Flowcharts and Cambridge pseudocode

Sub-systems, and inputs, processes, outputs and storage

A system is a set of parts that work together to do a job. Each part is a sub-system. A sub-system can be split again until each part is small enough to code easily.

Example: a supermarket self-checkout is a system. Scanning, weighing, payment and printing the receipt are its sub-systems.

Every problem can also be decomposed into four parts.

Inputs: the data going in. Processes: what is done to the data. Outputs: the results that come out. Storage: data that must be kept for later.

Structure diagrams

A structure diagram shows how a system is broken down into sub-systems. The whole system is in the top box.

Each level below shows smaller parts. Read it top to bottom. Boxes on one level are usually read from left to right.

It shows what the parts are, not the order of every step or the decisions.

There is a diagram for this step in the IGCSE library.

Flowcharts: the six symbols

A flowchart shows the steps of an algorithm as shapes joined by arrows. Each shape has one meaning.

Follow the flow lines from START to STOP. A decision diamond has two exits, labelled Yes and No.

A loop is drawn as a flow line that goes back up to an earlier step.

There is a diagram for this step in the IGCSE library.

Pseudocode: Cambridge's planning language

Pseudocode is a way of writing an algorithm that looks like code but is not tied to one language.

Cambridge has its own rules. Keywords are in CAPITALS. Assignment uses `←`. Comments start with `//`. Not equal is `<>`.

Paper 2 answers must be in pseudocode. Program code only earns marks in the 15-mark scenario question.

DECLARE Mark : INTEGER
OUTPUT "Enter your mark: "
INPUT Mark
IF Mark >= 25
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

mark = int(input("Enter your mark: "))
if mark >= 25:
    print("Pass")
else:
    print("Fail")

Stating the purpose

The purpose of an algorithm is the job it does overall. It is the reason the algorithm exists.

To find it, look at the inputs, then look at what is output at the end. The purpose links the two.

Write one sentence: 'The algorithm inputs … and outputs …'.

CONSTANT CorrectPin ← "4821"
DECLARE Guess : STRING
DECLARE Attempts : INTEGER
Attempts ← 0
REPEAT
  OUTPUT "Enter your PIN: "
  INPUT Guess
  Attempts ← Attempts + 1
UNTIL Guess = CorrectPin OR Attempts = 3
IF Guess = CorrectPin
  THEN
    OUTPUT "Card unlocked"
  ELSE
    OUTPUT "Card blocked"
ENDIF

CORRECT_PIN = "4821"
attempts = 0
guess = ""
# Python has no REPEAT ... UNTIL, so the loop condition is reversed
while guess != CORRECT_PIN and attempts < 3:
    guess = input("Enter your PIN: ")
    attempts = attempts + 1
if guess == CORRECT_PIN:
    print("Card unlocked")
else:
    print("Card blocked")

Describing the processes

Sometimes a question asks you to describe the processes. Now you do go through the steps, but in plain words and in order.

Mention the loop, what is checked, and what is changed or output. Use the identifiers from the algorithm.