Python Program to Reverse all Prime Numbers in Array Without Affecting Elements

Given an array, our task is to Reverse all prime numbers in an array without affecting other elements in Python.

Examples:

Input : arr = {1, 2, 3, 4, 5, 6}
Output : {1,5,3,4,2,6}
Explanation: Prime elements in given array are 2,3,5. After reversing the modified array 
will be {1,5,3,4,2,6}

Approach:

• Start traversing the array from left to right.
• Maintain another array for storing prime numbers.
• If the current element is prime then append the current element to the prime array.
• Start traversing the array again and replace the prime element with the element present in the prime array from the last.
• Finally, return the modified array.

Below is the implementation of the above approach

Python3

from math import sqrt
# Function to check element is prime


def isPrime(x):
# flag maintains status if x is prime
    prime = 0
    if(x > 1):
        for i in range(2, int(sqrt(x)) + 1):
            if (x % i == 0):
                prime = 1
                break
        if (prime == 0):
            return True
        else:
            return False
    else:
        return False


def reversePrimes(arr):
    n = len(arr)
    # array to store prime elements
    prime = []
    for i in range(n):
        if isPrime(arr[i]):
            prime.append(arr[i])
    # variable to store index of prime array
    k = len(prime)-1
    # Traversing again to modify the array
    for i in range(n):
        if isPrime(arr[i]):
            arr[i] = prime[k]
            k = k-1
    return arr


print(reversePrimes([1, 2, 3, 4, 5, 6]))

Output:

[1, 5, 3, 4, 2, 6]

Complexity Analysis:

Let x be the largest prime number in the array.

Time complexity: O(n*sqrt(x))

Space Complexity: O(n) for storing prime elements