Flowcharts

🎯 Syllabus & Goals 3 min

Cambridge 7.2 · Design methods — flowcharts Cambridge 7.9 · Flowchart symbols Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Name and draw the six standard flowchart symbols and state what each is for.
2. Follow a flowchart with given data and state its output.
3. Draw a flowchart that uses a decision for both selection and a repeat loop.

Textbook: Chapter 7, §7.2.3 Flowcharts (pp. 262–264)

Recap / Warm-Up 5 min

Last lesson a structure diagram showed what the parts are. It did not show the order things happen. A flowchart does exactly that, for one sub-system at a time.

Quick starter

Write the steps to make a cup of tea as a numbered list. Which step involves a decision?

🧠 Key Concept 14 min

1 · What a flowchart shows

A flowchart shows, as a diagram, the steps needed to complete a task and the order in which they happen. Those steps, in that order, are an algorithm. Flowcharts are a clear way to communicate how a system or sub-system works.

2 · The standard symbols

3 · Rules for drawing a good flowchart

• Exactly one START and at least one STOP.
• Every symbol is joined by arrowed flow lines — never a bare line.
• A decision is a question with two exits, labelled yes and no.
• A loop is drawn by a flow line going back up to an earlier symbol.
• Short variable names (N, D) are fine if the text would not fit in the box.

Worked Example 12 min

Worked example 1 · A smart kettle that switches itself off

A smart kettle heats water, checks the temperature every second, and switches off with a beep at 100 °C.

1. Trace with sensor readings 97, 99, 100. Temp = 97 → 97 >= 100? no → wait → read again.the no-path is the loop, so it must lead back to the input.
2. Temp = 99 → no → wait → read again.
3. Temp = 100 → 100 >= 100? yes → heater off → Beep → STOP.>= includes 100 itself. With > the kettle would keep boiling at 100 °C.

Worked example 2 · Swimming-pool group tickets

A pool charges $6 per swimmer. Groups of 5–7 get 10% off; groups of 8 or more get 25% off. A booking must be for 1 to 10 swimmers. Draw the flowchart, then write the same algorithm as pseudocode.

The same algorithm in Cambridge pseudocode

REPEAT
  OUTPUT "How many?"
  INPUT N
UNTIL N >= 1 AND N <= 10
IF N < 5
  THEN
    D ← 0
  ELSE
    IF N < 8
      THEN
        D ← 0.1
      ELSE
        D ← 0.25
    ENDIF
ENDIF
Cost ← N * 6 * (1 - D)
OUTPUT "Cost: $", Cost

n = 0
while not (n >= 1 and n <= 10):
    n = int(input("How many? "))
if n < 5:
    d = 0
elif n < 8:
    d = 0.1
else:
    d = 0.25
cost = n * 6 * (1 - d)
print("Cost: $", round(cost, 2))

How many? 12
How many? 8
Cost: $ 36.0

1. Input 12. 12 <= 10 is false, so the decision is no → back to the prompt.the loop rejects data outside 1–10 and asks again.
2. Input 8. Valid → 8 < 5? no → 8 < 8? no → D ← 0.25.
3. Calculate. 8 × 6 = 48; 1 − 0.25 = 0.75; 48 × 0.75 = 36.brackets are worked first, so the discount is taken off the full price.
4. Check other groups: 3 swimmers → $18; 5 → 30 × 0.9 = $27; 10 → 60 × 0.75 = $45.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

A flowchart shows an algorithm's steps and their order using standard symbols. Decisions have two labelled exits and give both selection and repetition. Every flowchart has a START and a STOP.

Flowchart

A diagram showing the steps required for a task and the order in which they are performed.

Algorithm

An ordered set of steps to solve a problem.

Terminator

The symbol at the start and end of a flowchart.

Decision

A diamond holding a yes/no question, with two labelled exits; used for selection and repetition.

Input/output symbol

A parallelogram used for both entering data and showing information.

Predefined process

A process defined elsewhere (a sub-routine), shown by name only.

Homework 1 min

Task (≤ 15 min): A cinema charges $8 for an adult and $5 for a child (under 13). Draw a flowchart that inputs an age, rejects ages below 0 or above 120 (asking again), and outputs the correct price. [5]