IB Computer Science HL · Year 2 · Lesson 5

Loops, Functions, Modularization and Strings

Construct Python solutions that combine substring manipulation, repetition, functions, modularization and scope.

Paper 250 minutesB2.1.2 · B2.3.3 · B2.3.4

Answer first. Reveal only after you have committed to a response.

Course home IA hub 2027 case study

Today’s targets

What you need to be able to do

2 / 10

construct programs that extract and manipulate substrings;
choose and construct counted and conditional loops appropriately;
use Boolean or relational conditions within loops;
construct reusable functions with inputs and return values;
explain how modularization and local/global scope support maintainable programs.

B2.1.2B2.3.3B2.3.4

Paper 2 habitRead the command term and the data given. A technically correct answer still needs to answer the exact task.

Page 2 of 10

Learn + retrieve

Rapid Recall Deck

3 / 10

Attempt each card first. If a term is new, flip and study it, then answer it again without looking. Mark only secure recall as “Know it”.

Recall deck answers

What is a substring? A contiguous sequence of characters taken from within a larger string.
How does Python slicing select a substring? Use start and end indices such as text[2:6]; the start is included and the end is excluded.
What is a counted loop best suited to? A known or countable number of repetitions, commonly implemented with for and range in Python.
What is a conditional loop best suited to? Repeating while a condition remains True when the number of repetitions is not known in advance.
Why must a while loop normally change something related to its condition? Without progress toward making the condition False, the loop may become infinite.
What does a function provide? A reusable block of code that can receive inputs, perform a task and optionally return a result.
What is modularization? Dividing a program into well-structured, reusable components with clear responsibilities.
Why does scope matter? It controls where a variable can be accessed and helps prevent unintended interference between parts of a program.
What is a local variable? A variable created inside a function or block whose access is limited to that scope.
What is one benefit of modular code? It can be easier to test, reuse, maintain and reason about because responsibilities are separated.

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B2.1.2 · Construct

Extract and manipulate substrings

4 / 10

code = "IBCS-HL-2027"

subject = code[0:4]
level = code[5:7]
year = code[8:12]

display = subject + " " + level + " " + year
updated = code.replace("HL", "SL")

Manipulation vocabulary: the display statement uses concatenation to join string parts, while replace() alters part of the original string.

Trace, then construct

Trace — state the values of subject, level, year, display and updated.

Construct — write one expression that extracts 2027 and one expression that produces "IBCS/HL/2027".

Trace: subject = "IBCS", level = "HL", year = "2027", display = "IBCS HL 2027", updated = "IBCS-SL-2027".

Possible constructions:

code[8:12]
code.replace("-", "/")

Page 4 of 10

B2.3.3 · Loops

Counted or conditional?

5 / 10

Counted loop

for i in range(5):
    print(i)

Use when the repetitions are known or can be generated from a count/range.

Conditional loop

attempts = 0
while attempts < 3:
    print("Try")
    attempts += 1

Use when repetition continues while a condition remains True.

Choose the appropriate loop

Scenario A processes exactly 30 sensor readings. Scenario B repeatedly asks for a password until it is correct or three attempts have been used.

Scenario A — choose counted or conditional and justify.

Scenario B — choose counted or conditional and state what condition should control repetition.

A: a counted loop is natural because exactly 30 readings must be processed. B: a conditional loop is natural because repetition depends on whether the password is still incorrect and attempts remain, for example while not correct and attempts < 3.

Page 5 of 10

Loop conditions

Selection inside repetition

6 / 10

scores = [72, 41, 88, 59, 95]
passes = 0

for score in scores:
    if score >= 60 and score <= 100:
        passes += 1

print(passes)

Trace the loop

State the value of passes after each score is processed and the final printed output.

Track one variable — passes.

Apply both relational tests joined by and.

After 72 → 1; after 41 → 1; after 88 → 2; after 59 → 2; after 95 → 3. Final output: 3.

Page 6 of 10

B2.3.4 · Functions

Modularize a repeated task

7 / 10

ProblemA program needs to calculate a percentage in several different places. Copying the same formula repeatedly makes later changes harder to manage.

Construct a reusable function

Write percentage(part, whole) so it returns 0 when whole == 0; otherwise it returns the percentage value.

Inputs — part and whole.

Selection — handle the zero denominator.

Return — send the result back to the caller.

def percentage(part, whole):
    if whole == 0:
        return 0
    return (part / whole) * 100

Why modularize?A single well-named function can be tested once, reused in multiple places and updated without editing every duplicate formula.

Page 7 of 10

Scope

Trace local and global variables

8 / 10

tax_rate = 0.08

def total_with_tax(price):
    tax = price * tax_rate
    total = price + tax
    return total

answer = total_with_tax(50)
print(answer)

Trace the call

Global — identify variables available outside the function.

Local during the call — identify price, tax and total values.

Output — state the printed result.

Globals: tax_rate = 0.08 and, after the call, answer = 54.0. Locals: price = 50, tax = 4.0, total = 54.0. The program prints 54.0.

Page 8 of 10

Integrated construct

Combine strings, loops and functions

9 / 10

RequirementGiven codes = ["IBCS-HL-2027", "IBDT-SL-2027", "IBCS-SL-2027"], return how many codes begin with the substring "IBCS". Do not use count().

Construct `count_ibcs(codes)`

Function — receive the List as an input.

Loop — traverse every code.

Substring test — compare code[0:4] with "IBCS".

Local counter — update and return it.

def count_ibcs(codes):
    count = 0
    for code in codes:
        if code[0:4] == "IBCS":
            count += 1
    return count

For the given data, the function returns 2.

Page 9 of 10

Homework

Finish the learning cycle

10 / 10

IA — main task

Refine IA Criterion B/C planning so repeated tasks are decomposed into clear modules or functions where appropriate. Do not force modularization where it does not fit the solution.

IA check complete

Syllabus — short retrieval

Redo the integrated function on page 9 without Reveal, then change it so it counts codes ending in "2027".

Retrieval complete

Designed by David Xu

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End of Lesson 5.

Loops, Functions, Modularization and Strings

What you need to be able to do

Rapid Recall Deck

Extract and manipulate substrings

Trace, then construct

Counted or conditional?

Counted loop

Conditional loop

Choose the appropriate loop

Selection inside repetition

Trace the loop

Modularize a repeated task

Construct a reusable function

Trace local and global variables

Trace the call

Combine strings, loops and functions

Construct count_ibcs(codes)

Finish the learning cycle

IA — main task

Syllabus — short retrieval

Construct `count_ibcs(codes)`