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
One column per variable, plus one for output.
Doing this before you start is what stops you losing track halfway through a loop.
2 — One row per step, always
Write a new row every time any value changes, even when the change looks trivial.
Skipping rows to save time is the single most common cause of a wrong trace.
3 — At a diamond, write the test out
Do not decide the branch in your head. Write "is 4 at most 3? No" and then follow that arrow.
The boundary cases — where the values are equal — are where most errors happen.
4 — Output is a column too
Give output its own column and add to it only when an output box is reached.
Many questions ask only "what is printed", and the trace table is where that answer already is.
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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