# Python Program to Check Prime Number

A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.

### Method 1: Using conditional statements

In the example below, the number called MyNum is checked for prime number by dividing it with all natural numbers starting from 2 to N - 1.

```MyNum = 17
n = 0
for i in range(2,MyNum):
if MyNum % i == 0:
n = n + 1
break
if n == 0:
print(MyNum,"is a prime number.")
else:
print(MyNum,"is not a prime number.")
```

The above code will give the following output:

```17 is a prime number.
```

### Method 2: Using function

In the example below, a function called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.

```def primenumber(MyNum):
n = 0
i = 2
for i in range(2,MyNum//2+1):
if MyNum % i == 0:
n = n + 1
break
if n == 0:
print(MyNum,"is a prime number.")
else:
print(MyNum,"is not a prime number.")

```

The above code will give the following output:

```21 is not a prime number.
```

### Method 3: Optimized Code

• Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
• Except from 2 and 3, every prime number can be represented into 6k ± 1.
```def primenumber(MyNum):
n = 0
i = 2
if MyNum == 2 or MyNum == 3:
print(MyNum,"is a prime number.")
elif MyNum % 6 == 1 or MyNum % 6 == 5:
while i*i <= MyNum:
if MyNum % i == 0:
n = n + 1
break
i = i + 1
if n == 0:
print(MyNum,"is a prime number.")
else:
print(MyNum,"is not a prime number.")
else:
print(MyNum,"is not a prime number.")

```21 is not a prime number.