Check for a Prime Number program in python

In this article we will look at how to check if a number is a prime number using python. This is a fairly common basic programming exercise in many languages.

First lets look at a  basic definition of what a prime number is

Definition

A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number.

For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 4 is composite because it is a product (2 × 2) in which both numbers are smaller than 4

The first 25 prime numbers (all the prime numbers less than 100) are as follows

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97

No even number greater than 2 is prime because any such a number can be expressed as the product . Therefore, every prime number other than 2 is an odd number.

Similarly, when written in the usual decimal system, all prime numbers larger than 5 end in 1, 3, 7, or 9.

The numbers that end with other digits are all composite: decimal numbers that end in 0, 2, 4, 6, or 8 are even, and decimal numbers that end in 0 or 5 are divisible by 5

Now lets delve into a python program

Python Example

Save the following as isaprime.py or similar

# A function for checking if  anumber is a prime number 
def CheckPrime(num):  
    # Checking that given number is greater than 1  
    if num > 1:  
        # Iterating over the given number with for loop  
        for j in range(2, int(num/2) + 1):  
            # If the given number is divisible or not  
            if (num % j) == 0:  
                print(num, "is not a prime number")  
                break  
        # it is a prime number  
        else:  
            print(num, "is a prime number")  
    # If the number is 1  
    else:  
        print(num, "is not a prime number")
        
# Get input from the user  
num = int(input("Enter an input number:"))

# Print the result  
CheckPrime(num)  

Now run he program, I used thonny. You can use some of the 25 prime numbers as test data.

Here are a few test runs

Python 3.7.9 (bundled)
>>> %Run isaprime.py
Enter an input number:1
1 is not a prime number
>>> %Run isaprime.py
Enter an input number:2
2 is a prime number
>>> %Run isaprime.py
Enter an input number:58
58 is not a prime number
>>> %Run isaprime.py
Enter an input number:59
59 is a prime number
>>>

Link

This is on GitHub if you would rather look at the code there and download

https://github.com/programmershelp/maxpython/blob/main/code%20example/isaprime.py