Why does my trace table always go wrong on the last iteration?
Because you are almost certainly checking the condition at the wrong moment, and the last row is the only place that difference shows up.
Worked answers
Computer Science questions, worked through
The programming-and-theory route: how machines represent and move data, plus writing and tracing algorithms by hand and at a keyboard. The exam tests a narrower, more precise skill than the one most keen programmers have been practising.
- 3 worked answers
- 3 topics
Algorithms and tracing
Data representation
How do I write a negative number in two's complement without getting confused?
There is a two-step method that always works, and most of the confusion comes from students half-remembering a third step that does not exist.
Programming and pseudocode
I wrote working Python and got half marks. Why?
Because the paper asked for pseudocode and marks the structure, not whether the code would run. A correct Python answer can miss several structural marks while…
Where Computer Science marks actually go
These answers are written around the failure modes below rather than around the topic list. That is deliberate: most students who are stuck in this subject know more content than their grade suggests, and lose marks to one of these instead.
- Writing code that works but cannot be explained in the terms the mark scheme wants
- Tracing an algorithm on paper under time pressure without a computer to check it
- Losing marks on binary, hexadecimal and two's complement arithmetic that was never properly drilled
- Treating the theory paper as memorisation when it tests applied understanding
More on how we teach Computer Science, or browse questions in other subjects.
Who these are written for
Students who want to build things and are comfortable with the logic and maths underneath. It is the stronger choice for engineering, computer science or any degree with a quantitative core.
How to get something out of a worked answer
Reading one of these is worth very little on its own, which is true of any explanation and is the reason revision by rereading feels productive and mostly is not. The method that works is slightly uncomfortable and takes about ten minutes per question.
Read the question, close the page, and attempt it on paper. Get it wrong if that is what happens: being wrong and then finding out why is the part that sticks. Then read the worked answer and, rather than checking whether your final answer matched, find the exact step where your version diverged from ours. That step is the thing to practise, and it is usually one specific decision rather than a whole topic.
Then find two similar questions from your own materials and do them cold. If the same step goes wrong again, you have found a genuine gap. If it does not, you have just fixed something. The wider method is here.
Why we publish these at all
Because we do not publish reviews or testimonials, and a parent choosing a tutor needs something to judge. Review widgets are cheap and star ratings are unfalsifiable, so we would rather hand you the actual teaching and let you decide whether it is any good. If these explanations are clear, that is the most honest sample of what you would be paying for that we can give you. More on that policy here.