Flowcharts

AQA 4.3.2 — Flowcharts

Retrieval: the previous lesson evaluated an algorithm written in plain English. Plain English is easy to write but easy to misread — a flowchart represents the same kind of algorithm visually, using standard shapes, so its structure is unambiguous at a glance.

Standard symbols

Representing sequence, selection and iteration

• Sequence: shapes connected one after another by a single arrow each.
• Selection: a diamond with two outgoing arrows (e.g. "Yes"/"No"), each leading to a different path that may later rejoin.
• Iteration: an arrow loops back from a later shape to an earlier one, usually guarded by a decision diamond that eventually breaks the loop.

Common mistake

Using a rectangle (process) where a diamond (decision) is needed, or vice versa — a decision must have two clearly labelled exit paths; a process shape has only one path out. Missing arrowheads, or arrows whose direction is ambiguous, are also a common reason a flowchart loses marks even when the underlying logic is correct.

Worked example — interpreting a described flowchart

Consider a flowchart described as: Start → Input a number → Decision: is the number negative? → (Yes) Output "Invalid", End → (No) Decision: is the number even? → (Yes) Output "Even", End → (No) Output "Odd", End.

Trace this for the input 7: not negative, so continue; not even, so the final output is "Odd". Trace it again for -3: negative, so the output is immediately "Invalid" — the second decision is never reached at all.

Trace it yourself

Using the same described flowchart above, state the output for each of these inputs: 4, -1, 9, 0.

(4 → "Even". -1 → "Invalid". 9 → "Odd". 0 → "Even" - zero is not negative, and it is even.)

Challenge

Design (describe in words, symbol by symbol, or sketch on paper) a flowchart for a simple number-guessing game: input a guess, compare it to a fixed target number, and loop back to ask for another guess until the guess is correct, then output "Correct!" and end. Make sure your loop's exit condition is a genuine decision diamond, not just an assumption that it will eventually stop.

Looking ahead: the next lesson (Pseudocode) represents the exact same kind of algorithm in structured text instead of shapes — you'll translate between the two directly.