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

Text File | 1987-11-21 | 18KB | 455 lines

                       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. There are actually additional attributes available  in
        C to allow more flexible I/O.  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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                              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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                  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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        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 can use a "char" variable for this purpose.
        There is a problem that could develop here if we happened to
        use  an "unsigned char" however, because C returns  a  minus
        one for an EOF which an "unsigned char" type variable is not
        capable of containing.  An "unsigned char" type variable can
        only have the values of zero to 255, so it will return a 255
        for a minus one.  This is a very frustrating problem to  try
        to  find.   The  program can never find  the  EOF  and  will
        therefore  never  terminate  the  loop.   This  is  easy  to
        prevent,  always  use  an "char" type variable  for  use  in
        returning an EOF.

             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
        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".


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                          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.
        Note  that some compilers apparently clear the buffer  after
        printing  so you may get an extra blank line instead of  two
        lines with "lines." on them.

             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 that we read a  complete
        line.

             We  are  using "fgets" which reads in an  entire  line,
        including  the newline character into a buffer.   The buffer


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        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.  Most C compilers use the reserved  "filename"  of
        "PRN" that instructs the compiler to send the output to  the
        printer.   There are other names that are used  occationally
        such  as "LPT", "LPT1", or "LPT2".  Check the  documentation
        for your particular compiler.

             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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             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.  Note that good programming  practice
        would include checking both file pointers to assure that the
        files were opened properly.

             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.

        3.   Modify ANYFIE.C to test if the file exists and print  a
             message  if it doesn't.  Use a method similar  to  that
             used in READCHAR.C.




























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