In this program, we have a number given by the user. We have to print whether that number is prime or not. We will code prime number program in python.

## Prime Number Program in Python using while loop

In this python program, we will check whether the given number is prime or not. We will use the while loop to check that the given number is divisible by any number between 2 and the square root of the given number. It will reduce our time complexity from O(n) to O(sqrt(n)).

```
num=int(input("Enter a number: "))
check=True
i=2
while i*i<=num:
if(num%i==0):
check=False
break
i+=1
if(check and num!=1):
print(str(num)+" is a Prime Number")
else:
print(str(num)+" is not a Prime Number")
```

Output:

`Enter a number: 1616 is not a Prime Number`

## Using for loop

In this program, we will make a python program to check prime numbers. It will print whether the given number is prime or not.

```
# check given number is prime or not
num=int(input("Enter a number: "))
check=True
for i in range(2,num):
if(num%i==0):
check=False
break
if(check and num!=1):
print(str(num)+" is a Prime Number")
else:
print(str(num)+" is not a Prime Number")
```

Output:

`Enter a number: 997997 is a Prime NumberEnter a number: 1515 is not a Prime Number`

## Using function

```
def prime_checker(number):
f=1
for i in range (2,number):
if(number%i==0):
f=0
break
if f:
print("It's a prime number.")
else:
print("It's not a prime number.")
n = int(input("Check this number: "))
prime_checker(number=n)
```

## Approach for Python program to check prime number:

We will write a python program to find whether a given number is prime or not.

Prime Number is a number that is divisible only by one and itself.

Let’s take a number here, so let’s say 11:

We will try to divide this number by two because all the numbers are divisible by one, so we will start with 2. We will check if 11 can be divided by two; if yes, then it’s not a prime number; if it’s not divisible by “2,” then we’ll go for three, then we’ll go for four, and so on till 10.

Read also: Factorial Program in Python.

We start from 2, and we’ll end with the given number minus 1. For example: if we take a number that is 17, then we will end at 16.

Every time you have to check if you find the number which is divisible by any other number between 2 & one less than the given number, then the given number is not a prime number. We can print that the given number is a prime number when we find that the given number is not divisible by the last number (which is number-1).

### About this post:

This post is all about the prime number program in python. If you have doubts related to this program, let me know in the comment section.