must check before
we print just like in the last program.
Last of course, we close the file.
HOW TO USE A VARIABLE FILENAME
Load and display the file ANYFILE.C for an example of
reading from any file. This program asks the user for the
filename desired, reads in the filename and opens that file
for reading. The entire file is then read and displayed on
the monitor. It should pose no problems to your
understanding so no additional comments will be made.
Compile and run this program. When it requests a
filename, enter the name and extension of any text file
available, even one of the example C programs.
HOW DO WE PRINT?
Load the last example file in this chapter, the one
named PRINTDAT.C for an example of how to print. This
program should not present any surprises to you so we will
move very quickly through it.
Once again, we open TENLINES.TXT for reading and we
open PRN for writing. Printing is identical to writing data
to a disk file except that we use a standard name for the
filename. There are no definite standards as far as the
name or names to be used for the printer, but some of the
usual names are, "PRN", "LPT", "LPT1", and "LPT2". Check
your documentation for your particular implementation.
Some of the newest compilers use a predefined file
pointer such as "stdprn" for the print file. Once again,
check your documentation.
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Chapter 10 - File Input/Output
The program is simply a loop in which a character is
read, and if it is not the EOF, it is displayed and printed.
When the EOF is found, the input file and the printer output
files are both closed.
You can now erase TENLINES.TXT from your disk. We will
not be using it in any of the later chapters.
PROGRAMMING EXERCISES
1. Write a program that will prompt for a filename for a
read file, prompt for a filename for a write file, and
open both plus a file to the printer. Enter a loop that
will read a character, and output it to the file, the
printer, and the monitor. Stop at EOF.
2. Prompt for a filename to read. Read the file a line at
a time and display it on the monitor with line numbers.
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