Exercise 05-23:
Write a program that reads a number and checks whether it is a prime number. To check, calculate the rest of the division of the number by 2 and then by all the odd numbers up to the number read. If the remainder of one of these divisions equals zero, the number is not prime. Note that 0 and 1 are not prime and that 2 is the only even prime number.
Answer:
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# Python From Scratch
# Author: Nilo Ney Coutinho Menezes
# Editora Novatec (c) 2010-2025 - LogiKraft 2025
# Site: https://pythonfromscratch.com
# ISBN: 978-85-7522-949-1 (Paperback), 978-85-7522-950-7 (hardcover), 978-85-7522-951-4 (ebook)
#
# File: chapter 05/exercise-05-23.py.py
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n = int(input("Enter a number:"))
if n < 0:
print("Invalid number. Please enter only positive values")
if n == 0 or n == 1:
print(f"{n} is a special case.")
else:
if n == 2:
print("2 is prime")
elif n % 2 == 0:
print(f"{n} is not prime, as 2 is the only even prime number.")
else:
x = 3
while x < n:
if n % x == 0:
break
x = x + 2
if x == n:
print(f"{n} is prime")
else:
print(f"{n} is not prime, as it is divisible by {x}")