Procedures, Functions & Parameters

🎯 Syllabus & Goals 3 min

Cambridge 8.1.6 · Procedures and functions Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Define and call a procedure, with and without parameters, in pseudocode and Python.
2. Define a function that uses RETURN, and call it on the right-hand side of an expression.
3. Explain the difference between a procedure and a function, and between a parameter and an argument.

Textbook: Chapter 8, §8.1.6 (pp. 321–325)

Recap / Warm-Up 5 min

Last lesson: nesting — loops and selection inside each other. Programs are getting long. Today you learn to package code into named blocks that can be re-used.

Quick starter

You have already used ready-made routines. What value does LENGTH("Robot") give back — and did you need to know how it works inside?

🧠 Key Concept 14 min

Programs often repeat the same group of statements. Instead of copying them each time, you define them once as a subroutine — a named procedure or function — and call it whenever it is needed.

1 · Procedures — with and without parameters

PROCEDURE Stars
  OUTPUT "************"
ENDPROCEDURE

PROCEDURE StarLine(Number : INTEGER)
  DECLARE Line : STRING
  DECLARE Counter : INTEGER
  Line ← ""
  FOR Counter ← 1 TO Number
    Line ← Line & "*"
  NEXT Counter
  OUTPUT Line
ENDPROCEDURE

CALL Stars
CALL StarLine(7)

def stars():
    print("************")

def star_line(number):
    for counter in range(number):
        print("*", end="")    # end="" keeps the stars on one line
    print()                   # then finish the line

stars()
star_line(7)

************
*******

A procedure call is a standalone statement: CALL StarLine(7). The value 7 is the argument; inside the procedure it is stored in the parameter Number. Python calls a procedure a void function.

2 · Functions — they RETURN a value

FUNCTION Celsius(Temperature : REAL) RETURNS REAL
  RETURN (Temperature - 32) / 1.8
ENDFUNCTION

MyTemp ← Celsius(212)

def celsius(temperature):
    return (temperature - 32) / 1.8

my_temp = celsius(212)
print(my_temp)

100.0

3 · The header line

The first line of a definition is its header. It contains the name, each parameter with its data type, and — for a function only — the data type of the value returned.

Worked Example 12 min

Example 1 · Procedures with zero and two parameters

A scoreboard program prints a banner, a title and lines of different lengths and symbols.

PROCEDURE Banner
  OUTPUT "=============="
ENDPROCEDURE

PROCEDURE DrawLine(Length : INTEGER, Symbol : CHAR)
  DECLARE Line : STRING
  DECLARE Counter : INTEGER
  Line ← ""
  FOR Counter ← 1 TO Length
    Line ← Line & Symbol
  NEXT Counter
  OUTPUT Line
ENDPROCEDURE

CALL Banner
OUTPUT " SCORE BOARD"
CALL Banner
CALL DrawLine(5, '*')
CALL DrawLine(3, '-')

# A procedure with no parameters, then one with two parameters
def banner():
    print("==============")

def draw_line(length, symbol):
    line = ""
    for counter in range(length):
        line = line + symbol
    print(line)

banner()
print(" SCORE BOARD")
banner()
draw_line(5, "*")
draw_line(3, "-")

==============
 SCORE BOARD
==============
*****
---

1. The definitions come first but do nothing until called.defining a subroutine is not the same as running it.
2. CALL DrawLine(5, '*') passes 5 into Length and '*' into Symbol.arguments match parameters by position.
3. The loop builds "*****", outputs it, and control returns to the next call.the second call re-uses the same code with different data.

Example 2 · A function with two parameters

A flooring company works out the total area of two rooms, then the cost at $12.50 per square metre.

CONSTANT PricePerSqMetre ← 12.50
DECLARE Kitchen, Lounge, TotalArea : REAL

FUNCTION RectangleArea(Length : REAL, Width : REAL) RETURNS REAL
  RETURN Length * Width
ENDFUNCTION

Kitchen ← RectangleArea(4, 3.5)
Lounge ← RectangleArea(5, 4)
TotalArea ← Kitchen + Lounge
OUTPUT "Total floor area: ", TotalArea, " square metres"
OUTPUT "Flooring cost: $", TotalArea * PricePerSqMetre

# A function with two parameters returns one value
PRICE_PER_SQ_METRE = 12.50

def rectangle_area(length, width):
    return length * width

kitchen = rectangle_area(4, 3.5)
lounge = rectangle_area(5, 4)
total_area = kitchen + lounge
print("Total floor area:", total_area, "square metres")
print("Flooring cost: $", total_area * PRICE_PER_SQ_METRE)

Total floor area: 34.0 square metres
Flooring cost: $ 425.0

How the marks are earned when you write a function: correct header with parameters and types (1), RETURNS with the type (1), a correct RETURN statement (1), ENDFUNCTION (1), and a call used in an expression (1).

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

Define once, call many times. A procedure does a job and is called with CALL. A function does a job and RETURNs one value, so it is used inside an expression. Arguments in the call fill the parameters in the header — at most two at IGCSE.

Procedure

A set of programming statements grouped under a single name that can be called to perform a task, rather than including a copy of the code every time.

Function

Like a procedure, except that it returns a value back to the program that called it.

Parameter

A variable in a procedure or function declaration that stores the value of an argument passed to it.

Argument

The actual value passed to a procedure or function when it is called.

RETURN

The keyword in a function that specifies the value sent back to the calling expression.

Homework 1 min

Task (≤ 15 min): Write a pseudocode function IsEven(Number : INTEGER) RETURNS BOOLEAN. Then write a FOR loop that uses it to output only the even numbers from 1 to 20. [5]

FUNCTION IsEven(Number : INTEGER) RETURNS BOOLEAN
  IF MOD(Number, 2) = 0
    THEN
      RETURN TRUE
    ELSE
      RETURN FALSE
  ENDIF
ENDFUNCTION

DECLARE Counter : INTEGER
FOR Counter ← 1 TO 20
  IF IsEven(Counter) = TRUE
    THEN
      OUTPUT Counter
  ENDIF
NEXT Counter