# 9. Putting Things Together

• What have we seen so far?

• Values

• Types

• Variables

• Print

• Input

• Functions

• Booleans

• Logic

• If/Else

• 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

1. Start with an empty function and have it return some arbitrary constant value (e.g., `0`)

2. Run the function and verify that it does what you expect

3. Add one or two lines of code

4. Run the function and verify that it does what you expect

5. Repeat

• 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

Problem

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:

• Age

• 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

Step 1

• Read the problem

Step 2

• 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

Step 3

• Chip away at the problem

Note

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.

#### 9.2.1.1. Input

```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))
```

Note

In the above example, we would want to verify it is doing what we expect. Since user input is a little difficult to test with `assert`, we can simply `print` out the data and confirm that it is doing what we expect. For example, the following code could be added to the above example.

```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 `total_charge` calculation

• If you were to run the example code, it would not work since `total_charge` is 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

#### 9.2.1.2. 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

#### 9.2.1.3. 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_start` is by no means difficult to understand, `total_kms` is even clearer

#### 9.2.1.4. 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)
```

#### 9.2.1.5. 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 `average_kms_per_day` we wrote

``` 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)
```

Note

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.

#### 9.2.1.6. Revisit Calculating the Total Charge

• With the functions we wrote, solving the big `calculate_total_charge` becomes simpler

``` 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
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

• The `if` for the rental classification simply evaluates the corresponding cost calculation

• The `if` for the age adds an additional \$10/day

• Let’s try: Google colab.

Note

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.

Activity

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?