My 'lowest common multiple' program hangs, and doesn't output an answer

Question

I have been trying to get more into programming, so I've been trying to make a simple program that takes two numbers as input, and computes the lowest common multiple. I did this in Python because I don't know how to take input in Java. What happens now is the program just hangs after I input my numbers, and nothing happens. Any pointers here would be greatly appreciated. Thank you.

#LCM Calculator
#Author: Ethan Houston
#Language: Python
#Date: 2013-12-27
#Function: Program takes 2 numbers as input, and finds the lowest number
# that goes into each of them

def lcmCalculator(one, two):
    """ takes two numbers as input, computes a number that evenly 
        divides both numbers """
    counter = 2 #this is the number that the program tried to divide each number by.
                #it increases by 1 if it doesn't divide evenly with both numbers.
    while True:
        if one % counter == 0 and two % counter == 0:
            print counter
            break
        else:
            counter += 1

print "\nThis program takes two numbers and computes the LCM of them...\n"

first_number = input("Enter your first number: ")
second_number = input("Enter your second number: ")

print lcmCalculator(first_number, second_number)

Answer 1

Your logic is a bit off. This line:

if one % counter == 0 and two % counter == 0:

needs to be rewritten like this:

if counter % one == 0 and counter % two == 0:

Also, your function should return counter instead of print it. This has two advantages:

1. It will keep the script from printing None at the end (the function's default return value).

2. It allows you to condense these two lines:

   print counter
   break
   

   into just one:

   return counter
   

Finally, as @FMc noted in a comment, you can improve the efficiency of the function by doing two things:

1. Starting counter at the smaller of the function's two arguments.

2. Incrementing counter by this value.

---

Below is a version of your script that addresses all this:

#LCM Calculator
#Author: Ethan Houston
#Language: Python
#Date: 2013-12-27
#Function: Program takes 2 numbers as input, and finds the lowest number
# that goes into each of them

def lcmCalculator(one, two):
    """ takes two numbers as input, computes a number that evenly 
        divides both numbers """
    counter = min_inp = min(one, two)
    while True:
        if counter % one == 0 and counter % two == 0:
            return counter
        else:
            counter += min_inp

print "\nThis program takes two numbers and computes the LCM of them...\n"

first_number = input("Enter your first number: ")
second_number = input("Enter your second number: ")

print lcmCalculator(first_number, second_number)

---

Oh, and one more thing. input in Python 2.x evaluates its input as real Python code. Meaning, it is dangerous to use with uncontrolled input.

A better approach is to use raw_input and then explicitly convert the input into integers with int:

first_number = int(raw_input("Enter your first number: "))
second_number = int(raw_input("Enter your second number: "))

Answer 2

Try this:

#!/usr/local/cpython-2.7/bin/python

def lcmCalculator(one, two):
    """ takes two numbers as input, computes a number that evenly
        divides both numbers """
    counter = 2 #this is the number that the program tried to divide each number by.
                #it increases by 1 if it doesn't divide evenly with both numbers.
    while True:
        if counter % one == 0 and counter % two == 0:
            break
        else:
            counter += 1
    return counter

print "\nThis program takes two numbers and computes the LCM of them...\n"

first_number = int(input("Enter your first number: "))
second_number = int(input("Enter your second number: "))

print lcmCalculator(first_number, second_number)

Answer 3

You need to let the loop end if it finds no factor, instead of while True:

def lcmCalculator(one, two):

    counter = 2    
    while counter <= min(one, two):
        if one % counter == 0 and two % counter == 0:
            return counter
        else:
            counter += 1

    return "No common factor found"

print "\nThis program takes two numbers and computes the LCM of them...\n"

first_number = input("Enter your first number: ")
second_number = input("Enter your second number: ")

print lcmCalculator(first_number, second_number)