Explain where in the construction of an algorithm it is appropriate to use a function

11.3 Structured Programming – Using Functions

Learning Objective

Explain at which stage in the construction of an algorithm it is appropriate to introduce a function, and justify how doing so improves the design, implementation, testing and maintenance of a program.

Syllabus Alignment (Cambridge International AS & A Level Computer Science 9618)

Why Use Functions? (Benefits)

• Encapsulation – isolates a specific task and hides implementation details.
• Re‑use – the same code can be called from many places without duplication.
• Readability – the main algorithm reads like a high‑level description of the problem.
• Testability – each function can be unit‑tested independently (AO3).
• Maintainability – a change to a task is made in one location only, reducing the risk of new errors.

When to Introduce a Function (Decision Checklist)

1. Repeated Action: identical step‑sequences appear two or more times.
2. Complex Sub‑task: a portion of the algorithm involves several steps and is conceptually distinct.
3. Abstraction Needed: hiding internal steps makes the main flow clearer.
4. Parameterisation: the same logic must operate on different data values.
5. Testing Requirement: the sub‑task should be verified in isolation before integration.

Typical Points in the Development Process to Insert Functions

Cambridge‑Style Pseudocode Conventions for Functions

Simple Example

FUNCTION max(a: INTEGER, b: INTEGER) → INTEGER
IF a > b THEN
RETURN a
ELSE
RETURN b
END IF
END FUNCTION

Scope, Declaration and Parameter Passing

• Local variables are declared inside a function and cease to exist when the function returns.
• Global variables / constants are declared outside any function; they are accessible to all functions unless shadowed.
• Parameters are passed by value unless the syllabus explicitly allows reference passing. The called function receives a copy, so changes to the parameter do not affect the original variable.

Function with Multiple Parameters and a Return Value

FUNCTION swapIfGreater(x: INTEGER, y: INTEGER) → LIST OF INTEGER
IF x > y THEN
RETURN [y, x]          // returns a list with the values swapped
ELSE
RETURN [x, y]
END IF
END FUNCTION

Interaction with Control Structures

A function may contain any standard construct (IF, CASE, FOR, WHILE). Conversely, a function can be called from within those constructs.

FUNCTION isPrime(n: INTEGER) → BOOLEAN
IF n < 2 THEN RETURN FALSE END IF
FOR i FROM 2 TO FLOOR(SQRT(n))
IF n MOD i = 0 THEN RETURN FALSE END IF
END FOR
RETURN TRUE
END FUNCTION
FOR i FROM 1 TO 20
IF isPrime(i) THEN OUTPUT i, " is prime" END IF
END FOR

Worked Example – Computing the Median of a List

Problem statement: For each data set read from input, output its median. The steps “sort the list” and “pick the middle element” are repeated for every set.

Version A – No functions (pseudocode)

READ sets
FOR s FROM 1 TO sets
READ size
READ size numbers INTO list
// bubble sort
FOR i FROM 1 TO size-1
FOR j FROM i+1 TO size
IF list[i] > list[j] THEN SWAP list[i], list[j] END IF
END FOR
END FOR
// pick median
IF size MOD 2 = 1 THEN
median ← list[(size+1)/2]
ELSE
median ← (list[size/2] + list[size/2+1]) / 2
END IF
OUTPUT median
END FOR

Version B – With functions (Cambridge‑style pseudocode)

FUNCTION sortAscending(data: LIST OF INTEGER) → LIST OF INTEGER
FOR i FROM 1 TO LENGTH(data)-1
FOR j FROM i+1 TO LENGTH(data)
IF data[i] > data[j] THEN SWAP data[i], data[j] END IF
END FOR
END FOR
RETURN data
END FUNCTION
FUNCTION median(data: LIST OF INTEGER) → REAL
SET sorted ← sortAscending(data)
SET n ← LENGTH(sorted)
IF n MOD 2 = 1 THEN
RETURN sorted[(n+1)/2]
ELSE
RETURN (sorted[n/2] + sorted[n/2+1]) / 2
END IF
END FUNCTION
// Main program
READ sets
FOR s FROM 1 TO sets
READ size
READ size numbers INTO numbers
OUTPUT median(numbers)
END FOR

Benefits observed

• The main loop now mirrors the problem description – “read data, output median”.
• Sorting and median extraction are each isolated, making them easy to test.
• If a more efficient sorting algorithm is later required, only sortAscending needs to be changed.

Testing Functions (Unit Testing)

1. Write a set of test cases for each function before integrating it.
2. For sortAscending, test:

   • An already‑sorted list.
   • A reverse‑sorted list.
   • A list containing duplicate values.

3. For median, test:

   • Odd‑length lists (e.g., [3,1,4] → 3).
   • Even‑length lists (e.g., [2,8,5,7] → 6.0).
   • Lists with negative numbers and zeros.

4. Record expected vs. actual output; any discrepancy indicates a fault to be fixed before the main program runs.

Assessment Objective (AO) Mapping for This Topic

Practical Implementation Tips (Paper 4 – Programming)

• Choose a language you are comfortable with (Java, Python, Visual Basic). All three support functions/sub‑routines and map cleanly onto Cambridge pseudocode.
• Follow the language‑specific syntax for:

  • Function header (e.g., public static int max(int a, int b) in Java).
  • Parameter passing – most exam languages use pass‑by‑value for primitives and pass‑by‑reference for objects/arrays.
  • Returning a value – use return and ensure the declared return type matches.

• Remember to keep the main routine short; it should orchestrate calls to your functions, not contain detailed logic.
• When testing on the exam computer, write a small “driver” program that calls each function with known inputs and prints the results – this satisfies the unit‑testing requirement.

A‑Level Extension Topics (Relevant to Functions)

• Recursion (20.2) – functions that call themselves (e.g., factorial, binary‑search).
• Exception handling (20.2) – using TRY…CATCH inside functions to manage run‑time errors.
• Abstract Data Types (19.2) – designing functions that operate on custom structures such as stacks, queues or linked lists.
• Algorithmic efficiency (14.2) – comparing sorting functions (bubble vs. insertion vs. merge) and discussing Big‑O notation.
• Database access (21.2) – encapsulating SQL queries inside functions for modular data retrieval.

Suggested Diagram

Key Take‑aways

• Introduce a function during the design stage when a task can be identified as a distinct, reusable unit.
• Use functions to replace repeated code, hide complexity, and enable independent testing.
• Follow Cambridge pseudocode conventions: clear header, parameter list, return statement, proper indentation.
• Observe scope rules – variables declared inside a function are local; parameters are passed by value unless otherwise specified.
• Unit‑test each function before integrating it into the main program to satisfy AO3 requirements.