Two answers to one problem

A corner shop (a small local shop) wants a program. It must show the price of an item and add the delivery fee. Two students each write one.

Both programs give the right answer today. So which is better? This lesson gives you a way to decide and to say why.

Key words

Word	What it means	Example
Requirement	Something the program must do.
Suitable	Right for the job and its requirements.
Constant	A value that never changes while the program runs. Its name is in capitals.	DELIVERY_FEE
Maintainability	How easy it is to fix or change the code later.
Presentation	How the output looks to the user.	£2.50 and not 2.5
Data type	The kind of thing a value is.	string, integer
Data structure	A way to store a group of values.	list, dictionary
Efficient	Uses little time and little memory.
Variable	A name that holds a value.
Dictionary	A group of pairs. Each pair is a key and a value. You find the value by its key.	{"tea": 1.2}
Function	A named piece of code that does a job.

The cells students fill in are left blank in this preview.

Solution A

Code: python

item = "tea"
if item == "tea":
    price = 1.2
elif item == "coffee":
    price = 2.5
elif item == "cake":
    price = 3.0
if price < 30:
    price = price + 3.5
print(price)

# Run it. Then read the code and look for problems.

Solution B

Code: python

PRICES = {"tea": 1.2, "coffee": 2.5, "cake": 3.0}
DELIVERY_FEE = 3.5
FREE_DELIVERY_FROM = 30.0

def order_total(item):
    total = PRICES[item]
    if total < FREE_DELIVERY_FROM:
        total = total + DELIVERY_FEE
    return total

# {:.2f} shows a number with 2 decimal places.
print("Total: £{:.2f}".format(order_total("tea")))

How to judge a solution

Do not say "B is nicer". Give reasons. Use a list of questions. Each question looks at one part of the code.

A good judgement names the part, says what you see, and says why it matters. For example: "B uses a dictionary, so adding an item is one line."

The judging questions

Question	What to look for
Does it meet the requirements?	Does it do everything the user asked, and nothing wrong?
Is it efficient in time?	Does it do more steps than it needs?
Is it efficient in storage?	Does it keep more data in memory than it needs?
Are the data structures right?	A list for items in order. A dictionary for finding a value by its name
Are the data types, variables and constants right?	The right type for each value. Fixed values named once as constants
Is the presentation clear?	Does the output look right: pound sign, two decimals, a message
Is it maintainable?	Meaningful names, short functions, no repeated code

The cells students fill in are left blank in this preview.

A judgement of A and B

Question	Solution A	Solution B
Meets the requirements	Yes for three items	Yes for three items
Adding a new item	Needs a new elif	One new line in PRICES
Fees and limits	The numbers 3.5 and 30 are typed in the code	Named once as constants
Presentation	Prints 4.7	Prints £4.70
Maintainability	Weak: the fee may be typed in many places	Strong: change the constant once
Verdict	Works, but hard to change later	More suitable

The cells students fill in are left blank in this preview.

Try it: the wrong data type

Code: python

phone_text = "07700900123"
phone_number = int(phone_text)

print(phone_text)
print(phone_number)

# The integer lost its first 0.
# A phone number is not for adding up, so it should be a string.