COMPUTER PROGRAMMING (CT 101) – Chapter 9

9.1 Binary and Text Files in C

• Text Files: Store data as a sequence of human-readable characters. Special translations, like converting \n to a carriage return-line feed (CR-LF) on Windows, happen automatically. They are easy to create and read but less efficient for storing complex data.
• Binary Files: Store data as a raw sequence of bytes, exactly as it is in the computer’s memory. They are not human-readable but are more efficient and compact for storing numbers, structures, and other non-textual data. No character translation occurs.

9.2 File Opening Modes

The fopen() function uses a mode string to determine how a file should be opened.

9.3 Defining, Opening, and Closing a File

1. Define a File Pointer: A special pointer of type FILE is needed to interact with a file.
   FILE *fp;
2. Open the File: Use the fopen() function, which returns a FILE pointer on success or NULL on failure.
   fp = fopen("data.txt", "w");
3. Close the File: Use the fclose() function to save changes and release the file. This is a crucial step.
   fclose(fp);

9.4 Input-Output Operations on Files

Character I/O (fgetc, fputc)

For reading/writing one character at a time.

• fgetc(fp): Reads a character.
• fputc(char, fp): Writes a character.

String I/O (fgets, fputs)

For reading/writing lines of text.

• fgets(str, size, fp): Reads a line.
• fputs(str, fp): Writes a string.

Formatted I/O (fscanf, fprintf)

Work like scanf and printf but for files.

• fprintf(fp, "...", ...): Writes formatted data.
• fscanf(fp, "...", ...): Reads formatted data.

Record I/O (fread, fwrite)

For reading/writing blocks of binary data, like structures.

• fwrite(&var, size, n, fp): Writes n items of size from &var.
• fread(&var, size, n, fp): Reads n items of size into &var.

9.5 Overview of Random File Access

Allows you to move the file pointer to any position in the file without reading sequentially.

• ftell(fp): Returns the current position of the file pointer (in bytes from the start).
• rewind(fp): Moves the file pointer back to the beginning of the file.
• fseek(fp, offset, position): Moves the pointer.
  • offset: Number of bytes to move.
  • position: Can be SEEK_SET (from start), SEEK_CUR (from current position), or SEEK_END (from end).

9.6 Error Handling

It’s vital to check if a file was opened successfully. fopen() returns NULL if it fails. Always check for this to prevent your program from crashing.

FILE *fp;
fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    printf("Error: Could not open file.\n");
    exit(1); // Terminate the program
}

Assignment Solution: Author Database Program

This program demonstrates creating a structure, writing multiple records to a binary file, and using fseek() for random access to read a specific record.

#include <stdio.h>
#include <stdlib.h>

// Structure to hold author details
struct Author {
    char name[100];
    char nationality[50];
    int num_books;
};

int main() {
    FILE *fp;
    struct Author author;
    char choice;
    int record_num, total_records;

    fp = fopen("authors.dat", "ab+");

    if (fp == NULL) {
        printf("Error: Cannot open file.\n");
        exit(1);
    }

    do {
        printf("\n--- Enter New Author Details ---\n");
        while(getchar() != '\n'); // Clear input buffer
        
        printf("Enter author's name: ");
        fgets(author.name, sizeof(author.name), stdin);

        printf("Enter author's nationality: ");
        fgets(author.nationality, sizeof(author.nationality), stdin);

        printf("Enter number of books published: ");
        scanf("%d", &author.num_books);

        fwrite(&author, sizeof(struct Author), 1, fp);

        printf("\nAdd another record? (y/n): ");
        scanf(" %c", &choice);

    } while (choice == 'y' || choice == 'Y');

    printf("\n--- Retrieve a Specific Record ---\n");

    fseek(fp, 0, SEEK_END);
    total_records = ftell(fp) / sizeof(struct Author);
    printf("There are %d records in the file.\n", total_records);

    if (total_records > 0) {
        printf("Enter record number to display (1 to %d): ", total_records);
        scanf("%d", &record_num);

        if (record_num >= 1 && record_num <= total_records) {
            fseek(fp, (record_num - 1) * sizeof(struct Author), SEEK_SET);
            fread(&author, sizeof(struct Author), 1, fp);

            printf("\n--- Record #%d ---\n", record_num);
            printf("Author Name: %s", author.name);
            printf("Nationality: %s", author.nationality);
            printf("Books Published: %d\n", author.num_books);
        } else {
            printf("Error: Invalid record number.\n");
        }
    }

    fclose(fp);
    printf("\nFile closed.\n");

    return 0;
}