Dynamic Array in C

A Dynamic Array in C programming is a variable-sized array whose memory size is allocated at runtime from the Heap rather than at compile-time on the Stack. Unlike static arrays with fixed dimensions (such as int arr[100];), dynamic arrays allow programmers to request exactly the amount of memory needed and resize it on the fly using standard C memory management functions: malloc(), calloc(), realloc(), and free().

Static Array vs. Dynamic Array in C

Feature	Static Array	Dynamic Array
Memory Location	Stack segment	Heap segment
Size Determination	Compile-time (fixed constant)	Run-time (user input)
Resizability	Cannot be resized	Can be grown/shrunk via `realloc()`
Deallocation	Automatic when out of scope	Manual via `free()` function
Memory Efficiency	Can cause memory wastage or stack overflow	Optimal memory utilization

How to Allocate a Dynamic Array in C

Dynamic memory is requested using a pointer and the malloc() function declared inside <stdlib.h>:


int *arr;
int n = 5;

/* Allocate memory for 5 integers */
arr = (int*) malloc(n * sizeof(int));

/* CRUCIAL: Always verify if allocation succeeded */
if (arr == NULL) {
    printf("Memory Allocation Failed!\n");
    exit(1);
}

Resizing with `realloc()`

When an array runs out of space, the realloc() function expands or contracts the existing block without losing its prior data:


/* Resize array from 5 to 10 elements */
int *temp = (int*) realloc(arr, 10 * sizeof(int));
if (temp != NULL) {
    arr = temp; /* Successfully reallocated */
}

Comprehensive Working Example: Dynamic Array with Resizing

This complete program accepts n initial numbers, dynamically doubles the array size, appends extra numbers, and deallocates memory:


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

int main()
{
    int *arr, n, i, new_size;

    printf("Enter initial number of elements: ");
    scanf("%d", &n);

    /* 1. Allocate initial dynamic array */
    arr = (int*) malloc(n * sizeof(int));
    if(arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }

    printf("Enter %d integers:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }

    /* 2. Display initial elements */
    printf("\nInitial Dynamic Array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    /* 3. Resize array to hold additional elements */
    new_size = n + 2;
    arr = (int*) realloc(arr, new_size * sizeof(int));
    if(arr == NULL) {
        printf("Reallocation failed!\n");
        return 1;
    }

    printf("\nEnter %d more integers:\n", 2);
    for(i = n; i < new_size; i++) {
        scanf("%d", &arr[i]);
    }

    /* 4. Display combined array */
    printf("\nExpanded Dynamic Array: ");
    for(i = 0; i < new_size; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    /* 5. Clean up memory */
    free(arr);
    arr = NULL; /* Prevent dangling pointer */
    printf("\nMemory successfully freed.\n");

    return 0;
}

Best Practices to Avoid Fatal Bugs

Never forget free(): Forgetting to release allocated heap memory causes Memory Leaks, eventually slowing down or crashing your system.

Reset pointers to NULL: After calling free(ptr), set ptr = NULL; to avoid accessing a dangerous dangling pointer.

Always check for NULL: Operating systems may deny allocation requests if RAM is exhausted.

Practice Session for Beginner Students

Review these dry-runs, multiple-choice questions, and lab challenges to master heap memory in C.

Part 1: Spot the Bug & Predict the Output

Problem 1: What is the fatal bug in this code snippet?


void createArray() {
    int *p = (int*)malloc(100 * sizeof(int));
    /* Some operations */
}
int main() {
    createArray();
    return 0;
}

Answer & Explanation: Memory Leak! The pointer p is a local variable inside createArray(). When the function returns, p is destroyed from the stack, but the 400 bytes on the heap remain allocated with no remaining pointer referencing them. The memory cannot be freed.

Part 2: Multiple Choice Questions (BCA Exam Special)

Q1. Which library header must be included to use malloc() and free()?
A) <stdio.h>
B) <stdlib.h>
C) <string.h>
D) <conio.h>
Correct Answer: B) <stdlib.h>.

Q2. What does malloc() return if there is insufficient memory available?
A) 0 (NULL pointer)
B) -1
C) A garbage pointer
D) Terminates program automatically
Correct Answer: A) NULL.

Q3. What is the key difference between malloc() and calloc()?
A) malloc() is faster and initializes memory to zero
B) calloc() initializes all allocated bytes to zero; malloc() leaves garbage values
C) calloc() allocates on stack; malloc() on heap
D) There is no difference
Correct Answer: B) calloc() initializes memory to zero; malloc() leaves garbage values.

Part 3: Hands-on Lab Assignments for Beginners

Assignment 1: Write a C program to find the sum and average of N floating-point numbers stored in a dynamically allocated array.
Logic Hint: Read n, allocate using float *arr = (float*)malloc(n * sizeof(float));, iterate with a loop, and calculate sum.

Part 4: Frequently Asked University Viva Questions

1. What is heap memory in C?
Answer: The heap is an unstructured pool of free memory managed by the operating system, used by programs for dynamic runtime memory allocation.

2. Can realloc() move the allocated block to a completely new memory location?
Answer: Yes. If there is not enough contiguous memory adjacent to the existing block, realloc() copies the data to a new location with sufficient space, frees the old block, and returns the new pointer.