Python Program to Check Prime Number

To understand this example, you should have the knowledge of the following Python programming topics:

• Python if…else Statement
• Python for Loop
• Python break and continue

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A positive integer greater than 1 which has no other factors except 1 and the number itself is called a prime number. 2, 3, 5, 7 etc. are prime numbers as they do not have any other factors. But 6 is not prime (it is composite) since, 2 x 3 = 6.

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Example 1: Using a flag variable

# Program to check if a number is prime or not

num = 29

# To take input from the user
#num = int(input("Enter a number: "))

# define a flag variable
flag = False

if num == 0 or num == 1:
print(num, "is not a prime number")
elif num > 1:
# check for factors
for i in range(2, num):
    if (num % i) == 0:
        # if factor is found, set flag to True
        flag = True
        # break out of loop
        break
# check if flag is True
if flag:
    print(num, "is not a prime number")
else:
    print(num, "is a prime number")</code></pre>


Run Code



Output



29 is a prime number



In this program, we have checked if num is prime or not. Numbers less than or equal to 1 are not prime numbers. Hence, we only proceed if the num is greater than 1.



We check if num is exactly divisible by any number from 2 to num - 1. If we find a factor in that range, the number is not prime, so we set flag to True and break out of the loop.



Outside the loop, we check if flag is True or False.




If it is True, num is not a prime number.



If it is False, num is a prime number.




Note: We can improve our program by decreasing the range of numbers where we look for factors.



In the above program, our search range is from 2 to num - 1.



We could have used the range, range(2,num//2) or range(2,math.floor(math.sqrt(num)+1)). The latter range is based on the fact that a composite number must have a factor less than or equal to the square root of that number. Otherwise, the number is prime.



You can change the value of variable num in the above source code to check whether a number is prime or not for other integers.



In Python, we can also use the for...else statement to do this task without using an additional flag variable.