Week 10: File Input and Output (File I/O)

Overview

Welcome to Week 10 of C programming! This week, we will learn how to work with files in C using the standard file I/O library. By the end of this tutorial, you will:

• Open and close files using fopen() and fclose()
• Read from and write to text files using fprintf() and fscanf()
• Understand file positioning using fseek() and ftell()
• Use binary I/O functions fwrite() and fread()

Time Breakdown

• Text file write and read
• File position and stream control
• Binary file I/O
• Exercises & Q/A

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Examples

Example 1: Write to a File

Create a text file and write a line of text into it using fopen() and fprintf().

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }
    fprintf(fp, "Hello, File!\n");
    fclose(fp);
    return 0;
}

Example Output:

example.txt created with content: Hello, File!

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Example 2: Read from a File

Open a file in read mode and read one line of text using fgets().

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        printf("File not found.\n");
        return 1;
    }
    char str[100];
    fgets(str, sizeof(str), fp);
    printf("Read from file: %s", str);
    fclose(fp);
    return 0;
}

Example Output:

Read from file: Hello, File!

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Example 3: Use fseek and ftell

Move the file pointer to the end of a file and determine the file’s size using fseek() and ftell().

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) return 1;
    fseek(fp, 0, SEEK_END);
    long pos = ftell(fp);
    printf("File size: %ld bytes\n", pos);
    fclose(fp);
    return 0;
}

Example Output:

File size: 13 bytes

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Example 4: Save & Load Structure in Binary File

Store a structure in a binary file using fwrite()

#include <stdio.h>

struct Student {
    char name[20];
    int score;
};

int main(void) {
    struct Student s1 = {"Alice", 90};
    FILE *fp = fopen("student.dat", "wb");
    fwrite(&s1, sizeof(struct Student), 1, fp);
    fclose(fp);
    return 0;
}

Example Output:

Binary file student.dat created with one student record.

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Exercises

Exercise 1: Write Names to File

1. Ask the user to input 3 names.
2. Write them line by line to names.txt.
3. Open the file and print all names.

Example Output:

Enter 3 names:
Alice Bob Charlie
names.txt contents:
Alice
Bob
Charlie

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Exercise 2: Read Integers and Sum

1. Create a file nums.txt with integers (space-separated). (In the created nums.txt file: ex: 10 20 30)
2. Read them using fscanf() and print the total sum.

Example Output:

Total sum = 60

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Exercise 3: Read Last Character

1. Open a text file and move to the last byte using fseek().
2. Print the last character.

Example Output:

Last character: !

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Exercise 4: Store Multiple Students

1. Define a Student structure with name and score.
2. Store 2 student records to a binary file.
3. Reopen and print both students.

Example Output:

Student 1: Alice 90
Student 2: Bob 85

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Bonus Challenge: Text Summary Tool

• Create text.txt file and try writing anything you want
• Read a text file and count:
  • Number of characters
  • Number of words (space-separated)
  • Number of lines

Example Output:

Characters: 35
Words: 6
Lines: 2