In this post, We will be going to see different ways to write a code in Python for checking whether a number is Prime or not, provided number > 1.

Prime Number is a natural number which is greater than 1 and that has no positive divisors other than 1 and number itself.

Examples :

Input:  n = 17
Output: 17 is a Prime Number

Input:  n = 21
Output: 17 is not a Prime Number

Code 1: Run the loop from 1 to the given number and count the factors if factors are greater than 2 then it is not a prime otherwise prime.

# input number
number = 17

# maxCount variable set to 2
# because prime numbers
# have only 2 divisors
# i.e. 1 and itself
maxCount  = 2

# initialise a variable
count = 0

# loop from 1 to number
for i in range(1, number + 1) :
    
    # if number is divisible by any
    # number (i) then increment 
    # count variable by 1
    if number % i == 0 :
        count += 1

# if value of count is greater than 
# maxCount value then number is not
# prime otherwise prime
if count > maxCount :
    print(number,"is not a Prime Number")

else:
    print(number,"is a Prime Number")

Output :

 17 is a Prime Number

Code 2: Run the loop from 2 to half of its number if any number in between divides the given number then the given number is not a prime otherwise prime.

# input number and flag assignment
# in one line
number, flag = 17, 0

# loop from 2 to half of the
# given number
for i in range(2, number // 2 + 1) :
   
    # if number is divisible by any
    # number (i) in b/w 2 to half of 
    # its number then set falg to 1
    # and break out of the loop
    if number % i == 0 :
        flag = 1
        break

# if flag is 1 then
# number is not prime
# otherwise prime
if flag :
    print(number,"is not a Prime Number")

else:
    print(number,"is a Prime Number")

Output :

 17 is a Prime Number

Code 3: Use the concept of for-else loop.

# input number
number = 17

# loop from 2 to half of the
# given number
for i in range(2, number // 2 + 1) :
    
    # if number is divisible by any
    # number (i) in b/w 2 to half of 
    # its number then it is not
    # a prime number
    if number % i == 0 :
        print(number,"is not a Prime Number")
        break
        
# if not divisible then it is
# a prime number       
else :
    print(number,"is a Prime Number")

Output :

 17 is a Prime Number

Code 4:

# main code
if __name__ == "__main__" :
    
    # input number
    number = 17

    # loop from 2 to half of the
    # given number
    for i in range(2, number // 2 + 1) :

        # if number is divisible by any
        # number (i) in b/w 2 to half of 
        # its number then it is not
        # a prime number
        if number % i == 0 :
            print(number,"is not a Prime Number")
            break

    # if not divisible then it is
    # a prime number
    else :
        print(number,"is a Prime Number")

Output :

 17 is a Prime Number

Code 5: Create a user defined function for checking number is prime or not.

# define a user defined 
# function for checking
# a number is prime or 
# not
def checkPrime(number) :

    # loop from 2 to half of the
    # given number
    for i in range(2, number // 2 + 1) :

        # if number is divisible by any
        # number (i) in b/w 2 to half of 
        # its number then it is not
        # a prime number
        if number % i == 0 :
            print(number,"is not a Prime Number")
            break

    # if not divisible then it is
    # a prime number
    else :
        print(number,"is a Prime Number")

# main code
if __name__ == "__main__" :
    
    # input number
    number = 17
    
    # function calling
    checkPrime(number)

Output :

 17 is a Prime Number