← IGCSE Computer Science Paper 2

Explaining a flowchart

Follow the flow lines from Start, say what each symbol does and what each decision tests (Yes and No paths), then state the overall purpose.

Part of: IGCSE Computer Science 0478 — 7c Pseudocode and Flowcharts — Module 93, ACARA Digital Technologies Years 9–10 — Algorithms with Logical Operators — Module 47

Understanding Explaining a flowchart in IGCSE Computer Science Paper 2

What it does: Spotting a loop is a sub-theme of explaining a flowchart: a Yes path returning to an earlier symbol repeats those steps. A full explanation names what is input (Mark) and what is output (Grade), then states the overall purpose of the flowchart. Always begin at Start and follow the arrows in order, so you describe the steps in the same order the program runs. A decision symbol tests a condition. It does not change a value. Say what is tested and where each path leads. Parallelograms are used for both input and output. The words inside show which it is. 'Enter' means input and 'Display' means output.

Common mistakes: The decision must test Age >= 18 so that the Yes path (Adult) is only taken by people aged 18 or over. With <= 18, young children would follow the Yes path and be called adults. Data enters the flowchart through an INPUT symbol. OUTPUT Mark would display a value that has not been read yet, so the decision has nothing to test. It should be INPUT Mark.

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This module's key terms are also in a printable crossword (Modules 232–241) and word search (Modules 232–253).

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Learning

Duo

What is the first step when explaining a flowchart?

Start with the last symbol and work backwardsStart at the Start terminator and follow the flow lines

Matches

Explanation: 'The diamond reads a number from the user, then the flowchart ends.'

↓

A diamond does not read input. It is a decision that tests a condition and has Yes and No paths. Input uses a parallelogram.

Applying

Meanings

Term

Explaining a flowchart

Meaning

Follow the flow lines from Start, say what each symbol does and what each decision tests (Yes and No paths), then state the overall purpose.

Practice

Start: INPUT Mark

Testing

Gaps

-> OUTPUT "Pass"

Check Answer

Spots

2INPUT Age

3Age <= 18 ?

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Quick Questions

1. What is the first step when explaining a flowchart?

A. Start at the Start terminator and follow the flow lines ✓

B. Start with the last symbol and work backwards

Always begin at Start and follow the arrows in order, so you describe the steps in the same order the program runs.

2. A decision diamond contains 'Is Age >= 18?'. How should you describe it?

A. It tests whether Age is 18 or more, then the Yes path or No path is followed ✓

B. It sets Age to 18 and moves on

A decision symbol tests a condition. It does not change a value. Say what is tested and where each path leads.

3. Which describes the Yes and No paths leaving a decision symbol?

A. Both paths always lead to the same step and are run one after the other

B. Each path is labelled and leads to different steps depending on the result of the test ✓

Only one path is followed each time. Which one depends on whether the condition is true (Yes) or false (No).

4. A flow line leaves a decision and goes back up to a symbol earlier in the flowchart. What does this show?

A. A loop, because some steps are repeated ✓

B. The program has finished

A flow line that returns to an earlier symbol means those steps repeat. Say what condition makes the loop repeat or stop.

5. A parallelogram contains 'Enter Mark'. How do you name this in your explanation?

A. An output: the program displays a Mark

B. An input: the user enters a Mark ✓

Parallelograms are used for both input and output. The words inside show which it is. 'Enter' means input and 'Display' means output.

Match the Pairs

Decision diamond: Is Total > 100? → A decision that tests the value of Total. One flow line leaves for Yes and another leaves for No.

Start -> INPUT Num -> Count <- Count + 1 -> Stop → Follow the flow lines in order: read in Num, add 1 to Count, then end the flowchart.

A flow line that goes back up to an earlier symbol shows a loop, so the same steps repeat until a decision sends the flow on. → Decision: Count < 10? Yes -> back to the INPUT symbol, No -> OUTPUT Total

Naming the inputs and outputs: the parallelogram symbols show the data entered and the results displayed. → INPUT Mark ... OUTPUT Grade

Explanation: 'The diamond reads a number from the user, then the flowchart ends.' → A diamond does not read input. It is a decision that tests a condition and has Yes and No paths. Input uses a parallelogram.

Explanation: 'The flowchart adds up numbers and outputs the total.' (no mention of the decision or loop) → The explanation skips what the decision tests, the Yes and No paths, and the loop, so it does not follow every flow line.

Key Term

Explaining a flowchart

Follow the flow lines from Start, say what each symbol does and what each decision tests (Yes and No paths), then state the overall purpose.

Fill in the Blanks

1. Mark >= 50? Yes -> OUTPUT "Pass" No -> OUTPUT "Fail"

Hint: Type the label on the flow line followed when the decision's condition is true.

2. Mark >= 50? Yes -> OUTPUT "Pass" No -> OUTPUT "Fail"

Hint: Type the keyword used in the output symbol to show a message.

3. START INPUT Age Age >= 18?

Hint: Type the keyword for the symbol that reads a value from the user.

4. Count ← 1 Total ← Total + Count Count <= 5? Yes -> back to line 2

Hint: Type the relational operator for 'less than or equal to'. This decision creates a loop.

5. INPUT Num Total ← Total + Num OUTPUT Total

Hint: Type the variable that stores the running total of the numbers.

Spot the Error

Question 1

A START

B INPUT Age

C Age <= 18 ?

D Yes: OUTPUT Adult

E No: OUTPUT Child

F END

Line C — The decision must test Age >= 18 so that the Yes path (Adult) is only taken by people aged 18 or over. With <= 18, young children would follow the Yes path and be called adults.

Question 2

A START

B OUTPUT Mark

C Mark >= 50 ?

D Yes: OUTPUT Pass

E No: OUTPUT Fail

Line B — Data enters the flowchart through an INPUT symbol. OUTPUT Mark would display a value that has not been read yet, so the decision has nothing to test. It should be INPUT Mark.

Question 3

A START

B Count ← 1

C OUTPUT Count

D Count ← Count - 1

E Count > 5 ? No: go to line 3

F Yes: END

Line D — The loop ends when Count > 5, so Count must go up each time round. Subtracting 1 makes Count smaller, the test never becomes Yes and the loop never ends. It should be Count ← Count + 1.

Question 4

A START

B Total ← 0

C INPUT Num

D Total ← Total * Num

E Num = 0 ? No: go to line 3

F Yes: OUTPUT Total

Line D — The loop going back to the INPUT symbol is a totalling loop, so each Num must be added to Total. Multiplying would not give a running total. It should be Total ← Total + Num.

Question 5

A END

B INPUT Price

C Price > 100 ?

D Yes: OUTPUT Expensive

E No: OUTPUT Cheap

Line A — To explain a flowchart you begin at the Start terminator. The first symbol must be START, not END, because END is where the flow finishes.

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