File Handling in C: Modes, Text vs Binary, Operations & Real-World Applications

Core Concepts: Files in C

1. What is a File?

A file is a named collection of related data stored on secondary memory (e.g., hard disk, USB). Files persist even after the program terminates, unlike variables stored in RAM.

Why use files?

• Persistence: Save data for later use (e.g., user accounts, transaction history).
• Efficiency: Faster access than user input/output for large datasets.
• Sharing: Multiple programs can read/write the same file (e.g., student.txt for a university system).

2. Types of Files

A. Text Files vs. Binary Files

Visual: Text vs. Binary File Structure

In the Real World

1. eSewa Transaction Logs

   • Idea Used: Text files (eSewa_transactions.txt) store every payment record (user ID, amount, timestamp) in ASCII format.
   • Why? Human-readable for audits; simple to parse for generating receipts.
   • Code Tie-In: Later, you’ll write a program to filter transactions by date (using fscanf and fprintf).

2. Ncell Customer Database

   • Idea Used: Binary files (customer_data.bin) store SIM records (phone number, balance, usage history) as raw bytes.
   • Why? Faster access for real-time balance checks; compact storage for millions of records.

3. Daraz Order Queue

   • Idea Used: Temporary text files (pending_orders.txt) manage unfulfilled orders. The system reads/writes to this file when orders are placed or shipped.
   • Worked Example: Simulate Daraz’s order processing:

     // Write an order to pending_orders.txt
     FILE *fp = fopen("pending_orders.txt", "a");
     fprintf(fp, "OrderID: %d, User: %s, Status: Pending\n", 101, "user123");
     fclose(fp);
     

     Trace:

     Step	File Content (pending_orders.txt)
     Initial	(empty)
     After Step	OrderID: 101, User: user123, Status: Pending

3. File Opening Modes

C uses fopen() to open files with specific modes. Memorize these 6 modes:

Visual: File Modes in Action

flowchart TD
    A["fopen(\"data.txt\", \"w\")"] -->|"Truncates"| B["Empty File"]
    A --> C["Creates if missing"]
    D["fopen(\"data.txt\", \"a\")"] -->|"Appends"| E["Existing Content\nNew Data"]
    F["fopen(\"data.txt\", \"r+\")"] -->|"Read/Write"| G["Existing Content\nModified"]

4. File Operations

A. Opening and Closing Files

#include <stdio.h>
int main() {
    FILE *fp;
    fp = fopen("example.txt", "r");  // Open for reading
    if (fp == NULL) {
        printf("Error opening file!");
        return 1;
    }
    // File operations here...
    fclose(fp);  // Always close to free resources
    return 0;
}

Trace:

B. Reading/Writing Data

1. Character-by-Character (fgetc, fputc)

char ch;
ch = fgetc(fp);  // Reads 1 character
fputc('A', fp);  // Writes 1 character

2. Line-by-Line (fgets, fputs)

char line[100];
fgets(line, 100, fp);  // Reads a line (including newline)
fputs("Hello\n", fp);  // Writes a line

3. Formatted I/O (fscanf, fprintf)

int id;
char name[50];
fscanf(fp, "%d %s", &id, name);  // Reads formatted data
fprintf(fp, "%d %s", 101, "Alice"); // Writes formatted data

Visual: Reading a Student Record

C. Binary File Operations

For binary files, use:

• fread() / fwrite() for raw bytes.
• No formatting (e.g., no %d for integers).

Example: Storing a Struct in Binary

typedef struct {
    int id;
    char name[50];
} Student;

Student s = {101, "Alice"};
FILE *fp = fopen("students.bin", "wb");
fwrite(&s, sizeof(Student), 1, fp);  // Write binary
fclose(fp);

Trace:

5. Error Handling

Always check if fopen() returns NULL:

FILE *fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    perror("Error");  // Prints system error (e.g., "No such file")
    return 1;
}

Common Errors:

• ENOENT: File does not exist (use "a" mode to create it).
• EACCES: Permission denied (check file permissions).

6. Practical Example: Filtering Prime Numbers

Problem: Given numbers.txt with integers, write a program to extract primes into primes.txt.

Solution:

#include <stdio.h>
#include <stdbool.h>

bool isPrime(int n) {
    if (n <= 1) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int main() {
    FILE *in = fopen("numbers.txt", "r");
    FILE *out = fopen("primes.txt", "w");
    int num;

    while (fscanf(in, "%d", &num) == 1) {
        if (isPrime(num)) {
            fprintf(out, "%d ", num);
        }
    }
    fclose(in);
    fclose(out);
    return 0;
}

Trace:

Real-World Tie-In:

• NEPSE Stock Data: Imagine stock_prices.txt contains daily closing prices. Your program could filter and save only days when the price was above a threshold (e.g., >2000).

7. Common Pitfalls & Best Practices

Exam Tip

1. Define Modes Clearly: In exams, list all 6 modes with examples (e.g., "w" truncates, "a" appends).
2. Code Structure:
   • Always include #include <stdio.h>.
   • Check fopen() return value.
   • Close files with fclose().
3. Common Questions:
   • Copy a file: Use fgetc/fputc in a loop.
   • Delete a record: Read all lines except the target, rewrite the file.
   • Binary vs. text: Compare storage, speed, and use cases (see table above).
4. Trace Tables: For programs like prime filtering, show the state of files before and after processing.

Example Answer Snippet (for "delete a record"):

// Delete record with ID=101 from student.txt
FILE *temp = fopen("temp.txt", "w");
FILE *in = fopen("student.txt", "r");
int id; char name[50];

while (fscanf(in, "%d %s", &id, name) == 2) {
    if (id != 101) {  // Skip record to delete
        fprintf(temp, "%d %s\n", id, name);
    }
}
fclose(in); fclose(temp);
remove("student.txt"); rename("temp.txt", "student.txt");