Python Program for Prime Number

What is Prime Number?

A positive natural number greater than 1, which only divisible by itself and 1 is known as a prime number. 

For example, 23 is a prime number because it is only divisible by 1 and itself whereas 24 is not a prime number because it is divisible by 1,2,3,4,6,8,12 and itself.

In this tutorial, you will learn how to write a python program to check whether a number is a prime number or not.

Python Program to Check Prime Number

Approach of Program

• The first thing you need to check in the program is that the input variable must be greater than 1 as mentioned above prime number is greater than 1.
• The second thing you need to check is if the input num is exactly divisible by any number from 2 to num – 1. If in case you find a factor in that range, then the number is not prime or else the number is prime.

1) Check Prime Number Using For Loop

# Program to check if a number is prime or not
# Input from the user
num = int(input("Enter a number: "))

# If number is greater than 1
if num > 1:
   # Check if factor exist  
   for i in range(2,num):
       if (num % i) == 0:
           print(num,"is not a prime number")
           break
   else:
       print(num,"is a prime number")
       
# Else if the input number is less than or equal to 1
else:
   print(num,"is not a prime number")

OUTPUT:

Enter a number: 9
9 is not a prime number

Enter a number: 23
23 is a prime number

Explanation

In the above code, the input() method is used for obtaining ‘num’ value from the user. We know number less than or equal to 1 are not prime numbers thus we only perform an operation on the value if ‘num’ greater than 1.

If ‘num’ is greater than 1 is true the for loop is executed. This loop checks the numbers between 2 and the number entered by the user. For every number within this range, another if statement is executed with the code if (number % i) == 0. If this condition is True, a string is printed using the statement print(num, is not a prime number). Otherwise, a print statement print(num, is a prime number) is printed. The last else statement is executed when the number entered is less than or equal to 1.

According to the output, the user has entered 9 as the number. As it is not a prime number, the string “9 is not a prime number“ is printed to the screen. But when 23 is entered, the string “23 is a prime number“ is printed.

2) Check Prime Number Using While Loop 

# Program to check if a number is prime or not

# Input from the user
num = int(input('Please enter a number:'))
# Declaring and Initialization of two integer type variable
i = 2
flag = 0
while i<num:
    if num%i == 0:
        #If Yes,update flag value
        flag = 1
        print (num,"is NOT a prime number!");
    #Updating the value of i on every iteration by 1
    i = i + 1
#checking the value of flag
if flag == 0:
    #If Yes, Then it is a prime number
    print (num,"is a prime number!");

OUTPUT :

Enter a number: 9
9 IS NOT A PRIME NUMBER

Enter a number: 23
23 IS A PRIME NUMBER 

Explanation

In the program, the input method is used for fetching a number from the user to evaluate if it is a prime number. After converting the value to an integer, the value is stored in the variable num. Then, a variable i is assigned a value 2. The flag variable is assigned a value 0.

In the next line, A while loop is executed that runs as long as the i variable is less than the num variable. Inside the while loop, an if statement checks the modulation value of the number divided by the i variable. If the modulation value is 0, then the flag variable is assigned the value of 1. Then a break statement moves the control out of the loop.

Outside the loop, the i variable is incremented by 1. The last part of the code checks the flag value. If the value of the flag variable is equal to 0, a print statement displays “is a prime number” along with the number.

If the flag value is 1, the string “9 is not a prime number” is displayed.

So, when the user enters 9, the string is not prime a prime number” is printed. But, when the number 23 is entered, the string “23 is a prime number” is printed.

Conclusion

As a prime number is supposed to be a positive integer, the program checks the condition at the beginning. So, it is best to enter a positive value for checking whether it is prime or not.