Read Data From Files

READ DATA FROM FILES

C provides the following set of functions to read data from a file.

• fscanf()

• fgets()

• fgetc ()

• fread()

In this section, we will read about these functions.

fscanf ()

The fscanf() is used to read formatted data from the stream. The syntax of the fscanf() can be given

int fscanf (FILE *stream, const char *format,...);

The fscanf() is used to read data from the stream and store them according to the parameter format into the locations pointed by the additional arguments. However, these additional arguments must point to the objects that have already occupied memory. These objects are of type as specified by their corresponding format tag within the format string.

Similar to the format specifiers used in scanf(), in fscanf() also the format specifiers is a c string that begins with an initial percentage sign (%). The format specifier is used to specify the type and format of the data that has to be obtained from the stream and stored in the memory locations pointed by the additional arguments. The prototype of a format specifier can be given as

[=% [*] [width] [modifiers] type=], where

'*' is an optional argument that suppresses assignment of the input field. It indicates that data should be read from the stream and ignored (not stored in the memory location).

width specifies the maximum number of characters to be read. However, fewer characters will be read if the fscanf function encounters a white space or an unconvertible character.

modifiers can be h, 1, or L for the data pointed by the corresponding additional arguments. Modifier h is used for short int or unsigned short int, 1 is used for long int, unsigned long int, or double values. Finally, L is used for long double data values.

type specifies the type of data that has to be read. It also indicates how this data is expected to be read from the user.

The type specifiers for fscanf function are given in Table 9.2.

The fscanf function has some additional arguments. Each of the additional arguments must point to an object of the type specified by its corresponding tag within the format string, in the same order.

Note

The fscanf function is similar to the scanf function, except that the first argument of £scanf specifies a stream from which to read, whereas scanf can only read from standard input.

Let us look at an example which illustrates the use of fscanf (). Here, we will not give the complete program but just a partial program to demonstrate the use of fscanf().

#include <stdio.h>

main()

{

FILE *fp;

char name [80];

int roll_no;

fp = fopen("Student.DAT", "r");

if (fp==NULL)

{

printf("\n The file could not be opened");

exit (1);

printf("\n Enter the name and roll number of the student: ");

// READ FROM KEYBOARD

fscanf (stdin, "%s %d", name, &roll_no);

 /* read from keyboard */

printf("\n NAME: %s \t ROLL NUMBER = %d", name, roll_no);

// READ FROM FILE Student.DAT fscanf(fp, = "%s %d", name, &roll_no);

printf("\n NAME: %s \t ROLL NUMBER = %d", name, roll_no);

fclose(fp);

return 0;

}

}

Output

Enter the name and roll number of the student: 01 Zubin

NAME: Zubin ROLL NUMBER = 01

NAME: Goransh ROLL NUMBER = 03

Note

If you want to use fprintf () to write on the screen, then specify stdout instead of specifying any other file pointer.

fgets()

The function fgets() stands for file get string. The fgets() function is used to get a string from a stream. The syntax of fgets() can be given as

char *fgets (char *str, int size, FILE *stream);

The fgets() function reads at most one less than the number of characters specified by size (gets size - 1 characters) from the given stream and stores them in the string str. The fgets() terminates as soon as it encounters either a newline character, EOF, or any other error. However, if a newline character is encountered it is retained. When all the characters are read without any error, a '\0' character is appended to the end of the string.

The gets () and fgets() functions are almost same except that gets () has an infinite size and a stream of stdin. Another difference is that when gets() encounters a newline character, it does not retain it, i.e., the newline character (if any) is not stored in the string.

On successful completion, fgets() will return str. However, if the stream is at EOF, the EOF indicator for the stream will be set and fgets() will return a null pointer. In case, fgets() encounters any error while reading, the error indicator for the stream will be set and null pointer will be returned. Look at the program code given below which demonstrates the use of fgets().

#include <stdio.h>

main ()

{

FILE *fp;

char str[80];

fp = fopen("ABC.DAT", "r");

if (fp==NULL)

{

printf("\n The file could not be opened");

exit(1);

}

/* the file will read 79 characters

during each iteration and prints them on the screen */

while (fgets (str, 80, fp) != NULL)

printf("\n %s", str);

printf("\n\n File Read. Now closing the file");

fclose(fp);

return 0;

}

Output

Abdceweeferrttet gfejjherroiew tjketjer

fddfgdfgfd

File Read. Now closing the file

fgetc ()

The fgetc() function returns the next character from stream, EOF if the end of file is reached, or if there is an error. The syntax of fgetc () can be given as

int fgetc (FILE *stream);

fgetc () returns the character read as an int or return EOF to indicate an error or end of file.

fgetc() reads a single character from the current position of a file (file associated with stream). After reading the character, the function increments the associated file pointer (if defined) to point to the next character. However, if the stream has already reached the end of file, the EOF indicator for the stream is set. Look at the following program code which demonstrates the use of fgets() function.

#include <stdio.h>

main()

{

FILE *fp;

char str[80];

int i, ch;

fp = fopen("Program. C", "r");

if (fp = =NULL)

{

printf("\n The file could not be opened");

exit(1);

}

// Read 79 characters and store them in str

ch= fgetc (fp);

for (i=0; (i < 79) && (feof (fp) = = 0); i++)

{

str[i] = (char) ch;

ch = fgetc (stream);

// reads character by character

}

str[i] = '\0';

// append the string with a null character

printf("\n %s", str);

}

fclose(fp);

}

Output

#include <stdio.h>

main().....

displays either first 79 characters or less characters if the file contains less than

79 characters

The feof() is used to detect the end of file. We will read more on this function later in this chapter.

fread()

The fread() function is used to read data from a file. Its syntax can be given as

int fread(void *str, size_t size, size_t num, FILE *stream);

The function fread() reads num number of objects (where each object is size bytes) and places them into the array pointed to by str. The data is read from the given input stream.

Upon successful completion, fread() returns the number of bytes successfully read. The number of objects will be less than num if a read error or end-of-file is encountered. If size or num is 0, fread() will return 0 and the contents of str and the state of the stream remain unchanged. In case of error, the error indicator for the stream will be set.

The fread() function advances the file position indicator for the stream by the number of bytes read.

Note

The fread() function does not distinguish between end-of-file and error. The programmer must use feof and ferror to determine which of the two has occurred.

Look at the program given below which illustrates the use of fread().

#include <stdio.h>

main ()

{

FILE *fp;

char str[11];

fp = fopen("Letter.TXT", "r+");

if (fp==NULL)

{

printf("\n The file could not be opened");

exit(1);

}

fread (str, 1, 10, fp);

/* In the str 10 objects of byte are read from the file pointd by fp */

str [10]='\0';

printf("\n First 9 characters of the file are: %s", str);

fclose(fp);

}

Output

First 9 characters of the file are: Hello how

Since fread() returns the number of bytes successfully read, we can also modify the above program to print the number of bytes read. This would be helpful to know how many characters were read.

#include <stdio.h>

main ()

{

FILE *fp;

char str[80];

size_t bytes_read;

fp = fopen("Letter.TXT", "r+");

if (fp==NULL)

printf("\n The file could not be opened");

exit(1);

}

bytes_read = fread (str, 1, 79, fp);

str [bytes_read+1]='\0';

// explicitly store null character at the maths end of str

printf("\n First %d characters of the file are: %s", bytes_read, str);

fclose(fp);

}

Output

Output will depend on the contents of the file. Assuming 14 characters were read, the output can be given as Hello how r u?

This program assumes that you have created a file Letter.TXT that conatins 14 characters which has been displayed above

The function fread() does not check for overflow in the receiving area of memory. It is the programmer's job to ensure that the memory pointed to by 'str' must be large enough to hold the number of objects being read.

If you have opened a stream for updating and later you want to switch from reading to writing or vice versa, you must first use the fseek () or rewind () function. However, if you have been reading and have reached end-of-file, then you can immediately switch to writing. We will discuss the fseek() and rewind functions later in this chapter.