How do Dynamic arrays work?
Last Updated : 11 Jul, 2025
A Dynamic array (vector in C++, ArrayList in Java) automatically grows when we try to make an insertion and there is no more space left for the new item. Usually the area doubles in size. A simple dynamic array can be constructed by allocating an array of fixed-size, typically larger than the number of elements immediately required. The elements of the dynamic array are stored contiguously at the start of the underlying array, and the remaining positions towards the end of the underlying array are reserved, or unused. Elements can be added at the end of a dynamic array in constant time by using the reserved space until this space is completely consumed. When all space is consumed, and an additional element is to be added, the underlying fixed-sized array needs to be increased in size. Typically resizing is expensive because you have to allocate a bigger array and copy over all of the elements from the array you have overgrow before we can finally append our item.
Approach: When we enter an element in array but array is full then you create a function, this function creates a new array double size or as you wish and copy all element from the previous array to a new array and return this new array. Also, we can reduce the size of the array. and add an element at a given position, remove the element at the end default and at the position also.
Key Features of Dynamic Array
Add Element: Add element at the end if the array size is not enough then extend the size of the array and add an element at the end of the original array as well as given index. Doing all that copying takes O(n) time, where n is the number of elements in our array. That's an expensive cost for an append. In a fixed-length array, appends only take O(1) time. But appends take O(n) time only when we insert into a full array. And that is pretty rare, especially if we double the size of the array every time we run out of space. So in most cases appending is still O(1) time, and sometimes it's O(n) time. In dynamic array you can create fixed-size array when required added some more element in array then use this approach: 
Delete Element: Delete an element from array, default remove() method delete an element from end, simply store zero at last index and you also delete element at specific index by calling removeAt(i) method where i is index. removeAt(i) method shift all right element in the left side from the given index. 
Resize of Array Size: When the array has null/zero data (aside from an element added by you) at the right side of the array, meaning it has unused memory, the method shrinkSize() can free up the extra memory. When all space is consumed, and an additional element is to be added, then the underlying fixed-size array needs to increase in size. Typically resizing is expensive because you have to allocate a bigger array and copy over all of the elements from the array you have outgrown before we can finally append our item. 
Simple code for dynamic array. In below code then the array will become full of size we copy all element to new double size array(variable size array).sample code is below
C++ #include <iostream> using namespace std; class DynamicArray { private: // Pointer to store array created // using new keyword int* array = NULL; // Size of array int size; // Container size int capacity; public: // Default constructor with size 1 DynamicArray() { capacity = 1; size = 0; array = new int[capacity]; } // Taking size from the user DynamicArray(int capacity) { this->capacity = capacity; array = new int[capacity]; size = 0; } // Returns the size of Array // i.e Total elements stored currently int getSize() { return size; } // Returns the size of container int getCapacity() { return capacity; } // Inserting element after last stored index void push_back(int value) { // check is array having size to store element or // not if (size == capacity) { // if not then grow the array by double growArray(); } // insert element array[size] = value; // increment the size or last_index+1 size++; } // Deleting element at last stored index void pop_back() { // Replace the last index by 0 array[size - 1] = 0; // Decrement the array size size--; // Reduce if the container half element of its // capacity if (size == (capacity / 2)) { shrinkArray(); } } // Increase the array size by double of current capacity void growArray() { // Creating new array of double size int* temp = new int[capacity * 2]; capacity = capacity * 2; // copy element of old array in newly created array for (int i = 0; i < size; i++) { temp[i] = array[i]; } // Delete old array delete[] array; // Assign newly created temp array to original array array = temp; } // Reduce the size of array by half void shrinkArray() { // Creating new array of half size capacity = size; int* temp = new int[capacity]; // copy element of old array in newly created array for (int i = 0; i < size; i++) { temp[i] = array[i]; } // Delete old array delete[] array; // Assign newly created temp array to original array array = temp; } // Searching element in the given array int search(int key) { for (int i = 0; i < size; i++) { if (array[i] == key) { // If element found return its index return i; } } // Return -1 if element not found; return -1; } // Insert element at given index void insertAt(int index, int value) { // check is array having size to store element or // not if (size == capacity) { // if not then grow the array by double growArray(); } for (int i = size - 1; i >= index; i--) { array[i + 1] = array[i]; } array[index] = value; size++; } // Delete element at given index void deleteAt(int index) { for (int i = index; i < size; i++) { array[i] = array[i + 1]; } // Replace the last index by 0 array[size - 1] = 0; // Decrement the array size size--; // Reduce if the container half element of its // capacity if (size == (capacity / 2)) { shrinkArray(); } } // To Print Array void printArrayDetails() { cout << "Elements of array : "; for (int i = 0; i < size; i++) { cout << array[i] << " "; } cout << endl; cout << "No of elements in array : " << size << ", Capacity of array :" << capacity << endl; } bool isEmpty() { if (size == 0) { return true; } else { return false; } } }; int main() { DynamicArray da; da.push_back(1); da.push_back(2); da.push_back(3); da.push_back(4); da.push_back(5); da.push_back(6); da.push_back(7); da.push_back(8); da.push_back(9); da.push_back(10); da.push_back(11); da.printArrayDetails(); da.shrinkArray(); cout << "\nCapacity of array after shrinking : " << da.getCapacity() << endl; cout << "\nAfter inserting at index 3 " << endl; da.insertAt(3, 50); da.printArrayDetails(); cout << "\nAfter delete last element "; da.pop_back(); da.printArrayDetails(); cout << "\nAfter deleting at index 3 "; da.deleteAt(3); da.printArrayDetails(); cout << "\nSearching 5 in array "; int index = da.search(5); if (index != -1) { cout << "Element found at index : " << index << endl; } else { cout << "Element not found " << endl; } return 0; } // This code is contributed by Aniket Tiwari // Java program deals with all operation of a dynamic array // add, remove, resize memory of array is the main feature public class DynamicArray { // create three variable array[] is a array, // count will deal with no of element add by you and // size will with size of array[] private int array[]; private int count; private int size; // constructor initialize value to variable public DynamicArray() { array = new int[1]; count = 0; size = 1; } // function add an element at the end of array public void add(int data) { // check no of element is equal to size of array if (count == size) { growSize(); // make array size double } // insert element at end of array array[count] = data; count++; } // function makes size double of array public void growSize() { int temp[] = null; if (count == size) { // temp is a double size array of array // and store array elements temp = new int[size * 2]; { for (int i = 0; i < size; i++) { // copy all array value into temp temp[i] = array[i]; } } } // double size array temp initialize // into variable array again array = temp; // and make size is double also of array size = size * 2; } // function shrink size of array // which block unnecessary remove them public void shrinkSize() { int temp[] = null; if (count > 0) { // temp is a count size array // and store array elements temp = new int[count]; for (int i = 0; i < count; i++) { // copy all array value into temp temp[i] = array[i]; } size = count; // count size array temp initialize // into variable array again array = temp; } } // function add an element at given index public void addAt(int index, int data) { // if size is not enough make size double if (count == size) { growSize(); } for (int i = count - 1; i >= index; i--) { // shift all element right // from given index array[i + 1] = array[i]; } // insert data at given index array[index] = data; count++; } // function remove last element or put // zero at last index public void remove() { if (count > 0) { array[count - 1] = 0; count--; } } // function shift all element of right // side from given index in left public void removeAt(int index) { if (count > 0) { for (int i = index; i < count - 1; i++) { // shift all element of right // side from given index in left array[i] = array[i + 1]; } array[count - 1] = 0; count--; } } public static void main(String[] args) { DynamicArray da = new DynamicArray(); // add 9 elements in array da.add(1); da.add(2); da.add(3); da.add(4); da.add(5); da.add(6); da.add(7); da.add(8); da.add(9); // print all array elements after add 9 elements System.out.println("Elements of array:"); for (int i = 0; i < da.size; i++) { System.out.print(da.array[i] + " "); } System.out.println(); // print size of array and no of element System.out.println("Size of array: " + da.size); System.out.println("No of elements in array: " + da.count); // shrinkSize of array da.shrinkSize(); // print all array elements System.out.println("Elements of array " + "after shrinkSize of array:"); for (int i = 0; i < da.size; i++) { System.out.print(da.array[i] + " "); } System.out.println(); // print size of array and no of element System.out.println("Size of array: " + da.size); System.out.println("No of elements in array: " + da.count); // add an element at index 1 da.addAt(1, 22); // print Elements of array after adding an // element at index 1 System.out.println("Elements of array after" + " add an element at index 1:"); for (int i = 0; i < da.size; i++) { System.out.print(da.array[i] + " "); } System.out.println(); // print size of array and no of element System.out.println("Size of array: " + da.size); System.out.println("No of elements in array: " + da.count); // delete last element da.remove(); // print Elements of array after delete last // element System.out.println("Elements of array after" + " delete last element:"); for (int i = 0; i < da.size; i++) { System.out.print(da.array[i] + " "); } System.out.println(); // print size of array and no of element System.out.println("Size of array: " + da.size); System.out.println("No of elements in array: " + da.count); // delete element at index 1 da.removeAt(1); // print Elements of array after delete // an element index 1 System.out.println("Elements of array after" + " delete element at index 1:"); for (int i = 0; i < da.size; i++) { System.out.print(da.array[i] + " "); } System.out.println(); // print size of array and no of element System.out.println("Size of array: " + da.size); System.out.println("No of elements in array: " + da.count); } } # Python 3 program deals with all operation of a dynamic array # add, remove, resize memory of array is the main feature class DynamicArray: # create three variable array[] is a array, # count will deal with no of element add by you and # size will with size of array[] array = None count = 0 size = 0 # constructor initialize value to variable def __init__(self): self.array = [0] * (1) self.count = 0 self.size = 1 # function add an element at the end of array def add(self, data): # check no of element is equal to size of array if (self.count == self.size): self.growSize() # insert element at end of array self.array[self.count] = data self.count += 1 # function makes size double of array def growSize(self): temp = None if (self.count == self.size): # temp is a double size array of array # and store array elements temp = [0] * (self.size * 2) i = 0 while (i < self.size): # copy all array value into temp temp[i] = self.array[i] i += 1 # double size array temp initialize # into variable array again self.array = temp # and make size is double also of array self.size = self.size * 2 # function shrink size of array # which block unnecessary remove them def shrinkSize(self): temp = None if (self.count > 0): # temp is a count size array # and store array elements temp = [0] * (self.count) i = 0 while (i < self.count): # copy all array value into temp temp[i] = self.array[i] i += 1 self.size = self.count # count size array temp initialize # into variable array again self.array = temp # function add an element at given index def addAt(self, index, data): # if size is not enough make size double if (self.count == self.size): self.growSize() i = self.count - 1 while (i >= index): # shift all element right # from given index self.array[i + 1] = self.array[i] i -= 1 # insert data at given index self.array[index] = data self.count += 1 # function remove last element or put # zero at last index def remove(self): if (self.count > 0): self.array[self.count - 1] = 0 self.count -= 1 # function shift all element of right # side from given index in left def removeAt(self, index): if (self.count > 0): i = index while (i < self.count - 1): # shift all element of right # side from given index in left self.array[i] = self.array[i + 1] i += 1 self.array[self.count - 1] = 0 self.count -= 1 @staticmethod def main(args): da = DynamicArray() # add 9 elements in array da.add(1) da.add(2) da.add(3) da.add(4) da.add(5) da.add(6) da.add(7) da.add(8) da.add(9) # print all array elements after add 9 elements print("Elements of array:") i = 0 while (i < da.size): print(str(da.array[i]) + " ", end="") i += 1 print() # print size of array and no of element print("Size of array: " + str(da.size)) print("No of elements in array: " + str(da.count)) # shrinkSize of array da.shrinkSize() # print all array elements print("Elements of array after shrinkSize of array:") i = 0 while (i < da.size): print(str(da.array[i]) + " ", end="") i += 1 print() # print size of array and no of element print("Size of array: " + str(da.size)) print("No of elements in array: " + str(da.count)) # add an element at index 1 da.addAt(1, 22) # print Elements of array after adding an # element at index 1 print("Elements of array after add an element at index 1:") i = 0 while (i < da.size): print(str(da.array[i]) + " ", end="") i += 1 print() # print size of array and no of element print("Size of array: " + str(da.size)) print("No of elements in array: " + str(da.count)) # delete last element da.remove() # print Elements of array after delete last # element print("Elements of array after delete last element:") i = 0 while (i < da.size): print(str(da.array[i]) + " ", end="") i += 1 print() # print size of array and no of element print("Size of array: " + str(da.size)) print("No of elements in array: " + str(da.count)) # delete element at index 1 da.removeAt(1) # print Elements of array after delete # an element index 1 print("Elements of array after delete element at index 1:") i = 0 while (i < da.size): print(str(da.array[i]) + " ", end="") i += 1 print() # print size of array and no of element print("Size of array: " + str(da.size)) print("No of elements in array: " + str(da.count)) # Driver code if __name__ == "__main__": DynamicArray.main([]) # This code is contributed by aadityaburujwale. // C# program deals with all operation // of dynamic array add, remove, resize // memory of array is the main feature using System; public class DynamicArray { // create three variable array[] is // a array, count will deal with no // of element add by you and // size will with size of array[] private int[] array; private int count; private int size; // constructor initialize value to variable public DynamicArray() { array = new int[1]; count = 0; size = 1; } // function add an element at the end of array public void add(int data) { // check no of element is equal to size of array if (count == size) { growSize(); // make array size double } // insert element at end of array array[count] = data; count++; } // function makes size double of array public void growSize() { int[] temp = null; if (count == size) { // temp is a double size array of array // and store array elements temp = new int[size * 2]; { for (int i = 0; i < size; i++) { // copy all array value into temp temp[i] = array[i]; } } } // double size array temp initialize // into variable array again array = temp; // and make size is double also of array size = size * 2; } // function shrink size of array // which block unnecessary remove them public void shrinkSize() { int[] temp = null; if (count > 0) { // temp is a count size array // and store array elements temp = new int[count]; for (int i = 0; i < count; i++) { // copy all array value into temp temp[i] = array[i]; } size = count; // count size array temp initialize // into variable array again array = temp; } } // function add an element at given index public void addAt(int index, int data) { // if size is not enough make size double if (count == size) { growSize(); } for (int i = count - 1; i >= index; i--) { // shift all element right // from given index array[i + 1] = array[i]; } // insert data at given index array[index] = data; count++; } // function remove last element or put // zero at last index public void remove() { if (count > 0) { array[count - 1] = 0; count--; } } // function shift all element of right // side from given index in left public void removeAt(int index) { if (count > 0) { for (int i = index; i < count - 1; i++) { // shift all element of right // side from given index in left array[i] = array[i + 1]; } array[count - 1] = 0; count--; } } // Driver code public static void Main() { DynamicArray da = new DynamicArray(); // add 9 elements in array da.add(1); da.add(2); da.add(3); da.add(4); da.add(5); da.add(6); da.add(7); da.add(8); da.add(9); // print all array elements after add 9 elements Console.WriteLine("Elements of array:"); for (int i = 0; i < da.size; i++) { Console.Write(da.array[i] + " "); } Console.WriteLine(); // print size of array and no of element Console.WriteLine("Size of array: " + da.size); Console.WriteLine("No of elements in array: " + da.count); // shrinkSize of array da.shrinkSize(); // print all array elements Console.WriteLine("Elements of array " + "after shrinkSize of array:"); for (int i = 0; i < da.size; i++) { Console.Write(da.array[i] + " "); } Console.WriteLine(); // print size of array and no of element Console.WriteLine("Size of array: " + da.size); Console.WriteLine("No of elements in array: " + da.count); // add an element at index 1 da.addAt(1, 22); // print Elements of array after adding an // element at index 1 Console.WriteLine("Elements of array after" + " add an element at index 1:"); for (int i = 0; i < da.size; i++) { Console.Write(da.array[i] + " "); } Console.WriteLine(); // print size of array and no of element Console.WriteLine("Size of array: " + da.size); Console.WriteLine("No of elements in array: " + da.count); // delete last element da.remove(); // print Elements of array after delete last // element Console.WriteLine("Elements of array after" + " delete last element:"); for (int i = 0; i < da.size; i++) { Console.Write(da.array[i] + " "); } Console.WriteLine(); // print size of array and no of element Console.WriteLine("Size of array: " + da.size); Console.WriteLine("No of elements in array: " + da.count); // delete element at index 1 da.removeAt(1); // print Elements of array after delete // an element index 1 Console.WriteLine("Elements of array after" + " delete element at index 1:"); for (int i = 0; i < da.size; i++) { Console.Write(da.array[i] + " "); } Console.WriteLine(); // print size of array and no of element Console.WriteLine("Size of array: " + da.size); Console.WriteLine("No of elements in array: " + da.count); } } /* This code contributed by PrinciRaj1992 */ class DynamicArray { // Pointer to store array created using new keyword #array; // Size of array #size; // Container size #capacity; // Default constructor with size 1 constructor() { this.#capacity = 1; this.#size = 0; this.#array = new Array(this.#capacity); } // Taking size from the user // constructor(capacity) { // this.#capacity = capacity; // this.#array = new Array(this.#capacity); // this.#size = 0; // } // Returns the size of Array i.e Total elements stored currently getSize() { return this.#size; } // Returns the size of container getCapacity() { return this.#capacity; } // Inserting element after last stored index push_back(value) { // check is array having size to store element or not if (this.#size == this.#capacity) { // if not then grow the array by double this.growArray(); } // insert element this.#array[this.#size] = value; // increment the size or last_index+1 this.#size++; } // Deleting element at last stored index pop_back() { // Replace the last index by 0 this.#array[this.#size - 1] = 0; // Decrement the array size this.#size--; // Reduce if the container half element of its capacity if (this.#size == Math.floor(this.#capacity / 2)) { this.shrinkArray(); } } // Increase the array size by double of current capacity growArray() { // Creating new array of double size let temp = new Array(this.#capacity * 2); this.#capacity = this.#capacity * 2; // copy element of old array in newly created array for (let i = 0; i < this.#size; i++) { temp[i] = this.#array[i]; } // Delete old array this.#array = null; // Assign newly created temp array to original array this.#array = temp; } // Reduce the size of array by half shrinkArray() { // Creating new array of half size this.#capacity = this.#size; let temp = new Array(this.#capacity); // copy element of old array in newly created array for (let i = 0; i < this.#size; i++) { temp[i] = this.#array[i]; } // Delete old array this.#array = null; // Assign newly created temp array to original array this.#array = temp; } // Searching element in the given array search(key) { for (let i = 0; i < this.#size; i++) { if (this.#array[i] == key) { // If element found return its index return i; } } // Return -1 if element not found; return -1; } // Insert element at given index insertAt(index, value) { // check is array having size to store element or not if (this.#size == this.#capacity) { // if not then grow the array by double this.growArray(); } for (let i = this.#size - 1; i >= index; i--) { this.#array[i + 1] = this.#array[i]; } this.#array[index] = value; this.#size++; } // Delete element at given index // Delete element at given index deleteAt(index) { for (let i = index; i < this.#size; i++) { this.#array[i] = this.#array[i + 1]; } // Replace the last index by 0 this.#array[this.#size - 1] = 0; // Decrement the array size this.#size--; // Reduce if the container half element of its // capacity if (this.#size == (this.#capacity / 2)) { shrinkArray(); } } // To Print Array printArrayDetails() { console.log("Elements of array : "); console.log(this.#array); console.log("No of elements in array : ", this.#size , ", Capacity of array :", this.#capacity); } isEmpty() { if (size == 0) { return true; } else { return false; } } }; const da = new DynamicArray(); da.push_back(1); da.push_back(2); da.push_back(3); da.push_back(4); da.push_back(5); da.push_back(6); da.push_back(7); da.push_back(8); da.push_back(9); da.push_back(10); da.push_back(11); da.printArrayDetails(); da.shrinkArray(); console.log("\nCapacity of array after shrinking : " , da.getCapacity()); console.log("\nAfter inserting at index 3 "); da.insertAt(3, 50); da.printArrayDetails(); console.log("\nAfter delete last element "); da.pop_back(); da.printArrayDetails(); console.log("\nAfter deleting at index 3 "); da.deleteAt(3); da.printArrayDetails(); console.log("\nSearching 5 in array "); let index = da.search(5); if (index >= 0) { console.log("Element found at index : ", index); } else { console.log("Element not found "); } // This code is contributed by akashish__ OutputElements of array : 1 2 3 4 5 6 7 8 9 10 11 No of elements in array : 11, Capacity of array :16 Capacity of array after shrinking : 11 After inserting at index 3 Elements of array : 1 2 3 50 4 5 6 7 8 9 10 11 No of elements in array : 12, Capacity of array :22 After delete last element Elements of array : 1 2 3 50 4 5 6 7 8 9 10 No of elements in array : 11, Capacity of array :11 After deleting at index 3 Elements of array : 1 2 3 4 5 6 7 8 9 10 No of elements in array : 10, Capacity of array :11 Searching 5 in array Element found at index : 4
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