Python program to check whether a number is Prime or not

Given a positive integer N. The task is to write a Python program to check if the number is prime or not.

Definition: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The first few prime numbers are {2, 3, 5, 7, 11, ….}.

Examples :

Input:  n = 11
Output: true

Input:  n = 15
Output: false

Input:  n = 1
Output: false

The idea to solve this problem is to iterate through all the numbers starting from 2 to (N/2) using a for loop and for every number check if it divides N. If we find any number that divides, we return false. If we did not find any number between 2 and N/2 which divides N then it means that N is prime and we will return True.

Below is the Python program to check if a number is prime:



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# Python program to check if 
# given number is prime or not
  
num = 11
  
# If given number is greater than 1
if num > 1:
      
   # Iterate from 2 to n / 2 
   for i in range(2, num//2):
         
       # If num is divisible by any number between 
       # 2 and n / 2, it is not prime 
       if (num % i) == 0:
           print(num, "is not a prime number")
           break
   else:
       print(num, "is a prime number")
  
else:
   print(num, "is not a prime number")

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Output:

11 is a prime number

Optimized Method
We can do following optimizations:

  1. Instead of checking till n, we can check till √n because a larger factor of n must be a multiple of smaller factor that has been already checked.
  2. The algorithm can be improved further by observing that all primes are of the form 6k ± 1, with the exception of 2 and 3. This is because all integers can be expressed as (6k + i) for some integer k and for i = ?1, 0, 1, 2, 3, or 4; 2 divides (6k + 0), (6k + 2), (6k + 4); and 3 divides (6k + 3). So a more efficient method is to test if n is divisible by 2 or 3, then to check through all the numbers of form 6k ± 1. (Source: wikipedia)
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    # A optimized school method based 
    # Python3 program to check
    # if a number is prime
      
      
    def isPrime(n) :
      
        # Corner cases
        if (n <= 1) :
            return False
        if (n <= 3) :
            return True
      
        # This is checked so that we can skip 
        # middle five numbers in below loop
        if (n % 2 == 0 or n % 3 == 0) :
            return False
      
        i = 5
        while(i * i <= n) :
            if (n % i == 0 or n % (i + 2) == 0) :
                return False
            i = i + 6
      
        return True
      
      
    # Driver Program 
    if (isPrime(11)) :
        print(" true")
    else :
        print(" false")
          
    if(isPrime(15)) :
        print(" true")
    else
        print(" false")
          
          
    # This code is contributed 
    # by Nikita Tiwari.

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    Main Article : Primality Test | Set 1 (Introduction and School Method)



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