9. Putting Things Together
What have we seen so far?
Each of the individual topics may feel simple on their own
The difficult part tends to be when putting these topics together to solve complex problems
9.1. Writing Bigger Programs
There is no single correct way to write programs, but there are some strategies
For now I recommend a bottom up, incremental approach
Start with an empty function and have it return some arbitrary constant value (e.g.,
Run the function and verify that it does what you expect
Add one or two lines of code
Run the function and verify that it does what you expect
This incremental strategy is great because a lot of the problem solving you will be doing will be incremental
Additionally, it helps you make sure everything is working along the way
If everything was working and you added two lines of code and suddenly it stops working, perhaps the issue is with the two new lines you wrote
We would still want to write tests for our completed functions, but you may find it difficult to debug a whole complete function when compared to one or two lines of code
9.2. Car Rental
Here we solve a bigger problem than we are used to, but we will follow the incremental approach
In fact, we will take it to another extreme
Instead of just writing a few lines of code, we will make each part a function we can test easily, regardless of how “simple” the part seems
A car rental place needs our help. They want a program to calculate how much a customer is to be charged based on their rental agreement, age, how far they drove, and how long they had the car.
We will get and record the customer’s:
Rental agreement classification code (B or D)
Number of days rented
Starting odometer reading
Ending odometer reading
If the classification code is B
Base charge of $20.00/day
Plus $0.30 for every km driven
If the classification is D
Base charge of $50.00/day
Plus $0.30 for every km driven above the 100km/day average allowance
All renters under the age of 25 are charged an additional $10.00/day
9.2.1. An Incremental Solution
Read the problem
Understand the problem
This cannot be understated — this is a big part of solving any problem
Half of the description is IO
We know how to do this, so we will start here
The other half of the description is the calculation
Chip away at the problem
Understand that the example below is only one possible implementation of a solution to this problem. There is literally an infinite number of ways one could go about solving this problem.
1age = int(input('Age: ')) 2classification = input('Classification Code: ') 3number_of_days = int(input('Number of Days Rented: ')) 4starting_kms = float(input('Odometer reading at start: ')) 5ending_kms = float(input('Odometer reading at end: ')) 6 7total_charge = # Some function to do the total charge calculation 8 9print('The total charge is: ' + str(total_charge))
In the above example, we would want to verify it is doing what we expect. Since user input is a little difficult to
assert, we can simply
1print(age, type(age)) 2print(classification, type(classification)) 3print(number_of_days, type(number_of_days)) 4print(starting_kms, type(starting_kms)) 5print(ending_kms, type(ending_kms))
The above example of reading user input seems to be sufficient for what we need; however, obviously we are far from solving the problem
Line 7 is currently non-functional; it is simply a placeholder for the actual
If you were to run the example code, it would not work since
total_chargeis currently not being assigned to anything
In other words, we need to actually write some function to do the actual calculation for us
The calculation may seem intimidating, but let’s take the same approach as above
We will write the code we can and leave comments for the parts we still need to tackle
126.96.36.199. Calculating The Total Charge
1def calculate_total_charge(some_number_of_parameters): 2 3 # Set up a variable for our total charge 4 total_charge = 0 5 6 # Calculate the number of kilometres traveled. 7 total_kms_traveled = 8 9 # Calculate the average number of kilometers travelled per day 10 average_kms = 11 12 # Calculate the charge based on rental code 13 if rental_code == 'B': 14 # Base charge of $20.00/days + $0.30 for every km driven 15 else: 16 # Base charge of $50.00/days + $0.30 for every km driven above the 100km/day average allowance 17 num_kms_above_allowance = 18 19 # if they're under 25, add additional charge 20 if something : 21 22 # Return the final total charge 23 return some_total_charge
Although it may feel like this is a rather silly function so far, it did help us outline what we need to know in order to solve the problem
We need to know the total kilometers travelled
We need to know the average kms/day
We need to know the number of kms driven above the 100km/day average allowance
We need to do the actual rental agreement classification calculation
We need to add the extra charge for people under 25
188.8.131.52. Total Kilometers
A function to calculate the total number of kms
What do we know?
1def total_kms(odometer_start: float, odometer_finish: float) -> float: 2 """ 3 This function calculates the total number of kilometers driven based 4 on starting and ending odometer readings. 5 6 @rtype: float 7 @param odometer_start: The number of kms the car had before renting 8 @param odometer_finish: The number of kms the car had after rending 9 @return: The total kms driven 10 """ 11 12 return odometer_finish - odometer_start 13 14assert 0 == total_kms(0, 0) 15assert 100 == total_kms(0, 100) 16assert -100 == total_kms(100, 0) 17assert 100.5 == total_kms(100.5, 201)
You may be thinking that turning this simple sub-problem (calculating the total kilometers) into a function is overkill
Perhaps you are right
But, it’s also really straightforward to confirm correctness of this function
It is solving an important sub-problem
It is facilitating our incremental development approach
Although the functionality and purpose of
odometer_finish - odometer_startis by no means difficult to understand,
total_kmsis even clearer
184.108.40.206. Average Kilometers Per Day
A Function to calculate the daily average number of kms
What do we know?
We have a function to calculate the total kms
We also know the number of days the car was rented
1def average_kms_per_day(num_days: float, num_kms: float) -> float: 2 """ 3 Calculate the average number of kilometers driven per day 4 over the rental period 5 6 @rtype: float 7 @param num_days: The total number of days the car was rented 8 @param num_kms: The total number of kilometers driven during the rental period 9 @return: The average number of kilometers driven per day 10 """ 11 12 return num_kms / num_days 13 14 15assert 0 == average_kms_per_day(1, 0) 16assert 1 == average_kms_per_day(1, 1) 17assert -1 == average_kms_per_day(-1, 1) 18assert 0.5 == average_kms_per_day(3, 1.5)
220.127.116.11. Kilometers Above Allowable Average
Now for something a little harder
Number of kms over the daily average allowance
What do we know?
Average kms/day given the function
1def num_kms_above_average(avg_num_kms: float) -> float: 2 """ 3 Calculates the number of kms the renter went over of their daily allowance. 4 We will use the customer's average daily kms. 5 6 @rtype: float 7 @param avg_num_kms: average number of kms driven per day 8 @return: The number of kms over 100 they went (return 0 if it's less than 100) 9 """ 10 11 # If the average kms traveled is above 100, 12 # return how much above, otherwise zero 13 if avg_num_kms > 100: 14 return avg_num_kms - 100 15 else: 16 return 0 17 18 19assert 0 == num_kms_above_average(100) 20assert 1 == num_kms_above_average(101) 21assert 0 == num_kms_above_average(99) 22assert 100 == num_kms_above_average(200)
If you were wondering why
num_kms_above_average had the two
return statements instead of having only one,
good observation; however, having two vs. one is not any more or less correct — it’s simply different.
Further, there is a good argument for making use of a constant instead of hard coding the
100 for the daily
average limit. Perhaps something like
AVERAGE_DAILY_LIMIT. Or maybe have the function include another parameter
for the limit as that would make it far more general.
Remember, with these small differences discussed, one is not more correct than the other. There is literally an infinite number of ways one could go about solving this problems.
18.104.22.168. Revisit Calculating the Total Charge
With the functions we wrote, solving the big
1def calculate_total_charge(num_days: float, age: float, rental_code: str, odometer_start: float, odometer_finish: float) -> float: 2 """ 3 Calculate how much the renter needs to be charged based on the classification, 4 the number of kms travelled and the age of the driver. 5 6 @rtype: float 7 @param num_days: Number of days the car was rented. 8 @param age: Age of the driver. 9 @param rental_code: The classification code (B ord D). 10 @param odometer_start: Odometer when the renter took the car. 11 @param odometer_finish: Odometer when the renter returned the car. 12 @return: The amount to charge the renter. 13 """ 14 # Set up a variable for our total charge 15 total_charge = 0 16 17 # Calculate the number of kilometres traveled. 18 total_kms_traveled = total_kms(odometer_start, odometer_finish) 19 20 # Calculate the average number of kilometers travelled per day 21 average_kms = average_kms_per_day(num_days, total_kms_traveled) 22 23 if rental_code == "B": 24 total_charge = 20.00 * num_days + 0.30 * total_kms_traveled 25 else: 26 total_charge = 50.00 * num_days + 0.30 * num_kms_above_average(average_kms) 27 28 # if they're under 25, add additional charge 29 if age < 25: 30 total_charge += 10 * num_days 31 32 # Return the final total charge 33 return total_charge 34 35 36assert 20 == calculate_total_charge(1, 30, "B", 0, 0) 37assert 50 == calculate_total_charge(1, 30, "D", 0, 0) 38assert 30 == calculate_total_charge(1, 20, "B", 0, 0) 39assert 60 == calculate_total_charge(1, 20, "D", 0, 0) 40assert 50 == calculate_total_charge(1, 30, "B", 0, 100) 41assert 50 == calculate_total_charge(1, 30, "D", 0, 100) 42assert 60 == calculate_total_charge(1, 20, "B", 0, 100) 43assert 60 == calculate_total_charge(1, 20, "D", 0, 100) 44assert 190 == calculate_total_charge(2, 30, "B", 0, 500) 45assert 145 == calculate_total_charge(2, 30, "D", 0, 500) 46assert 210 == calculate_total_charge(2, 20, "B", 0, 500) 47assert 165 == calculate_total_charge(2, 20, "D", 0, 500)
Take the time to go over all the parts of this function
If any part feels intimidating, slow down
The new functions were used to simplify much of the calculation
iffor the rental classification simply evaluates the corresponding cost calculation
iffor the age adds an additional $10/day
Let’s try: Google colab.
There was nothing stopping us from writing a function for the rental classification calculation or the age calculation. If you feel that would be better, then I would encourage you to do that. Again, assuming your implementation does what is required, it would not be any more or less correct than this implementation.
Think about how you would write this differently
Would you use all the same functions?
Would you change how the functions worked?
Would you move where you called the functions?
Would you add additional functions?
Would you use constants? Where?