3.15. Even Number of Customers

In this question, the task is for helping a shop owner who wants to know whether there are even number of customers in her shop at any time.

Whenever she needs the answer, she should call even_customers function with no argument and this function must return True or False according to the number of customers in the shop being even or odd respectively. To keep track of the incoming and outgoing customers, she will call incoming and outgoing functions. These functions do not take any arguments and do not return anything.

Hint: Think about using a global variable which is accessible to all three functions.

Sample I/O:

>>> from even_customers import *
>>> incoming()
>>> incoming()
>>> incoming()
>>> even_customers()
False
>>> outgoing()
>>> even_customers()
True

In these sample calls, as 3 consecutive calls of incoming function shows, 3 customers arrived to the shop. That’s why the first call of even_customers function returns False. Then a customer left, as indicated by the call of outgoing function. At that time, there were 2 customers in the shop, therefore, the second call of even_customers returns True.

Show the Answer

The solution will be constructed incrementally with requested functions and a common, global variable. First of all, let’s define this global variable:

counter = 0

Since the definition of this variable is not in any function, it will be in the global scope. Therefore, all of the functions will be able to reach this variable.

Now we can define incoming function as below:

def incoming():
    global counter
    counter += 1

This function needs to update global variable counter, and to be able to do that, it is declared with global keyword at the beginning of function.

The outgoing function can be defined in the same manner:

def outgoing():
    global counter
    if counter > 0: counter -= 1

Finally, the definition of even_customers function becomes like following:

def even_customers():
    return counter % 2 == 0

Note that the global variable, counter is used read-only and not updated here. Therefore declaration with global keyword is not necessary.