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Python Program to Check a Prime Number

Beginner · Numbers · CBSE class 11 practical list · CBSE class 12 practical list

A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself, like 2, 3, 5, 7 and 29. Numbers greater than 1 that are not prime are called composite. This program checks whether a number is prime by looking for any divisor.

Program

prime_number.py
num = 29 # or: num = int(input("Enter a number: "))
if num < 2:
print(num, "is neither prime nor composite")
else:
is_prime = True
# A divisor larger than the square root would pair with a smaller one,
# so checking up to the square root is enough.
for i in range(2, int(num ** 0.5) + 1):
if num % i == 0:
is_prime = False
break
if is_prime:
print(num, "is a prime number")
else:
print(num, "is a composite number")
Output
29 is a prime number

How it works

  1. Numbers below 2 are handled first, because 0 and 1 are neither prime nor composite.
  2. The loop tries every divisor from 2 up to the square root of num.
  3. If num % i == 0 for any i, the number has a divisor, so it is not prime and the loop stops with break.
  4. If no divisor is found, is_prime stays True and the program reports a prime number.

Another way to write it

Using a function

Using a function
def is_prime(n):
if n < 2:
return False
for i in range(2, int(n ** 0.5) + 1):
if n % i == 0:
return False
return True
for n in [2, 9, 17, 25, 29]:
print(n, "prime" if is_prime(n) else "not prime")
Output
2 prime
9 not prime
17 prime
25 not prime
29 prime

Putting the check in a function lets you test many numbers at once.

Try it yourself

Change the program to print every prime number between 1 and 50.

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