3.2 Pointers, Structures, and Data Files

1. Pointers

1. What is a Pointer?

   • A pointer is a variable that stores the memory address of another variable.

   • Syntax:

     int a = 10;
     int *p = &a;  // 'p' is a pointer to 'a', storing its address.
     printf("%d", *p); // Dereference 'p' to access the value of 'a': 10

2. Pointer Arithmetic

   • You can perform arithmetic operations on pointers to navigate through memory locations.

   • Valid operations:

     • Increment (p++): Moves to the next memory location of the same type.

     • Decrement (p--): Moves to the previous memory location.

     • Addition/Subtraction (p + n, p - n): Adjusts the pointer by n locations.

   • Example:

     int arr[] = {10, 20, 30};
     int *p = arr;     // Points to the first element of 'arr'.
     printf("%d\n", *p);  // Output: 10
     p++;               // Move pointer to the next element.
     printf("%d\n", *p);  // Output: 20

3. Passing Pointers to Functions

   • Pointers allow functions to modify the original value of variables.

   • Example:

     void increment(int *n) {
         (*n)++; // Increment the value pointed to by 'n'
     }
     
     int main() {
         int num = 5;
         increment(&num);  // Pass the address of 'num'.
         printf("%d", num);  // Output: 6
         return 0;
     }

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2. Structures

1. What is a Structure?

   • A structure is a user-defined data type that groups variables of different types under a single name.

   • Syntax:

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

   • Declaration:

     struct Student s1;
     s1.id = 101;
     strcpy(s1.name, "Alice"); // Assign string to 'name'
     printf("ID: %d, Name: %s\n", s1.id, s1.name);  // Output: ID: 101, Name: Alice

2. Pointer to a Structure

   • Accessing structure members using a pointer:

     struct Student s1 = {101, "Alice"};
     struct Student *ptr = &s1;
     printf("ID: %d, Name: %s\n", ptr->id, ptr->name);  // Output: ID: 101, Name: Alice

3. Array of Structures

   • Example:

     struct Student {
         int id;
         char name[50];
     };
     
     struct Student students[2] = {{101, "Alice"}, {102, "Bob"}};
     printf("Student 1: %s\n", students[0].name);  // Output: Student 1: Alice
     printf("Student 2: %s\n", students[1].name);  // Output: Student 2: Bob

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3. Unions

1. What is a Union?

   • A union is similar to a structure but uses shared memory for all its members. Only one member can hold a value at any given time.

   • Syntax:

     union Data {
         int i;
         float f;
     };
     
     union Data d;
     d.i = 5;
     printf("%d\n", d.i);  // Output: 5
     d.f = 3.14;
     printf("%f\n", d.f);  // Output: 3.140000

2. Key Difference from Structures

   • Structure: Allocates memory for all members.

   • Union: Allocates memory for the largest member only.

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4. File Operations

1. File Modes

   • "r": Open a file for reading.

   • "w": Open a file for writing (overwrites if the file exists).

   • "a": Open a file for appending (writes at the end without overwriting).

2. Basic File Handling Functions

   • fopen(): Opens a file.

   • fclose(): Closes a file.

   • fprintf() and fscanf(): Handles formatted input/output.

   • fread() and fwrite(): Handles binary data.

3. Examples

   • Writing to a File:

     include <stdio.h>
     int main() {
         FILE *file = fopen("data.txt", "w");  // Open file in write mode
         if (file == NULL) {
             printf("Error opening file!\n");
             return 1;
         }
         fprintf(file, "Hello, World!\n");  // Write to file
         fclose(file);  // Close file
         return 0;
     }

   • Reading from a File:

     #include <stdio.h>
     int main() {
         FILE *file = fopen("data.txt", "r");  // Open file in read mode
         char str[100];
         if (file == NULL) {
             printf("File not found!\n");
             return 1;
         }
         while (fscanf(file, "%s", str) != EOF) {  // Read words until EOF
             printf("%s\n", str);  // Print each word
         }
         fclose(file);  // Close file
         return 0;
     }

   • Binary File Handling:

     #include <stdio.h>
     struct Student {
         int id;
         char name[50];
     };
     
     int main() {
         FILE *file = fopen("student.dat", "wb");  // Open binary file for writing
         struct Student s = {101, "Alice"};
         fwrite(&s, sizeof(s), 1, file);  // Write structure to file
         fclose(file);  // Close file
     
         file = fopen("student.dat", "rb");  // Open binary file for reading
         struct Student readStudent;
         fread(&readStudent, sizeof(readStudent), 1, file);  // Read structure
         printf("ID: %d, Name: %s\n", readStudent.id, readStudent.name);  // Output: ID: 101, Name: Alice
         fclose(file);  // Close file
         return 0;
     }

4. Common File Handling Errors

   • Forgetting to close a file.

   • Trying to read a file that doesn’t exist.

   • Using the wrong file mode.

Conclusion

• Pointers, structures, unions, and file operations are essential concepts in C programming. Pointers provide direct memory manipulation and efficient data handling.

• Structures enable grouping of related data, while unions optimize memory usage by sharing storage among members.

• File operations facilitate data persistence and management through various modes and functions.