Files

Data Files

Many applications require that information be written to or read from an auxiliary memory device. Such information is stored on the memory device in the form of data files. Thus data files allow us to store information permanently, to access and alter that information whenever necessary. In C, an extensive set of library functions is available for creating and processing data files.

There are two different types of data files, called stream-oriented (or standard) data files, and system-oriented (or low-level) data files. Stream-oriented data files are generally easier to work with and are therefore more commonly used.

Stream-oriented data files can be subdivided into two categories: text files and unformatted data files. The text file consists of consecutive characters. These characters can be interpreted as individual data items, or as components of strings or numbers. The manner in which these characters are interpreted is determined either by the particular library functions used to transfer the information, or by format specifications within the library function.

The unformatted data files organize data into blocks containing contiguous bytes of information. These blocks represent more complex data structures, such as arrays and structures. A separate set of library function is available for processing these data files. These library functions provide single instructions that can transfer entire arrays or structures to or from data files.

System-oriented data files are more closely related to the computer’s operating system than stream-oriented data files. They are somewhat more complicated to work with though their use may be more efficient for certain kinds of applications. A separate set of procedures, with accompanying library functions, is required to process such data files.

There is also another type of file called binary file. These files are useful to handle file containing machine language contents, for example .exe or .com files. The files which are stored in binary format differ significantly from data files.

Why Data Files

Programs without data files accept input data from the keyboard at the time of execution and write output to the monitor. This type of I/O is called console I/O. The main limitation of this approach is that it is very difficult to handle a large volume of data. For large volume of data,

• It takes a lot of time to enter the entire data.
• If the same data is to be entered again at some later stage, again we have to enter the same data.
• If we exit from the program, all the data will evaporate.

Using data files enable us to save our data permanently to disk and retrieve them whenever we want.

Opening and Closing a Data file

When working with a stream-oriented data file, the first step is to establish a buffer area, where information is temporarily stored while being transferred between the computer’s memory and the data file. This buffer area allows information to be read from or written to the data file more rapidly than would otherwise be possible. This buffer area is established by writing

FILE *ptvar;

Where FILE (uppercase letters required) is a special structure type that established the buffer area, and ptvar is a pointer variable that indicates the beginning of the buffer area. The pointer ptvar is often referred to as a stream pointer, file pointer or simply stream.

A data file must be opened before it can be created or processed. This associates the file name with the buffer area. It also specifies how the data file will be utilized, i.e., as a read-only file, a write-only file, or a read/write file. The library function fopen is used to open a file. This function is typically written as

ptvar = fopen(file-name, file-type);

Where file-name and file-type are strings that represent the name of the data file and the manner in which the data file will be utilized. The file-type must be one of the strings as shown below. These are also called file opening modes.

File-Type	Meaning
“r”	Open an existing file for reading only.
“w”	Open a new file for writing only. If a file with the specified file name currently exists, it will be destroyed and a new file created in its place.
“a”	Open an existing file for appending (i.e., for adding new information at the end of the file). A new file will be created if the file with the specified file name does not exist.
“r+”	Open an existing file for both reading and writing.
“w+”	Open a new file for both reading and writing. If a file with the specified file name currently exists, it will be destroyed and a new file created in its place.
“a+”	Open an existing file for both reading and appending. A new file will be created if the file with the specified file name does not exist.

The fopen function returns a pointer to the beginning of the buffer area associated with the file. A NULL value is returned if the file cannot be opened for some reason.

Finally, a data file must be closed at the end of the program. This can be accomplished with the library function fclose. The syntax is

fclose(ptvar);

Reading and Writing a Data file

Normally files are used for one of the two purposes – reading or writing. To write data into a data file, we use fprintf function. For example, the program below writes characters A to Z each in a separate line in an output file named “output.dat”.

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

            int i;

            FILE *fp;

            fp = fopen("output.dat", "w");

            if(fp == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        for(i=65; i <= 90; i++)

                                    fprintf(fp, "%c\n", i); //writes data to the data file

                        fclose(fp);

            }

            getch();

}

To read data from a data file, we use fscanf function. For example, the following program reads the contents of a data file on a character-by-character basis, and displays them on the screen.

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

            char ch;

            FILE *fp;

            fp = fopen("output.dat", "r");

            if(fp == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        fscanf(fp, "%c", &ch);

                        while(!feof(fp)) { //continues while loop until end of file

                                    printf("%c", ch);

                                    fscanf(fp, "%c", &ch);

                        }

                        fclose(fp);

            }

            getch();

}

If character inputs are to be read from the file, getc and fgetc can also be used instead of fscanf statement. Similarly, to write data into a file, we can use putc and fputc instead of fprintf statement. For example, the program below uses getc and putc to copy contents from a file named “input.dat” into a file named “output.dat”. You can also use fgetc and fputc instead of getc and putc.

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

            FILE *fpin, *fpout;

            char ch;

            fpin = fopen("input.dat", "r");

            if(fpin == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        fpout = fopen("output.dat", "w");

                        if(fpout == NULL)

                                    printf("\nERROR - Cannot create the destination file\n");

                        else {

                                    while(!feof(fpin)) {

                                                ch = getc(fpin);

                                                putc(ch,fpout);

                                    }

                        }

            }

            fcloseall(); //close all open files

            getch();

}

Data files consisting entirely of strings can often be created and read more easily with programs that utilize special string-oriented library functions. We can use fgets, and fputs to read and write strings with data files. For example,

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

            FILE *fpin, *fpout;

            char str1[20];

            char *str2;

            fpin = fopen("input.dat", "r");

            if(fpin == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        fpout = fopen("output.dat", "w");

                        if(fpout == NULL)

                                    printf("\nERROR - Cannot create the destination file\n");

                        else {

                                    while(!feof(fpin)) {

                                                str2 = fgets(str1,10,fpin);

                                                fputs(str2,fpout);

                                    }

                        }

            }

            fcloseall(); //close all open files

            getch();

}

Example – Program to read all numbers from the input file “values.dat” and store the average of these numbers in an output file named as “average.res.”

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

            FILE *fpin, *fpout;

            float val, avg, sum = 0;

            int count = 0;

            fpin = fopen("values.dat", "r");

            if(fpin == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        while(!feof(fpin)) {

                                    fscanf(fpin, "%f", &val);

                                    sum = sum + val;

                                    count++;

                        }

            }

            avg = sum / count;

            fpout = fopen("average.res", "w");

            if(fpout == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else

                        fprintf(fpout, "Average=%f", avg);

            fclose(fpin);

            fclose(fpout);

            getch();

}

Unformatted Data files

Some applications involve the use of data files to store blocks of data, where each block consists of a fixed number of contiguous bytes. Each block will generally represent a complex data structure, such as a structure or an array. The library functions fread and fwrite are intended to be used in situations of this type. These functions are often referred to as unformatted read and write functions. Similarly, data files of this type are often referred to as unformatted data files.

Each of these functions requires four arguments: a pointer to the data block, the size of the data block, the number of data blocks being transferred, and the stream pointer. For example,

fread(&customer, sizeof(struct account), 1, fpt);

fwrite(&customer, sizeof(struct account), 1, fpt);

The program given below is used to write customer data (account number, account type, name, and balance) to a data file named “customer.dat”.

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

struct account {

                        int acct_no;

                        char acct_type;

                        char name[10];

                        float balance;

            };

            struct account customer;

            FILE *fpt;

            char ch;

            fpt = fopen("customer.dat", "w");

            if(fpt == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        printf("Enter customers' information:\n");

                        do {

                                    printf("Enter account number:");

                                    scanf("%d",&customer.acct_no);

                                    printf("Enter account type:");

                                    scanf(" %c",&customer.acct_type);

                                    printf("Enter name:");

                                    scanf(" %[^\n]",customer.name);

                                    printf("Enter balance:");

                                    scanf("%f",&customer.balance);

                                    fwrite(&customer,sizeof(struct account),1,fpt);

                                    printf("Do you wnat to add another record?(y/n)");

                                    scanf(" %c",&ch);

                        } while(ch != 'n');

            }

            fclose(fpt);

}

The program given below is used to read customer data (account number, account type, name, and balance) from a data file named “customer.dat”.

#include<stdio.h>

#include<conio.h>

#define NULL 0

void main() {

struct account {

                        int acct_no;

                        char acct_type;

                        char name[10];

                        float balance;

            };

            struct account customer;

            FILE *fpt;

            fpt = fopen("customer.dat", "r");

            if(fpt == NULL)

                        printf("\nERROR - Cannot open the destination file\n");

            else {

                        fread(&customer,sizeof(struct account),1,fpt);

                        while(!feof(fpt)) {

                                    printf("Account no = %d\n",customer.acct_no);

                                    printf("Account type = %c\n",customer.acct_type);

                                    printf("Customer name = %s\n",customer.name);

                                    printf("Balance = %f\n",customer.balance);

                                    fread(&customer,sizeof(struct account),1,fpt);

                        }

            }

            fclose(fpt);

            getch();

}