how to delete an element from an array c++

Remove a specific element from array

Let's write an algorithm to remove a specific element from the array.

Input

Array : {1, 20, 5, 78, 30}

Element : 78

Output

Array : {1, 20, 5, 30}

Input

Array : {1, 20, 5, 78, 30}

Element : 100

Output

Element Not Found

Algorithm

1. Find the given element in the given array and note the index.

2. If the element found,

Shift all the elements from index + 1 by 1 position to the left.

Reduce the array size by 1.

3. Otherwise, print "Element Not Found"

Visual Representation

Let's take an array of 5 elements.

1, 20, 5, 78, 30.

If we remove element 20 from the array, the execution will be,

1. We need to remove the element 20 (index 1) from the array.

2. Shift all the elements from index + 1 (index 2 to 4) by 1 position to the left.

arr[2] (value 5) will be placed in arr[1].

arr[3] (value 78) will be placed in arr[2].

arr[4] (value 30) will be placed in arr[3].

3. Finally, the new array.

Implementation Algorithm

1. Set index value as -1 initially. i.e. index = -1

2. Get key value from the user which needs to be deleted.

3. Search and store the index of a given key

[index will be -1, if the given key is not present in the array]

4. if index not equal to -1

Shift all the elements from index + 1 by 1 position to the left.

5. else

print "Element Not Found"

Animated Tutorial

Implementation

Example

              /*                              * Program  : Remove an element in the array                              * Language : C                              */              #include<stdio.h>              #define size 5              int              main() {              int              arr[size]              =              {1,              20,              5,              78,              30};              int              key, i, index              =              -              1;      printf("Enter element to delete              \n              ");     scanf("%d",&key);              /*                              * iterate the array elements using loop                              * if any element matches the key, store the index                              */              for(i              =              0; i              <              size; i++)     {              if(arr[i]              ==              key)         {             index              =              i;              break;         }     }              if(index              !=              -              1)     {              //shift all the element from index+1 by one position to the left              for(i              =              index; i              <              size              -              1; i++)             arr[i]              =              arr[i+              1];          printf("New Array : ");              for(i              =              0; i              <              size              -              1; i++)             printf("%d ",arr[i]);     }              else              printf("Element Not Found              \n              ");              return              0; }

Time Complexity Analysis - Remove a specific element from an array

Worst Case - O(N)

If the element which needs to be deleted is present in arr[0], we need to shift all the elements from index 1 to size - 1 by position to the left.

So it will take N - 1 iteration. For example, if the array has 100 elements the for loop will work for 99 times.

Hence the time complexity will be O(N - 1). Polynomially, O(N). Where N is the number of elements in the array.

Best Case - O(1)

If the element present at the last index, then the below for loop will not work. (We won't shift any element.)

            for(i            =            index; i            <            size            -            1; i++)     arr[i]            =            arr[i+            1];

We just reduce the array size by 1.

It will take constant time. Hence the time complexity will be O(1).

Useful Resources

1. https://www.log2base2.com/C/array/arrays-in-c.html [Why do we need array?]
2. https://www.log2base2.com/C/array/declaration-and-initialization-of-array-in-c.html [Declaration and Initialization of array]

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