Array in C is static in nature, so its size should be known at compile time and we can't change the size of the array after its declaration. Due to this, we may encounter situations where our array doesn't have enough space left for required elements or we allotted more than the required memory leading to memory wastage. To solve this problem, dynamic arrays come into the picture.
A Dynamic Array is allocated memory at runtime and its size can be changed later in the program.
We can create a dynamic array in C by using the following methods:
- Using malloc() Function
- Using calloc() Function
- Resizing Array Using realloc() Function
- Using Variable Length Arrays(VLAs)
- Using Flexible Array Members
1. Dynamic Array Using malloc() Function
The “malloc” or “memory allocation” method in C is used to dynamically allocate a single large block of memory with the specified size. It returns a pointer of type void which can be cast into a pointer of any form. It is defined inside <stdlib.h> header file.
Syntax:
ptr = (cast-type*) malloc(byte-size);
We can use this function to create a dynamic array of any type by simply allocating a memory block of some size and then typecasting the returned void pointer to the pointer of the required type.
Example:
ptr = (int*) malloc(100 * sizeof(int));
In the above example, we have created a dynamic array of type int and size 100 elements.
Note: It is to be noted that if malloc fails to allocate the required memory, it returns the NULL pointer. So, it is a good practice to check for NULL pointer to see if the memory is successfully allocated or not.
Example:
C
// C program to create dynamic array using malloc() function
#include <stdio.h>
#include <stdlib.h>
int main()
{
// address of the block created hold by this pointer
int* ptr;
int size;
// Size of the array
printf("Enter size of elements:");
scanf("%d", &size);
// Memory allocates dynamically using malloc()
ptr = (int*)malloc(size * sizeof(int));
// Checking for memory allocation
if (ptr == NULL) {
printf("Memory not allocated.\n");
}
else {
// Memory allocated
printf("Memory successfully allocated using "
"malloc.\n");
// Get the elements of the array
for (int j = 0; j < size; ++j) {
ptr[j] = j + 1;
}
// Print the elements of the array
printf("The elements of the array are: ");
for (int k = 0; k < size; ++k) {
printf("%d, ", ptr[k]);
}
}
return 0;
}
Output:
Enter size of elements:5
Memory successfully allocated using malloc.
The elements of the array are: 1, 2, 3, 4, 5,
2. Dynamic Array Using calloc() Function
The “calloc” or “contiguous allocation” method in C is used to dynamically allocate the specified number of blocks of memory of the specified type and initialized each block with a default value of 0.
The process of creating a dynamic array using calloc() is similar to the malloc() method. The difference is that calloc() takes arguments instead of one as compared to malloc(). Here, we provide the size of each element and the number of elements required in the dynamic array. Also, each element in the array is initialized to zero.
Syntax:
ptr = (cast-type*)calloc(n, element-size);
Example:
ptr = (int*) calloc(5, sizeof(float));
In the above example, we have created a dynamic array of type float having five elements.
Let's take another example to create a dynamic array using the calloc() method.
Example:
C
// C program to create dynamic array using calloc() function
#include <stdio.h>
#include <stdlib.h>
int main()
{
// address of the block created hold by this pointer
int* ptr;
int size;
// Size of the array
printf("Enter size of elements:");
scanf("%d", &size);
// Memory allocates dynamically using calloc()
ptr = (int*)calloc(size, sizeof(int));
// Checking for memory allocation
if (ptr == NULL) {
printf("Memory not allocated.\n");
}
else {
// Memory allocated
printf("Memory successfully allocated using "
"malloc.\n");
// Get the elements of the array
for (int j = 0; j < size; ++j) {
ptr[j] = j + 1;
}
// Print the elements of the array
printf("The elements of the array are: ");
for (int k = 0; k < size; ++k) {
printf("%d, ", ptr[k]);
}
}
return 0;
}
Output:
Enter size of elements:6
Memory successfully allocated using malloc.
The elements of the array are: 1, 2, 3, 4, 5, 6,
3. Dynamically Resizing Array Using realloc() Function
The “realloc” or “re-allocation” method in C is used to dynamically change the memory allocation of a previously allocated memory.
Using this function we can create a new array or change the size of an already existing array.
Syntax:
ptr = realloc(ptr, newSize);
Let's take an example to understand this properly.
Example:
C
// C program to resize dynamic array using realloc()
// function
#include <stdio.h>
#include <stdlib.h>
int main()
{
// address of the block created hold by this pointer
int* ptr;
int size = 5;
// Memory allocates dynamically using calloc()
ptr = (int*)calloc(size, sizeof(int));
if (ptr == NULL) {
printf("Memory not allocated.\n");
exit(0);
}
else {
printf("Memory successfully allocated using "
"calloc.\n");
}
// inserting elements
for (int j = 0; j < size; ++j) {
ptr[j] = j + 1;
}
printf("The elements of the array are: ");
for (int k = 0; k < size; ++k) {
printf("%d, ", ptr[k]);
}
printf("\n");
size = 10;
int *temp = ptr;
// using realloc
ptr = realloc(ptr, size * sizeof(int));
if (!ptr) {
printf("Memory Re-allocation failed.");
ptr = temp;
}
else {
printf("Memory successfully re-allocated using "
"realloc.\n");
}
// inserting new elements
for (int j = 5; j < size; ++j) {
ptr[j] = j + 10;
}
printf("The new elements of the array are: ");
for (int k = 0; k < size; ++k) {
printf("%d, ", ptr[k]);
}
return 0;
}
OutputMemory successfully allocated using calloc.
The elements of the array are: 1, 2, 3, 4, 5,
Memory successfully re-allocated using realloc.
The new elements of the array are: 1, 2, 3, 4, 5, 15, 16, 17, 18, 19,
To Know more about these above methods, please refer to the article - malloc, calloc,free in C
4. Variable Length Arrays(VLAs)
Variable length arrays or VLAs, are those arrays in which we can determine the size of the array at the run time. It allocates the memory in the stack and it's based on the local scope level.
The size of a variable length array cannot be changed once it is defined and using variable length array as its found down sides as compared to above methods.
Example:
C
// C program to demonstrate the use of VLAs
#include <stdio.h>
int main()
{
int n;
printf("Enter the size of the array: ");
scanf("%d", &n);
int arr[n];
printf("Enter elements: ");
for (int i = 0; i < n; ++i) {
scanf("%d", &arr[i]);
}
printf("Elements of VLA of Given Size: ");
for (int i = 0; i < n; ++i) {
printf("%d ", arr[i]);
}
return 0;
}
Output:
Enter the size of the array: 5
Enter elements: 1 2 3 4 5
Elements of VLA of Given Size: 1 2 3 4 5
To know more about variable length arrays, please refer to the article -Variable Length Arrays in C/C++.
5. Flexible Array Members
The flexible array members are the array that is defined inside a structure without any dimension and their size is flexible. This feature was introduced in C99 standard.
We can control the size of a flexible member using malloc() function.
There are a few rules to follow in the case of flexible array members:
- The array inside the structure should preferably be declared as the last member of the structure and its size is variable(can be changed at runtime).
- The structure must contain at least one more named member in addition to the flexible array member.
Let's take the following structure for example
struct student
{
int len;
int
};
Now to allocate memory, we will use malloc() function as shown below.
struct student *s = malloc(sizeof(*s) + 5 * sizeof(int));
Example:
C
// C program to demonstrate the use of Flexible Array Member
#include <stdio.h>
#include <stdlib.h>
// defining struct
typedef struct {
int len;
int arr[];
} fam;
int main()
{
// creating an array member of size 5
fam* fam1
= (fam*)malloc(sizeof(fam*) + 5 * sizeof(int));
// creating an array mebmer of size 10
fam* fam2
= (fam*)malloc(sizeof(fam*) + 10 * sizeof(int));
// inserting elements
for (int i = 0; i < 5; i++) {
fam1->arr[i] = i + 1;
}
for (int i = 0; i < 10; i++) {
fam2->arr[i] = i + 10;
}
// printing elements
printf("Array of Size 5:\n");
for (int i = 0; i < 5; i++) {
printf("%d, ", fam1->arr[i]);
}
printf("\n");
printf("Array of size 10:\n");
for (int i = 0; i < 10; i++) {
printf("%d, ", fam2->arr[i]);
}
return 0;
}
OutputArray of Size 5:
1, 2, 3, 4, 5,
Array of size 10:
10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
To know more about flexible array members, please refer to the article - Flexible Array Members Structure in C.
Dynamic Allocation of Two-Dimensional Array
We can also create a two-dimensional dynamic array in c. These are the following different ways to create a 2D dynamic array.
- Using a single pointer and a 1D array with pointer arithmetic
- Using an array of pointers
- Using a pointer to a pointer
- Using a double-pointer and one malloc call
- Using a pointer to Variable Length Array
- Using a pointer to the first row of VLA
To know more about the Dynamic Allocation of Two-Dimensional Arrays, refer to the article - How to dynamically allocate a 2D array in C?
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Properties of Array in CAn array in C is a fixed-size homogeneous collection of elements stored at a contiguous memory location. It is a derived data type in C that can store elements of different data types such as int, char, struct, etc. It is one of the most popular data types widely used by programmers to solve differe
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Multidimensional Arrays in C - 2D and 3D ArraysA multi-dimensional array in C can be defined as an array that has more than one dimension. Having more than one dimension means that it can grow in multiple directions. Some popular multidimensional arrays include 2D arrays which grows in two dimensions, and 3D arrays which grows in three dimension
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Initialization of Multidimensional Array in CIn C, multidimensional arrays are the arrays that contain more than one dimensions. These arrays are useful when we need to store data in a table or matrix-like structure. In this article, we will learn the different methods to initialize a multidimensional array in C. The easiest method for initial
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Pass Array to Functions in CPassing an array to a function allows the function to directly access and modify the original array. In this article, we will learn how to pass arrays to functions in C.In C, arrays are always passed to function as pointers. They cannot be passed by value because of the array decay due to which, whe
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How to pass a 2D array as a parameter in C?A 2D array is essentially an array of arrays, where each element of the main array holds another array. In this article, we will see how to pass a 2D array to a function.The simplest and most common method to pass 2D array to a function is by specifying the parameter as 2D array with row size and co
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What are the data types for which it is not possible to create an array?In C, an array is a collection of variables of the same data type, stored in contiguous memory locations. Arrays can store data of primitive types like integers, characters, and floats, as well as user-defined types like structures.However, there are certain data types for which arrays cannot be dir
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How to pass an array by value in C ?In C programming, arrays are always passed as pointers to the function. There are no direct ways to pass the array by value. However, there is trick that allows you to simulate the passing of array by value by enclosing it inside a structure and then passing that structure by value. This will also p
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Strings in CA String in C programming is a sequence of characters terminated with a null character '\0'. The C String is work as an array of characters. The difference between a character array and a C string is that the string in C is terminated with a unique character '\0'.DeclarationDeclaring a string in C i
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Array of Strings in CIn C, an array of strings is a 2D array where each row contains a sequence of characters terminated by a '\0' NULL character (strings). It is used to store multiple strings in a single array.Let's take a look at an example:C#include <stdio.h> int main() { // Creating array of strings for 3 str
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What is the difference between single quoted and double quoted declaration of char array?In C programming, the way we declare and initialize a char array can differ based on whether we want to use a sequence of characters and strings. They are basically same with difference only of a '\0' NULL character.Double quotes automatically include the null terminator, making the array a string l
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C String FunctionsC language provides various built-in functions that can be used for various operations and manipulations on strings. These string functions make it easier to perform tasks such as string copy, concatenation, comparison, length, etc. The <string.h> header file contains these string functions.Th
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C Pointers
C PointersA pointer is a variable that stores the memory address of another variable. Instead of holding a direct value, it has the address where the value is stored in memory. This allows us to manipulate the data stored at a specific memory location without actually using its variable. It is the backbone of
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Pointer Arithmetics in C with ExamplesPointer Arithmetic is the set of valid arithmetic operations that can be performed on pointers. The pointer variables store the memory address of another variable. It doesn't store any value. Hence, there are only a few operations that are allowed to perform on Pointers in C language. The C pointer
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C - Pointer to Pointer (Double Pointer)In C, double pointers are those pointers which stores the address of another pointer. The first pointer is used to store the address of the variable, and the second pointer is used to store the address of the first pointer. That is why they are also known as a pointer to pointer.Let's take a look at
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Function Pointer in CIn C, a function pointer is a type of pointer that stores the address of a function, allowing functions to be passed as arguments and invoked dynamically. It is useful in techniques such as callback functions, event-driven programs, and polymorphism (a concept where a function or operator behaves di
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How to Declare a Pointer to a Function?A pointer to a function is similar to a pointer to a variable. However, instead of pointing to a variable, it points to the address of a function. This allows the function to be called indirectly, which is useful in situations like callback functions or event-driven programming.In this article, we w
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Pointer to an Array | Array PointerA pointer to an array is a pointer that points to the whole array instead of the first element of the array. It considers the whole array as a single unit instead of it being a collection of given elements.Example:C #include<stdio.h> int main() { int arr[5] = { 1, 2, 3, 4, 5 }; int *ptr = arr;
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Difference between constant pointer, pointers to constant, and constant pointers to constantsIn this article, we will discuss the differences between constant pointer, pointers to constant & constant pointers to constants. Pointers are the variables that hold the address of some other variables, constants, or functions. There are several ways to qualify pointers using const. Pointers to
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Pointer vs Array in CMost of the time, pointer and array accesses can be treated as acting the same, the major exceptions being:  1. the sizeof operator sizeof(array) returns the amount of memory used by all elements in the array sizeof(pointer) only returns the amount of memory used by the pointer variable itself 2.
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Dangling, Void , Null and Wild Pointers in CIn C programming pointers are used to manipulate memory addresses, to store the address of some variable or memory location. But certain situations and characteristics related to pointers become challenging in terms of memory safety and program behavior these include Dangling (when pointing to deall
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Near, Far and Huge Pointers in CIn older times, the intel processors had 16-bit registers, but the address bus was 20-bits wide. Due to this, CPU registers were not able to hold the entire address at once. As a solution, the memory was divided into segments of 64 kB size, and the near pointers, far pointers, and huge pointers were
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restrict Keyword in CThe restrict keyword is a type qualifier that was introduced in the C99 standard. It is used to tell the compiler that a pointer is the only reference or access point to the memory it points to, allowing the compiler to make optimizations based on that information.Let's take a look at an example:C#i
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