The big idea: Instructions run one after another, in the order written.
So two correct instructions in the wrong order give a wrong program — and nothing warns you, because both lines are perfectly valid.
# Wrong order
total = total + price
price = price * 1.2 # tax added AFTER it was counted
# Right order
price = price * 1.2 # tax added first
total = total + price// Wrong order
total = total + price;
price = price * 1.2; // tax added AFTER it was counted
// Right order
price = price * 1.2; // tax added first
total = total + price;The general rule: A value must be set before it is used, and used before it is changed again.
Most sequence errors are one of those two, and both are invisible to the computer.
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Infinite loop
The thing the loop is waiting for never changes.
Usually the counter is increased inside a branch that does not always run, or not at all. The program does not crash — it simply never finishes.
Incorrect output
Everything runs, and the answer is wrong.
A total printed before the last value was added, or an average calculated before the final mark arrived. This is the quiet one.
Deadlock
Two parts of a program each hold something the other is waiting for, so neither can move.
One holds the file and wants the printer; the other holds the printer and wants the file. Both wait forever.
# Infinite loop: count only increases when the value is large
count = 0
while count < 5:
value = get_next()
if value > 10:
count = count + 1 # small values never advance it
# Incorrect output: printed before the last addition
for mark in marks:
print("Total so far:", total) # one value behind, every time
total = total + mark// Infinite loop: count only increases when the value is large
int count = 0;
while (count < 5) {
int value = getNext();
if (value > 10) {
count = count + 1; // small values never advance it
}
}
// Incorrect output: printed before the last addition
for (int mark : marks) {
System.out.println("Total so far: " + total); // one value behind, every time
total = total + mark;
}Avoiding deadlock: The standard fix is to make every part claim shared resources in the same fixed order — always the file first, then the printer.
If nobody can be holding the second while waiting for the first, the circle cannot form.
Trace it like this: one row per line, and each variable keeps its value until a line changes it.
Practice with exam-style questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
Before the loop
- Set up anything the loop will use: totals at 0, counters at their start
- Read anything needed to decide how many times to go round
- Setting a total inside the loop resets it every time
Inside the loop
- Do the work for one item
- Update whatever the loop's test depends on — every time round
- Ask: could this line ever fail to run?
After the loop
- Print or return the finished result
- Printing inside the loop shows a running value, not the final one
- Divide for an average here, once the count is complete
The three-question check: Is every value set before it is used? Does the loop's test always change? Is the result used after it is complete, not during?
Three questions catch nearly every sequence error there is.
How this is tested — you must say WHICH line is in the wrong place and what the effect is. It comes up two ways:
Paper 2 — working with code
- Identify and correct a sequence error, 3-5 marks
- Explain why a loop never ends
- Put given instructions into the right order
Paper 2 — the algorithmic-thinking question
- Reason about ordering with no code shown
- Explain how deadlock arises and how to prevent it
The classic trap: Saying "the order is wrong" without naming the line. Marks come from which line, where it should go, and what the wrong order produced — one value behind, a loop that never ends.
This should print the average mark but prints a smaller number. Find and correct the error.
total = 0
count = 0
for mark in marks:
count = count + 1
print(total / count)
total = total + mark
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
This should print the average mark but prints a smaller number. Find and correct the error.
int total = 0;
int count = 0;
for (int mark : marks) {
count = count + 1;
System.out.println((double) total / count);
total = total + mark;
}
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.