Four problems, one choice

Problem 1: a pizza app with many steps. Problem 2: a game with 100 zombies. Problem 3: a library that must remember who has which book. Problem 4: a payment rule that must never be wrong.

Does one design method fit all four? No. A good developer thinks first and then chooses. This is called critical thinking.

Key words

Word	What it means
Function	A small named job in a program.
Unit test	A small test that checks one small part of the code.
Design approach	A way of planning software before and while you build it.
Critical thinking	Asking questions, comparing choices and giving reasons for your decision.
Top-down design	Start with the whole system and split it into smaller jobs.
Object-oriented design	Build the program from objects (things that hold data and can act). A class is the plan for making an object.
Data model	A picture of the data you need and how it is organised.
TDD	Test-driven development. Write a failing test first, then the code.
BDD	Behaviour-driven development. Write the wanted behaviour in plain words first.
Functional design	Build the program from small functions that each do one job.
Project method	The plan for how a whole team runs a project, such as waterfall or agile.
Waterfall	A project method with steps one after another: plan, design, build, test.
Agile	A project method that works in short steps and welcomes change.
Sprint	A short, fixed time, often two weeks, in which an agile team finishes some work.

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Six approaches at a glance

Approach	It starts from	Good for	Example
Top-down (function-orientated)	The whole system, split into jobs	A system with many steps and data that moves	A pizza ordering system
Object-oriented	The things in the problem	Many similar things that share behaviour	A game with many characters
Data model	The data and how it links	Systems that store a lot of linked data	A school library
TDD	A failing unit test	Rules that must be exactly right	A fare calculator
BDD	A behaviour in plain words	Work with customers who are not developers	A student ticket feature
Functional	Small functions	Jobs that change data step by step	Sorting and filtering a list

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Approaches work together

The approaches do not compete. One project often uses several.

A game team may plan the whole game top-down. They use classes for the characters. They draw a data model for scores and players. They write tests first for the scoring rules.

You choose by asking questions about the problem.

Try it: three ideas in one program

Code: python

# A data model: a dictionary of prices
MENU = {"pizza": 8, "chips": 3}


# Function-orientated design: one small job
def total(items):
    cost = 0
    for item in items:
        cost = cost + MENU[item]
    return cost


# Object-oriented design: a class
class Order:
    def __init__(self, items):
        self.items = items

    def cost(self):
        return total(self.items)


# Test-driven: the test says what we want
def test_cost():
    assert Order(["pizza", "chips"]).cost() == 11


test_cost()
print("The test passes")