realloc function - Mini Project: Build & Apply

Using the realloc Function in C

📖 Scenario: You are managing a list of daily temperatures stored in a dynamic array in C. Sometimes, you need to add more days to the list, so you must resize the array safely.

🎯 Goal: Learn how to use the realloc function to resize a dynamic array in C while preserving existing data.

📋 What You'll Learn

Create a dynamic array of integers with initial size 3

Create a variable new_size to hold the new size of the array

Use realloc to resize the array to new_size

Print all elements of the resized array

💡 Why This Matters

🌍 Real World

Dynamic arrays are useful when you don't know the exact amount of data in advance, like reading sensor data or user input.

💼 Career

Understanding memory management and resizing arrays is important for systems programming, embedded development, and performance-critical applications.

1

Create the initial dynamic array

Write code to create a dynamic array called temps of integers with initial size 3 using malloc. Initialize the array with values 20, 22, and 19.

C

#include 
#include 

int main() {
    // Create dynamic array temps with size 3 and initialize values
    // Your code here

Hint

Use malloc to allocate memory for 3 integers and assign it to temps. Then assign values to each index.

2

Create a new size variable

Add a variable called new_size and set it to 5 to represent the new size of the array.

C

#include 
#include 

int main() {
    int *temps = malloc(3 * sizeof(int));
    temps[0] = 20;
    temps[1] = 22;
    temps[2] = 19;
    // Create new_size variable and set to 5
    // Your code here

Hint

Declare an integer variable new_size and assign it the value 5.

3

Resize the array using realloc

Use realloc to resize the temps array to new_size. Assign the result back to temps. Then initialize the new elements temps[3] and temps[4] with values 21 and 23.

C

#include 
#include 

int main() {
    int *temps = malloc(3 * sizeof(int));
    temps[0] = 20;
    temps[1] = 22;
    temps[2] = 19;
    int new_size = 5;
    // Resize temps using realloc to new_size
    // Initialize temps[3] = 21 and temps[4] = 23
    // Your code here

Hint

Use temps = realloc(temps, new_size * sizeof(int)) to resize. Then assign values to the new indexes.

4

Print all elements of the resized array

Use a for loop with variable i from 0 to new_size - 1 to print each element of temps on its own line. Then free the allocated memory.

C

#include 
#include 

int main() {
    int *temps = malloc(3 * sizeof(int));
    temps[0] = 20;
    temps[1] = 22;
    temps[2] = 19;
    int new_size = 5;
    temps = realloc(temps, new_size * sizeof(int));
    temps[3] = 21;
    temps[4] = 23;
    // Print all elements of temps using a for loop
    // Free the allocated memory
    // Your code here

Hint

Use a for loop to print each element with printf. Don't forget to call free(temps) at the end.

Practice

1. What does the realloc function do in C programming?

easy

A. It allocates a new memory block without preserving old data.

B. It frees the allocated memory block immediately.

C. It changes the size of a previously allocated memory block, preserving its content.

D. It copies data from one memory block to another.

Solution

1. Step 1: Understand the purpose of realloc

   The realloc function is used to resize an existing memory block allocated by malloc or calloc.

2. Step 2: Check data preservation behavior

   realloc preserves the existing data up to the minimum of old and new sizes.

3. Final Answer:

   It changes the size of a previously allocated memory block, preserving its content. -> Option C

4. Quick Check:

   realloc resizes memory and keeps data [OK]

Hint: Remember: realloc resizes and keeps old data safe [OK]

Common Mistakes:

• Thinking realloc frees memory without resizing
• Assuming realloc always allocates new memory without preserving data
• Confusing realloc with memcpy or free

2. Which of the following is the correct syntax to resize a memory block pointed by ptr to hold 50 integers using realloc?

easy

A. ptr = realloc(ptr, 50 * sizeof(int));

B. ptr = realloc(sizeof(int), 50 * ptr);

C. ptr = realloc(50, ptr * sizeof(int));

D. ptr = realloc(ptr, 50);

Solution

1. Step 1: Understand realloc parameters

   realloc takes a pointer and the new size in bytes. To hold 50 integers, size must be 50 times size of int.

2. Step 2: Check each option's correctness

   ptr = realloc(ptr, 50 * sizeof(int)); correctly multiplies 50 by sizeof(int). Others misuse parameters or sizes.

3. Final Answer:

   ptr = realloc(ptr, 50 * sizeof(int)); -> Option A

4. Quick Check:

   Size in bytes = count * sizeof(type) [OK]

Hint: Multiply count by sizeof(type) for realloc size [OK]

Common Mistakes:

• Passing count instead of bytes as size
• Swapping parameters in realloc
• Forgetting sizeof(int) when resizing

3. What will be the output of this code snippet?

int *arr = malloc(3 * sizeof(int));
arr[0] = 1; arr[1] = 2; arr[2] = 3;
arr = realloc(arr, 5 * sizeof(int));
arr[3] = 4; arr[4] = 5;
for(int i = 0; i < 5; i++) printf("%d ", arr[i]);
free(arr);

medium

A. 1 2 3 0 0

B. 1 2 3 4 5

C. Garbage values after 3 elements

D. Runtime error due to invalid access

Solution

1. Step 1: Analyze memory allocation and initialization

   Initially, 3 integers allocated and set to 1, 2, 3. Then resized to hold 5 integers.

2. Step 2: Assign new values and print

   After realloc, elements 3 and 4 are assigned 4 and 5. Printing all 5 elements outputs 1 2 3 4 5.

3. Final Answer:

   1 2 3 4 5 -> Option B

4. Quick Check:

   realloc preserves old data and new elements assigned [OK]

Hint: realloc keeps old data; new elements must be assigned [OK]

Common Mistakes:

• Assuming new elements are zeroed automatically
• Expecting garbage values without assignment
• Thinking realloc frees old data immediately

4. Identify the error in this code snippet:

int *ptr = malloc(10 * sizeof(int));
ptr = realloc(ptr, 20 * sizeof(int));
if (ptr == NULL) {
    printf("Memory allocation failed\n");
    free(ptr);
}

medium

A. Calling free(ptr) after ptr is NULL does not free the original memory block.

B. Missing check for malloc failure before realloc.

C. Incorrect size calculation in realloc.

D. No error; code is correct.

Solution

1. Step 1: Understand realloc return and NULL check

   If realloc returns NULL, original pointer is lost if not saved separately, and ptr becomes NULL.

2. Step 2: Analyze free(ptr) after NULL assignment

   Calling free(ptr) after ptr is NULL does not free the original memory block, leading to memory leak.

3. Final Answer:

   Calling free(ptr) after ptr is NULL does not free the original memory block. -> Option A

4. Quick Check:

   Check realloc result before overwriting pointer [OK]

Hint: Save realloc result before overwriting pointer [OK]

Common Mistakes:

• Not checking realloc return before assignment
• Freeing NULL pointer thinking it frees old memory
• Ignoring malloc failure before realloc

5. You have a dynamic array of integers allocated with malloc. You want to safely double its size using realloc without losing data or causing memory leaks. Which approach is correct?

hard

A. ptr = malloc(2 * current_size); memcpy(ptr, old_ptr, current_size); free(old_ptr);

B. ptr = realloc(ptr, 2 * current_size); // directly assign realloc result

C. free(ptr); ptr = malloc(2 * current_size); // allocate new memory after freeing

D. int *temp = realloc(ptr, 2 * current_size); if (temp) ptr = temp; else handle error;

Solution

1. Step 1: Understand safe realloc usage

   Directly assigning realloc result to original pointer risks losing original memory if realloc fails.

2. Step 2: Use temporary pointer to check realloc success

   Assign realloc to temp pointer, check if not NULL, then update original pointer. Else handle error safely.

3. Final Answer:

   int *temp = realloc(ptr, 2 * current_size); if (temp) ptr = temp; else handle error; -> Option D

4. Quick Check:

   Use temp pointer to avoid memory leaks on realloc failure [OK]

Hint: Use temp pointer for realloc to avoid losing original memory [OK]

Common Mistakes:

• Overwriting pointer before checking realloc result
• Freeing pointer before realloc causing data loss
• Not handling realloc failure properly