B2.3 Programming Constructs

Sequence, selection, loops and functions. All examples use Python.

Contents

B2.3.1: Sequence
B2.3.2: Selection
B2.3.3: Loops
B2.3.4: Functions and modularization
Practice questions

Construct	Meaning	Python
Sequence	Instructions run one after another, in order.	line by line
Selection	Choose a path depending on a condition.	`if` `elif` `else` `match`
Repetition (iteration)	Repeat a block of code.	`while` `for`

B2.3.1: Sequence

Sequence: instructions run in the order written. Wrong order = wrong result or crash.

total = price * quantity   # used before it exists
price = 20
quantity = 3
print("Total:", total)

NameError: name 'price' is not defined. Did you mean: 'print'?

Problem	What happens	Avoid it by
Infinite loop	End condition never met, e.g. counter never updated.	Update the loop variable inside the loop.
Deadlock	Two processes each hold a resource the other needs, so neither continues.	Sequence steps so resources are used and released at different times.
Incorrect input	Unexpected input crashes the program or gives wrong results.	Validate input (B2.1.3).

B2.3.2: Selection

Selection: a condition is tested and the program takes a different path depending on whether it is True or False.

Part	Runs when
`if condition:`	the condition is True
`elif condition:` (optional, any number)	every condition above was False and this one is True
`else:` (optional, last)	nothing above was True. The catch all.

Only the first True branch runs, so order matters. After age <= 5, the next check only needs age <= 12.

def suggest(age):
    if age < 0 or age > 100:
        return "Invalid age"
    elif age <= 5:
        return "Soft play centre"
    elif age <= 12:
        return "Trip to the park"
    elif age <= 18:
        return "Meet up with friends"
    else:
        return "Go to the movies"

for a in [3, 12, 13, 40, 150]:
    print(a, suggest(a))

3 Soft play centre
12 Trip to the park
13 Meet up with friends
40 Go to the movies
150 Invalid age

Comparison operators

Operator	Meaning	x = 100
`==`	equal to (two equals signs, one `=` assigns)	`x == 100` → True
`!=`	not equal to	`x != 50` → True
`>` `<`	greater than, less than	`x > 150` → False
`>=` `<=`	greater or equal, less or equal	`x >= 100` → True

They also work on strings (alphabetical, character by character), so "9" > "10" is True. Convert input with int() before comparing numbers.

Logical operators

Python	Java	True when	Example
`and`	`&&`	both conditions are True	it is sunny and warm
`or`	`\|\|`	at least one is True	it is sunny or warm
`not`	`!`	the condition is False	it is not raining

The textbook says ! is "not" in Python. Wrong: Python uses not.

Case statements: match

Compares one value against several options. Alternative to a long elif chain. Python: match, Java: switch. case _ = else.

day = "Sun"

match day:
    case "Sat" | "Sun":
        print("Weekend")
    case "Fri":
        print("Almost weekend")
    case _:
        print("Weekday")

Weekend

B2.3.3: Loops

	while loop	for loop
Type	Condition controlled (indefinite)	Count controlled (definite)
Use when	Number of repeats unknown	Number of repeats known, or every item in a list
Watch out	Checked before each pass, may run 0 times. Can loop forever.	`range(a, b)` stops before b

while

n = 1
steps = 0
while n <= 100:        # checked BEFORE every pass
    n = n * 2
    steps = steps + 1
print(n, "after", steps, "doublings")

128 after 7 doublings

Password with 3 attempts:

PASSWORD = "ninja"
attempts = 0
correct = False

while attempts < 3 and not correct:
    guess = input("Password: ")
    attempts = attempts + 1
    if guess == PASSWORD:
        correct = True
    else:
        print("Wrong password")

if correct:
    print("Logged in")
else:
    print("Locked out")

for

for i in range(1, 6):
    print(i, "* 10 =", i * 10)

1 * 10 = 10
2 * 10 = 20
3 * 10 = 30
4 * 10 = 40
5 * 10 = 50

print(list(range(5)))          # 0 up to 4
print(list(range(2, 6)))       # 2 up to 5
print(list(range(10, 0, -2)))  # count down in steps of 2

[0, 1, 2, 3, 4]
[2, 3, 4, 5]
[10, 8, 6, 4, 2]

Looping through a list (linear search):

team = ["Heather", "Toni", "Francesca", "Ben", "Syed",
        "Wes", "Toni", "Ife", "Chidi", "Taro"]

count = 0
for name in team:
    if name == "Toni":
        count = count + 1

print("Number of Tonis:", count)

Number of Tonis: 2

Nested loop: the inner loop runs fully for each pass of the outer one (used for 2D lists).

B2.3.4: Functions and Modularization

Modularization: splitting code into separate functions or modules, each doing one task. Function: a named block of code that performs a task when called.

Advantage	Why
Easier to read	Named blocks show what each part does.
Easier to test	Each function tested on its own.
Easier to reuse	Write once, call many times or import elsewhere.
Easier to update	Change it in one place only.
Teamwork	Modules built in parallel.

Anatomy of a function

def rectangle_volume(w, h, d):     # w, h, d are parameters
    volume = w * h * d             # local variable
    return volume                  # send the answer back

print(rectangle_volume(10, 20, 10))   # 10, 20, 10 are arguments

width = 3
height = 4
depth = 5
v = rectangle_volume(width, height, depth)
print("Volume:", v)

2000
Volume: 60

Term	Meaning	In the example
Function signature	The name plus the parameters it expects.	`rectangle_volume(w, h, d)`
Parameters (formal)	The placeholders in the definition that receive values.	`w, h, d`
Arguments (actual parameters)	The real values or variables sent in when the function is called.	`10, 20, 10` or `width, height, depth`
Return value	The result sent back to the line that called it.	`volume`
Local variable	Created inside the function, only exists while it runs.	`volume`
Global variable	Created outside any function, can be read anywhere.	`width`

return vs print

return sends a value back to use. print only displays it. No return gives None.

Libraries

Library: pre written, pre tested modules you import. Saves time, fewer bugs.

Python library	Used for
`random`	Pseudorandom numbers and choices
`math`	Mathematical functions (sqrt, pi, floor, ceil)
`datetime`	Dates and times
`os`	Access to the operating system (files, folders)

import random, math
random.choice(students)   # random item
random.randint(1, 6)      # 1 to 6 inclusive
math.sqrt(81)             # 9.0

Never name your file random.py: it would be imported instead of the library.

Practice Questions

With and, every condition must be True for the if statement to run its code [1]. With or, only one of the conditions needs to be True [1]. For example, if age >= 18 and has_ticket: needs both, while if day == "Sat" or day == "Sun": runs on either day [1].

Saves time: the code is already written [1], so the programmer does not have to write common functions such as random numbers or square roots from scratch [1].
Reliable: library code has already been tested by many users [1], so it is less likely to contain bugs than new code [1].

import math

def circle_area(radius):
    return math.pi * radius ** 2

print(round(circle_area(5), 2))

78.54

Marks: function defined with a sensible name [1], radius as a parameter [1], correct formula πr² [1], result returned [1].

The user is given clues one at a time and must guess the celebrity. The game ends when the name is guessed (or the clues run out).

ANSWER = "usain bolt"
clues = ["I am an athlete", "I am from Jamaica", "I run very fast",
         "I won 8 Olympic golds", "My pose looks like a lightning bolt"]

clue_number = 0
guessed = False

while clue_number < len(clues) and not guessed:
    print("Clue", clue_number + 1, ":", clues[clue_number])
    guess = input("Who am I? ").lower()
    if guess == ANSWER:
        guessed = True
    else:
        print("Not quite")
        clue_number = clue_number + 1

if guessed:
    print("Correct! You needed", clue_number + 1, "clues")
else:
    print("Out of clues. It was Usain Bolt")

Marks: clues stored in a list [1], loop that gives one clue per pass [1], input and comparison with the answer [1], loop ends when guessed or clues run out [1], correct output for win and lose [1].

price = input("User One, enter the price")
guess = input("User Two, enter your guess")
if guess > price:
    print("too high")

What goes wrong if the price is 10 and the guess is 9?

input() always returns a string, so "9" > "10" compares text and is True. The program says 9 is "too high". Fix: price = int(input(...)) and guess = int(input(...)).

Ninja Notes:

Sequence, selection, iteration: the three building blocks of every program.
if / elif / else runs only the first True branch. Order your conditions carefully.
= assigns, == compares. Convert input with int() before comparing numbers.
and = both, or = at least one, not = flip.
for = count controlled (known number). while = condition controlled (unknown number).
range(1, 6) gives 1 to 5. The stop value is never included.
Parameters are in the definition, arguments are in the call. return sends a value back.
Modularization makes code easier to read, test, reuse and update.