How to initialize a dynamic array in C++

Regular arrays or static arrays have a predetermined size or fixed size. Change in the size of regular arrays is not possible.

The memory size for static arrays determines at compile time. Dynamic arrays are different from static arrays. The scope for these arrays determines at runtime. The "new" keyword is used to declare the dynamic arrays.

Before understanding the concept of initializing a dynamic array, one must know about the array and dynamic array.

What is an array in C++?

In C++, a collection of data elements of the same data type set in a contiguous memory location is referred to as an array. These data elements are accessed through an index value.

Syntax

datatype array_name[n]= { data_element_1, .. ,data_element_n};

Example

#include <iostream>
using namespace std;
int main()
 {                                         
    int a[4] = { 1, 2, 3, 4};   //array name “a”
    cout<<a[1];                //Compiler creates an array with size 4
    return 0;
 }

Output:

2          

What is a Dynamic Array(C++)?

In C++, a dynamic array is an array in which the size of an array can be changed or modified. It is similar to a static array(regular array), But the array size alters during run time

The memory of a static array is predetermined at the time of array creation, whereas the memory of a dynamic array grows when the array size increments.

The new keyword in C++

A dynamic array is created using the new keyword, and the number of elements to be assigned is defined within the square brackets ([ ]). The datatype is also mentioned.

Syntax

pointer_variable = new data_type[ ];

The mentioned datatype should be a valid datatype of C++.

We can also delete a dynamic array after its creation.

Example 1 (initializing a dynamic array)

#include<iostream>
using namespace std;
int main() {
   int k,p;
   cout<<"Enter total number of data elements:"<<"\n";
   cin>>p;
   int *arr = new int(p);
   cout<<"Enter "<<p<<" data elements"<<endl;
   for(k = 0;k<p;k++) {
      cin>>arr[k];
   }
   cout<<"Entered data elements are: ";
   for(k = 0;k<p;k++) {
      cout<<arr[k]<<" ";
   }
   cout<<endl;
   delete (arr);
   return 0;
}

Output

Enter total number of data elements:
3                                                                                                              
Enter 3 data elements
1                                                                                                             
2
3
Entered data elements are: 1 2 3

In the above program, int *arr = new int(p) is used to initialize the array “arr”. By using delete(arr), memory is retrieved.

Explanation:

The iostream header file is included in the program to use all its functions; the std namespace is used for its classes without calling it. The main() function has the body of the function. Integer variables k and p are used. Integer "p" is used to know the user input for the number of elements, whereas k is used for iteration. Declaring a dynamic array of name "arr", the array "arr" holds the "p" number of integer elements. "int *arr = new int(p)” is used to declare the dynamic array by using the “new” keyword. Use for loop to iterate the array elements mentioned by the user. The program prints desired array of elements. The “endl" is a C++ keyword that means the end of the line. 

Here, the user takes p=3, and the three elements are {1,2,3}.

Example Program 2 (initializing a dynamic array):

In this program, the dynamic array is initialized to zero.

Syntax:

int *arr{ new int[length of the array]{} };

Here, the “length of the array” denotes the number of data elements of the array.

Program

#include<iostream>
using namespace std;
int main() {
   int k,p;
   cout<<"Enter total number of data elements:"<<"\n";
   cin>>p;
int *arr{ new int[p]{} };
  
   cout<<"Entered data elements are: ";
   for(k = 0;k<p;k++) {
      cout<<arr[k]<<" ";
   }
   cout<<endl;
   delete (arr);
   return 0;
}

Output

Enter total number of data elements:
5
Entered data elements are: 0 0 0 0 0

Explanation

The iostream header file is included in the program to use all its functions; the std namespace is used for its classes without calling it. The main() function has the body of the function. Integer variables k and p are used. Integer "p" is used to know the user input for the number of elements, whereas k is used for iteration. Declaring a dynamic array of name “arr”, the array “arr” holds the “p” number of integer elements. “int *arr{ new int[p]{} }”  is used to declare the dynamic array by using “new” keyword with 0 as the initial value. Use for loop to iterate the array elements mentioned by the user. The program prints desired array of elements. The “endl” is a C++ keyword that means the end of the line.  Here, user takes p=5 and the five elements are {0,0,0,0,0}.