Writing Data From Files

WRITING DATA TO FILES

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

• fprintf()

• fputs()

• fputc()

 • fwrite()

In this section, we will read about these functions.

fprintf()

The fprintf() is used to write formatted output to stream. The syntax of the fprintf() can be given as

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

The function writes data that is formatted as specified by the format argument to the specified stream. After the format parameter, the function can have as many additional arguments as specified in format.

The parameter format in the fprintf() is nothing but a C string that contains the text that has to be written on to the stream. Although not mandatory, the fprintf() can optionally contain format tags, that are replaced by the values specified in subsequent additional arguments and are formatted as requested.

Note

There must be enough arguments for format because if there are not, then result will be completely unpredictable. However, if by mistake you specify more number of arguments, the excess arguments will simply be ignored.

The prototype of the format tag can be given as

% [flags] [width] [.precision] [length] specifier

Each format specifier must begin with a % sign. The % sign is followed by:

Flags which specifies output justification such as decimal point, numerical sign, trailing zeros, or octal or hexa- decimal prefixes. Table 9.3 shows the different types of flags with their description.

Width specifies the minimum number of characters to print after being padded with zeros or blank spaces.

Precision specifies the maximum number of characters to print.

• For integer specifiers (d, i, o, u, x, X): precision flag specifies the minimum number of digits to be written. However, if the value to be written is shorter than this number, the result is padded with leading zeros. Otherwise, if the value is longer, it is not truncated.

• For character strings, precision specifies the maximum number of characters to be printed.

Length field can be explained as given in Table 9.4.

Specifier is used to define the type and the interpretation of the value of the corresponding argument.

The fprintf() may contain some additional parameters as well depending on the format string. Each argument must contain a value to be inserted instead of each% tag specified in the format parameter, if any. In other words, the number of arguments must be equal to the number of % tags that expect a value.

Look at the program given below which demonstrates the use of fprintf().

#include <stdio.h>

main()

{

FILE *fp;

int i;

char name [20];

float salary;

fp = fopen("Details.TXT", "w");

if (fp==NULL)

{

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

exit(1);

}

for (i=0;i<10;i++)

{

puts ("\n Enter your name: "); 

gets (name);

fflush(stdin);

puts("\n Enter your salary: ");                                                                   

scanf("%f", &salary);

fprintf(fp, " (%d) NAME: [%-10.10s] \t SALARY %5.2f", i, name, salary);

}

fclose(fp);

}

Programming Tip: If you open a file for writing using w mode, then the contents of the file will be deleted. If a file has to be used for reading as well as writing, then it must be opened in w+ mode.

Output

Enter your name: Aryan

Enter your salary: 50000

Enter your name: Anshita

Enter your salary: 65000

Enter your name: Saesha

Enter your salary: 70000

This example asks the user to enter the name and salary of 10 people. Each time the user enters the name and salary, the data read is written to Details. TXT. The names are written on new lines in the file. In this example, we have used three format tags:

• %d to specify a signed decimal integer.

• %-10.10s. Here - indicates that the characters must be left aligned. There can be minimum of 10 characters as well as maximum of 10 characters (.10) in the strings.

• %f to specify a floating point number.

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

fputs()

The opposite of fgets() is fputs (). The fputs () is used to write a line to a file. The syntax of fputs() can be given as

int fputs (const char *str, FILE *stream);

The fputs() writes the string pointed to by str to the stream pointed to by stream. On successful completion, fputs() returns 0. In case of any error, fputs() returns EOF.

#include <stdio.h>

main()

{

FILE *fp;

char feedback [100];

fp = fopen("Comments.TXT", "w");

if (fp==NULL)

{

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

exit (1);

}

printf("\n Provide feedback on this book: ");

gets (feedback);

fflush(stdin);

// feedback stored

fputs (feedback, fp);

fclose(fp);

}

Output

Provide feedback on this book: good

fputc()

The fputc() is just the opposite of fgetc () and is used to write a character to the stream.

int fputc(int c, FILE *stream);

The fputc() function will write the byte specified by C (converted to an unsigned char) to the output stream pointed to by stream. On successful completion, fputc() will return the value it has written. Otherwise, in case of error, the function will return EOF and the error indicator for the stream will be set.

#include <stdio.h>

main()

{

FILE *fp;

char feedback [100];

int i;

fp = fopen("Comments.TXT", "w");

if (fp==NULL)

{

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

exit (1);

}

printf("\n Provide feedback on this book: ");

gets (feedback);

for (i=0;i< feedback [i]; i++)

fputc (feedback [i], fp);

fclose(fp);

}

Output

Provide feedback on this book: good

Note

The standard file stdout is buffered in case the output unit is not the terminal. On the contrary, the standard file stderr is usually unbuffered. However, the settings of stdout and stderr can be changed using setbuf.

When an output stream is unbuffered, information appears on the destination device as soon as it is written. When it is buffered, characters are saved internally and then written out as a group. In order to force buffered characters to be output before the buffer is full, use the fflush().

Programming Tip: EOF is an integer type defined in stdio.h and has a value '–1'.

fwrite()

The fwrite() is used to write data to a file. The syntax of fwrite can be given as

int fwrite (const void *str, size t size, size_t count, FILE *stream);

The fwrite() function will write objects (number of objects will be specified by count) of size specified by size, from the array pointed to by ptr to the stream pointed to by stream.

The file-position indicator for the stream (if defined) will be advanced by the number of bytes successfully written. If an error occurs, the resulting value of the file- position indicator for the stream is unspecified.

On successful completion, the fwrite() function returns the number of objects successfully written. The number of objects will be less than count if an error is encountered. If size or count is 0, fwrite() will return o and the contents of the stream remains unchanged. In case of error, the error indicator for the stream will be set.

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

main (void)

{

FILE *fp;

size_t count;

char str[] = "GOOD MORNING";

fp = fopen("Welcome.txt", "wb");

if (fp==NULL)

{

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

exit(1);

}

count = fwrite(str, 1, strlen(str), fp);

printf("\n %d bytes were written to the file", count);

fclose(fp);

return 0;

}

Output

13 bytes were written to the file

Note

fwrite() can be used to write characters, integers, or structures to a file. However, fwrite() can be used only with files that are opened in binary mode.