Overview
Welcome to Week 12 of C programming! This week, we will explore dynamic allocation. By the end of this tutorial, you will:
- Understand malloc, calloc, realloc, and free
- Structure Pointer
Time Breakdown
- Basic concept of Dynamic allocation
- malloc, calloc, realloc, and free
- Structure Pointer
- Exercises & Q/A
1. Malloc
It allocates space dynamically in run time.
Example:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
| #include <stdio.h>
#include <stdlib.h>
int main() {
int *arr;
int n;
printf("size of array: ");
scanf("%d", &n);
arr = (int *)malloc(n * sizeof(int));
if (arr == NULL) {
printf(" failed to allocate\n");
return 1;
}
for (int i = 0; i < n; i++) {
arr[i] = i * 10;
}
printf("array : ");
for (int i = 0; i < n; i++) {
printf("%d ", arr[i]);
}
free(arr);
return 0;
}
|
Example Output:
1
2
| size of array: 5
array : 0 10 20 30 40
|
2. Calloc
It’s almost same with malloc but it resets space with 0.
Example:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
| #include <stdio.h>
#include <stdlib.h>
int main() {
int *arr;
int n;
printf("Size of array: ");
scanf("%d", &n);
arr = (int *)calloc(n, sizeof(int));
if (arr == NULL) {
printf(" failed to allocate\n");
return 1;
}
printf("array: ");
for (int i = 0; i < n; i++) {
printf("%d ", arr[i]);
}
free(arr);
return 0;
}
|
Example Output:
1
2
| Size of array: 5
array: 0 0 0 0 0
|
3. realloc
It tries to resize the existing memory space. If it can’t, it allocates a new space, copies the data, and frees the old one.
Example:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
| #include <stdio.h>
#include <stdlib.h>
int main() {
int *arr;
int n = 3;
arr = (int *)malloc(n * sizeof(int));
if (arr == NULL) {
printf("failed to allocate\n");
return 1;
}
for (int i = 0; i < n; i++) arr[i] = i + 1;
printf("original array: ");
for (int i = 0; i < n; i++) printf("%d ", arr[i]);
printf("\n");
n = 6;
int *temp = realloc(arr, n * sizeof(int));
if (temp == NULL) {
printf("\n failed to reallocate\n");
free(arr);
return 1;
}
arr = temp;
for (int i = 3; i < n; i++) arr[i] = (i + 1) * 10;
printf("new array:");
for (int i = 0; i < n; i++) printf("%d ", arr[i]);
printf("\n");
free(arr);
return 0;
}
|
Example Output:
1
2
| original array: 1 2 3
new array:1 2 3 40 50 60
|
4. Allocation Using Structure Pointer
Example: How to define and use a Pointer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
| #include <stdio.h>
#include <stdlib.h>
typedef struct {
char name[30];
int age;
} Person;
int main() {
Person *p;
int n;
printf("Number of People: ");
scanf("%d",&n);
p = (Person *)malloc(sizeof(Person) * n);
if (p == NULL) {
printf("failed to allocate\n");
return 1;
}
for(int i=0;i<n;i++){
printf("name: ");
scanf("%s", p[i].name);
printf("age: ");
scanf("%d", &p[i].age);
}
for(int i=0;i<n;i++){
printf("%dth name: %s, age: %d\n", i, p[i].name, p[i].age);
}
free(p);
return 0;
}
|
Example Output:
1
2
3
4
5
6
7
8
9
10
| Number of People: 3
name: Kim
age: 20
name: Lee
age: 10
name: Park
age: 24
0th name: Kim, age: 20
1th name: Lee, age: 10
2th name: Park, age: 24
|
Exercise 1: Make a dynamic array and find the largest and smallest number.
Write a C program to get a dynamic array of numbers and return the largest and smallest number in the array. You should use malloc() and free the memory at the end of your application using free().
Example Output
1
2
3
4
| Enter the size of the array: 3
Enter 3 elements of the array: 10 9 22
Largest element present in the given array is : 22
Smallest element present in the given array is : 9
|
Exercise 2: Utilizing realloc
Create a program that prompts the user to fill an array with a specified size and then allows the user to insert a number at any position.
Example Output
1
2
3
4
5
| Enter size of the array : 5
Enter elements in array : 0 1 2 3 4
Enter element to insert : 9
Enter the element position : 2
Array elements after insertion : 0 1 9 2 3 4
|
Exercise 3: Make a scoreboard
We want to create a C program that requests the number of enrolled subjects to get these subjects’ name and score. Then calculate the average grade in the overall semester. You should create a course structure with subject name and mark.
Example Output
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
| Enter the number of records: 4
Enter subject and marks:
Math 100
Enter subject and marks:
English 80
Enter subject and marks:
Korean 94
Enter subject and marks:
Science 77
Displaying Information:
Math 100
English 80
Korean 94
Science 77
Your average = 87
|