Finding and displaying all odd numbers using Python

Sometimes even an easiest task can be daunting especially when you have just started learning a programming language. This tutorial is for python programmers who have started learning the language.

What is an odd number?

Mathematically speaking, if a number is not fully divisible by 2 then it is an odd number. To check whether or not a given number is an odd number then we just need to look up to its modulus. In Python, to find modulus (remainder) of a division we use modulo operator (which is “%”).

You can read more in detail in the Wikipedia’s article.

How to use modulo operator?

Instead of using normal division operator “/”, we’ll use modulo operator “%” for modulo operation. Writing following line in Python shell will give us modulus (remainder) of the division of 9 / 2:

9 % 2

Output is as following:

Finding all odd numbers between 0 and 100

In Python, if a number is completely divisible by 2 then the modulus will be 0 for an even number. However, if modulus is 1 then that would be an odd number. With that knowledge in our hand, we can write a For-Loop to find odd numbers between 0 and let’s say 100.

for number in range(100):
    if number % 2 > 0:
        print("{0} is an odd number".format(number))

Find all odd numbers up to user’s input

Instead of fixed number for our For-Loop in above code, we can ask user to give us a number to calculate odd numbers up to that number:

max_number = int(input("Enter a number: "))
for number in range(max_number):
    if number % 2 > 0:
        print("{0} is an odd number".format(number))

Validation of user input

Although above code works but there is a caveat. We have to check the input of user. What if user mistakenly enter an invalid number (or a string) then our program will crash with error exception on function int(). For that we have to surround that part with Try-Except block. We can also compact our output by replacing newline with tab when writing each number on screen. Then our source code would be as following:

import sys

print("*** Find All Odd Numbers ***")

user_input = input("Please enter a number: ")
try:
    max_number = int(user_input)
except ValueError:
    print("Input was not a valid number.")
    sys.exit(1)

if max_number < 0:
    print("Please enter a positive number.")
else:
    print("Displaying odd numbers from 0 to {0}".format(max_number))
    for number in range(max_number):
        if number % 2 > 0:
            print("{0}".format(number), end='\t')

You can also find above code in Github’s Gist here.

You may also be interested in following articles:

• Find all even numbers using Java while also validating user input