The big idea: Tracing means working through an algorithm by hand, writing down every variable after every step.
It is slow and mechanical on purpose — the whole value is that you never guess what happens next.
A flowchart adding 1 to N, traced step by step. Use Next step to go at your own pace.
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Where it starts and stops
- Start / End — a rounded box, one at each end
- Exactly one Start; there may be more than one End
- Flowline — an arrow showing which way control goes
Doing things
- Input / Output — a slanted box: reading a value or showing one
- Process — a plain rectangle: a calculation or an assignment
- One action per box, so the trace has one row per box
Choosing and joining
- Decision — a diamond, with a question inside it
- Exactly two flowlines out, labelled Yes and No
- Connector — a small circle joining parts of a large chart, or a page to the next
The diamond is where loops live: A flowchart has no separate loop symbol.
A loop is a decision whose No branch carries on and whose Yes branch leads back up — or the reverse. Follow the arrows and the loop is obvious.
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1 — Draw the table first
2 — One row per step, always
3 — At a diamond, write the test out
4 — Output is a column too
Watch the order inside a loop: Whether the counter increases before or after the calculation changes the answer.
Trace what the chart actually shows, in the order the arrows go — not what you assume it meant.
How this is tested — you must produce a complete trace table and read the output from it. It comes up two ways:
Paper 2 — working with code
- Trace a flowchart and state the output, 4-6 marks
- Complete a partly-filled trace table
- Say what the algorithm does in general
Paper 2 — the algorithmic-thinking question
- Often the no-code question: a flowchart, a trace table and an output
- Sometimes: find the error a trace reveals
The classic trap: Getting the boundary wrong. When the test is "count at most N", the loop runs when they are equal — one extra time round. Write the comparison out at every diamond and you cannot slip.
A flowchart reads N, sets total to 0 and count to 1, then while count is at most N adds count to total and increases count by 1, finally printing total. Trace it for N = 4.
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