28. Dynamic Memory Allocation.md

Dynamic Memory Allocation

Video Link: https://youtu.be/Dn87Bna23TQ
Tutorial Link: https://www.programiz.com/c-programming/c-dynamic-memory-allocation

malloc() and free() Function

#include <stdio.h>

int main() {

  int n = 4;
  
  int* ptr;

  ptr = (int*) malloc(n * sizeof(int));

  if (ptr == NULL) {
      printf("Memory cannot be allocated");
      return 0;
  }

  printf("Enter input values:\n");
  for(int i = 0; i < n; ++i) {
      scanf("%d", ptr + i);
  }

  printf("Input Values\n");
  for(int i = 0; i < n; ++i) {
      printf("%d\n", *(ptr + i));
  }

  free(ptr);

  return 0;
}

Output

Enter input values:
12
21
13
9
Input Values
12
21
13
9

realloc() Function

#include <stdio.h>

int main() {

  int n = 4;
  
  int* ptr;

  ptr = (int*) malloc(n * sizeof(int));

  if (ptr == NULL) {
      printf("Memory cannot be allocated");
      return 0;
  }

  printf("Allocated Memory\n");
  for (int i = 0; i < n; ++i) {
      printf("%p\n", ptr + i);
  }

  n = 6;

  ptr = realloc (ptr, n * sizeof(int));

  printf("Newly Allocated Memory\n");
  for(int i = 0; i < n; ++i) {
      printf("%p\n", ptr + i);
  }

  return 0;
}

Output

Allocated Memory
0x559442b682a0
0x559442b682a4
0x559442b682a8
0x559442b682ac
Newly Allocated Memory
0x559442b682a0
0x559442b682a4
0x559442b682a8
0x559442b682ac
0x559442b682b0
0x559442b682b4

---

Programming Task

Create a program that dynamically changes the size of an array.

• Create a pointer array like this
  int* ages;
• Allocate 4 spaces for the array using the malloc() function
• Get 4 input values for the array.
• Print the array elements using the pointer.
• Resize the array to store 6 elements using realloc()
• Assign 32 and 59 as the 5th and 6th elements.
  Hint
  ages[4] = 32;
  ages[5] = 49;
• Then finally Print all 6 array elements.

Solution

#include <stdio.h>

int main() {
    
    int* ages;
    int n = 4;
    
    ages = (int*)malloc(n * sizeof(int));
    
    if(ages == NULL) {
        printf("Memory cannot be allocated");
        return 0;
    }
    
    printf("Enter input values:\n");
    for (int i = 0; i < n; ++i) {
        scanf("%d", ages+i);
    }
    
    printf("Input Values are:\n");
    for (int i = 0; i < n; ++i) {
        printf("%d\n", *(ages + i));
    }
    
    n = 6;
    
    ages[4] = 32;
    ages[5] = 49;
    
    ages = realloc(ages, n * sizeof(int));
    
    printf("After reallocation:\n");
    for(int i = 0; i < n; ++i) {
        printf("%d\n", *(ages+i));
    }
    
    free(ages);
    return 0;
}

Output

Enter input values:
11
14
13
15
Input Values are:
11
14
13
15
After reallocation:
11
14
13
15
32
49

---

Programiz Quiz

Q. What is the correct way to allocate memory to store 12 double data?

Options:

1. malloc(20 * double)
2. (double*) malloc(12 * sizeof(double))
3. (double) malloc(20 * sizeof(double))
4. (double*) malloc( sizeof(20 * int))

Answer: 2