C programming session 11

of 37Ver. 1.0
Programming in C
In this session, you will learn to:
Read and write contents in a file
Use random access in files
Objectives

of 37Ver. 1.0
Programming in C
Reading and Writing Contents in a File
File inputs-outputs is similar to input from/to the terminal.
Files are treated as streams of characters.
Function are available for single character as well as
multiple character input-output from/to files.

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Programming in C
Opening Files
A file needs to be opened to read or to write contents in it.
The fopen() function is used to open a file.
The fopen() function returns a pointer of the FILE type
data.
The fopen() function opens a file in a specific access
mode.
The various modes in which a file can be opened are:
r - Read-Only Mode
w - Write-Only Mode
a - Append Mode
r+ - Read + Write Mode
w+ - Write + Read Mode
a+ - Read + Append Mode

of 37Ver. 1.0
Programming in C
The FILE type pointer is:
Returned when a file is opened by using the fopen() function.
Used to manipulate a file.
Used to check whether a file has opened successfully.
The stdin, stdout, and stderr FILE pointers refer to
the standard input device (keyboard) and standard output
and error device (VDU).
FILE Type Pointers

of 37Ver. 1.0
Programming in C
The exit() Function
The exit() Function:
Is used to terminate a program execution.
Is used as shown in the following code snippet:
if (argc ! = 3)
{
print (“invalid arguments n”);
exit ();
}

of 37Ver. 1.0
Programming in C
Character Input-Output with Files
The functions used for character input-output with files are:
fgetc(): Reads one character at a time from a file, assigns it
to a character variable, and moves the file pointer to the next
character. It returns an integer type of value.
fputc(): Writes one character at a time in a file.

of 37Ver. 1.0
Programming in C
Closing Files
The fclose() function is used to close files.
Closing the file release the resources.
The syntax of the fclose() function is:
fclose (ptr1);
Where ptr1 is a FILE pointer.

of 37Ver. 1.0
Programming in C
Practice: 6.1
1. What does the following code do?
while((c = fgetc (fp)) != EOF) {
if ((c >= ‘a’) && (c <= ‘z’))
c -= 32;
fputc(c, stdout); }
1. Write a program called append, which appends the contents
of the first file to the second file specified on the command
line. The program should also terminate in the following
situations:
a. 2 arguments are not specified on the command line. In this
case, the following message must be displayed:
Usage: append file1 file2
b. In case the file to be read cannot be opened, the following
message may be displayed:
Cannot open input file

of 37Ver. 1.0
Programming in C
Solution:
Microsoft Word
Document
Practice: 6.1 (Contd.)

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Programming in C
Practice: 6.2
1. Point out the errors in the following code:
a. /* this program creates the file emp.dat */
main() {
FILE point;
fopen(“w”, “emp.dat”);
:
fclose(emp.dat);
}
b. /* this program reads the file emp.dat */
main() {
#include<stdio.h>
file*ptr;
ptr = fopen(emp.dat);
:
ptr= fclose();
}

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Programming in C
2. Given the following statements of a C program:
fopen(“man.txt”, “r”);
fclose(fileptr);
What will the FILE declaration statement of this program
be?
3. Point out the error(s) in the following code:
#include<stdio.h>
main() {
char emp;
FILE *pointer1;
pointer1= fopen(“man1.txt”,”w”);
while((inp = fgetc(pointer1)) != eof) {
printf(“?%c”, inp);
} }

of 37Ver. 1.0
Programming in C
4. The copy command of DOS copies the contents of the first
file named on the command line to the second file. Make
appropriate changes to the file-copy program so that it
works identical to the copy command.

of 37Ver. 1.0
Programming in C
Solution:
Microsoft Word
Document

of 37Ver. 1.0
Programming in C
Line Input and Output with Files
The functions used for line input and output with files are:
fgets():
Is used to read number of specified characters from a stream.
Reads number of characters specified – 1 characters.
Has the following syntax:
fgets(str, 181, ptr1);
Str – Character array for storing the string
181 – Length of the string to be read
ptr1- FILE pointer
fputs():
Is used to output number of specified characters to a stream.
fputs(str, ptr1);
Str – Character array to be written
ptr1- FILE pointer

of 37Ver. 1.0
Programming in C
Practice: 6.3
1. State whether True or False:
Files created using the fputs() function will always have
records of equal length.
2. Consider the following C statement to input a record from a
file called number-list:
fgets (line, MAXLEN, file_ind);
Given that MAXLEN is a #define and that all lines in the file
number-list are 25 characters long what will the declaration
statements for the parameters of fgets() be?
3. Assume that the file number_list contains the following
records:
120
1305

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Programming in C
Given that the file has been opened and the first input
statement executed is as follows:
fgets(line, 3, file_ind);
Which of the following will the array called line contain?
a. 1 followed by 0.
b. 12 followed by 0.
c. 120 followed by 0.
4. Match the following functions with the values they can
return:
a. fgets() 1. NULL
b. fgetc() 2. EOF
c. fopen() 3. FILE type pointer

of 37Ver. 1.0
Programming in C
If a function can return more than one type of these values,
state the conditions under which the values are returned.
5. A utility called hprint has to be written in C, which will
allow a user to display, on screen, any number of lines from
the beginning of any file. The user has to specify both the
number of lines and the file name on the command line in
the following format:
hprint number file-name
The maximum line length is 80 characters. The program
should handle possible errors.

of 37Ver. 1.0
Programming in C
Solution:
1. False. fputs() writes the contents of a string onto a file. So
even if a string has size 100, but contains only 20 characters
before a 0, only 20 characters get written.
2. The declarations are:
#define MAXLEN 26/* macro definition outside
main() */
char line[26];
3. b. fgets() will read either 3 - 1 characters , i.e. 2 characters,
or until it comes across a newline character. Since the newline
occurs after the third character, it will read in 2 characters from
the first record.

of 37Ver. 1.0
Programming in C
4. a. 1
b. 2
c. 1 and 3. (NULL in case file cannot be opened; FILE type
pointer in case of successful open)
5. The answer to this practice will be discussed in class. Work
out your solution.

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Programming in C
Formatted Input and Output with Files
The functions for formatted input and output with files are:
fscanf():
Scans and formats input from a stream.
Is similar to scanf().
int fscanf(FILE *Stream, const char
*format[,address,..]);
fprintf():
Sends formatted output to a stream.
Is similar to printf().
int fprintf(FILE *Stream, const char
*format[,address,..]);

of 37Ver. 1.0
Programming in C
Practice: 6.4
1. Rewrite the following printf() statement using the
function fprintf():
printf(“The test value is %d”, x);
2. The following statement is written to input 2 fields from the
keyboard:
scanf(“ %6s%d”, array, &num);
It is rewritten as:
fscanf(“%6s%d”, array, &num);
This statement is erroneous. Give the correct fscanf()
statement.

of 37Ver. 1.0
Programming in C
3. Write the appropriate statements to input fields from a
record of a file called alpha-doc, the first field being a
float value, and the second field a string of size 10. In
case the file does not have he required data, and the end-
of-file occurs, the following message should be displayed:
End of file encountered.

of 37Ver. 1.0
Programming in C
4. A utility called format is required to create a formatted report
from a file called manufact. This report is also to be stored
on disk with suitable report headings. The name of the file to
be created should be accepted during program execution.
The program should also ask for a report title, which should
appear after every 60 record of the file manufact.
The file manufact contains the following 3 fields separated
by space.
Field Size
Manufacturer Code 4
Name 20
Address 60
In the output file, the fields should be separated by one tab
character.

of 37Ver. 1.0
Programming in C
Solution:
Microsoft Word
Document

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Programming in C
Using Random Access in Files
A file can be accessed using sequential access or random
access.
In sequential access, the file is always accessed from the
beginning.
In random access the file can be accessed arbitrarily from
any position.

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Programming in C
The fseek () Function
The fseek() function:
Is used for repositioning the current position on a file opened
by the fopen() function.
rtn = fseek (file-pointer, offset, from-where);
Here:
int rtn is the value returned by fseek()(0 if successful and 1 if
unsuccessful).
file-pointer is the pointer to the file.
offset is the number of bytes that the current position will shift on a
file.
from-where is the position on the file from where the offset would be
effective.

of 37Ver. 1.0
Programming in C
The rewind () Function
The rewind() function:
Is used to reposition the current position to the beginning of a
file.
Is useful for reinitializing the current position on a file.
rewind(file-pointer);
Here:
file-pointer is the pointer returned by the function fopen().
After rewind() is executed, current position is always 1,
i.e. beginning of file.

of 37Ver. 1.0
Programming in C
Practice: 6.5
1. Write the equivalent of the function rewind() using
fseek().
2. Assume the following representation of the first 30 bytes of
a file.

of 37Ver. 1.0
Programming in C
What will the current position on the file be after the
following instructions are performed in sequence?
a. fp = fopen ("FOR DEMO.DAT", “r”);
b. fseek(fp, 29L, 1);
c. rewind(fp);
d. fgets(buffer, 20L, fp);
e. fseek(fp, 4L, 1);

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Programming in C
Solution:
1. fseek(fp, 0L, 0);
2. The following current positions are relative to the beginning of
the file:
a. 1
b. 30
c. 1
d. 20
e. 24

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Programming in C
Practice: 6.6
1. Write a function to update the field balance in the file
SAVINGS.DAT based on the following information.
If balance is Increment balance by
< Rs 2000.00 Rs 150.50
Between Rs. 2000.00 Rs 200.00
and Rs 5000.00
<Rs 5000.00 Rs 300.40
The structure of the file SAVINGS.DAT is as follows.
Account number Account holder's name Balance
(5 bytes) (20 bytes) (5 bytes)

of 37Ver. 1.0
Programming in C
Solution:
Microsoft Word
Document

of 37Ver. 1.0
Programming in C
Practice: 6.7
1. Go through the following program called inpcopy.c and its
error listing on compilation and then correct the program:
1 #include <stdio.h>
2 main()
3 {
4 file fp;
5 char c;
6
7 fp = fopen(“file”, w);
8
9 while (( c = fgetc(stdin)) != EOF)
10 fputc(c,fp);
11
12 fclose(fp);
13 }

of 37Ver. 1.0
Programming in C
Error listing:
"inpcopy/.c", line 4: file undefined
"inpcopy/.c". line 4: syntax error
"inpcopy/.c", line 7: fp undefined
"inpcopy/.c", line 7: w undefined
"inpcopy/.c", line 7: learning: illegal
pointer/integer combination, op = "inpcopy/.c",
line 9: c undefined

of 37Ver. 1.0
Programming in C
Solution:
1. Work out for your answer. The solution will be discussed in the
classroom session.

of 37Ver. 1.0
Programming in C
Summary
In this session, you learned that:
C treats file input-output in much the same way as input-output
from/to the terminal.
A file needs to be opened to read or to write contents in it.
The fopen() function is used to open a file.
C allows a number of modes in which a file can be opened.
When a file is opened by using the fopen() function, it
returns a pointer that has to be stored in a FILE type pointer.
This FILE type pointer is used to manipulate a file.
The exit() function is used to terminate program execution.
The fgetc() and fputc() functions are used for character
input-output in files.
After completing the I/O operations on the file, it should be
closed to releases the resources.

of 37Ver. 1.0
Programming in C
Summary (Contd.)
The fclose() function is used to close a file.
The fgets() and fputs() functions are used for string
input-output in files.
The fscanf() and fprintf() functions are used for
formatted input-output in files.
In sequential access, the file is always accessed from the
beginning.
In random access the file can be accessed arbitrarily from any
position.
C provides the fseek() function for random access.
The function rewind() is used to reposition the current
position to the beginning of a file.

C programming session 11

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