跳到主要内容

常见错误

🔴 Mistake 1: HeadPointer == TailPointer 判断 Full / Empty​

# ❌ Wrong — fails when queue is full
IF HeadPointer = TailPointer THEN
    OUTPUT "Queue is empty"
ENDIF

# ✅ Correct — use NumberOfItems
IF NumberOfItems = 0 THEN
    OUTPUT "Queue is empty"
ENDIF

In a circular queue, HeadPointer == TailPointer when the queue is both empty and full. You cannot distinguish the two states without NumberOfItems.

🔴 Mistake 2: Manual wrapping without MOD​

# ❌ Wrong — handwritten IF gets -1
IF TailPointer = QueueSize - 1 THEN
    TailPointer = 0
ELSE
    TailPointer = TailPointer + 1
ENDIF

# ✅ Correct — single MOD line
TailPointer = (TailPointer + 1) MOD QueueSize

The mark scheme insists on the MOD operator. Manual wrapping with IF is not accepted.

🔴 Mistake 3: Off-by-one in MOD direction​

# ❌ Wrong — should be (P + 1), not (P - 1)
TailPointer = (TailPointer - 1) MOD QueueSize

# ✅ Correct
TailPointer = (TailPointer + 1) MOD QueueSize

🔴 Mistake 4: Decrementing NumberOfItems before retrieval​

# ❌ Wrong — loses item reference
NumberOfItems = NumberOfItems - 1
Item = Queue[HeadPointer]

# ✅ Correct — retrieve first, then decrement
Item = Queue[HeadPointer]
NumberOfItems = NumberOfItems - 1

🔴 Mistake 5: Forgetting to wrap HeadPointer before dequeue​

# ❌ Wrong — HeadPointer never wraps
Item = Queue[HeadPointer]
HeadPointer = HeadPointer + 1

# ✅ Correct — wrap first, then access
HeadPointer = (HeadPointer + 1) MOD QueueSize
Item = Queue[HeadPointer]

🔴 Mistake 6: Not initialising pointers to -1​

# ❌ Wrong — starts at 0, first enqueue goes to index 1
HeadPointer = 0
TailPointer = 0

# ✅ Correct
HeadPointer = -1
TailPointer = -1
注意

While starting at 0 can work with a different algorithm (enqueue first, then wrap), the standard convention in 9618 is -1. Stick with -1 to avoid confusion.

🔴 Mistake 7: Full check using TailPointer == QueueSize - 1​

# ❌ Wrong — circular queue wraps around!
IF TailPointer = QueueSize - 1 THEN
    OUTPUT "Queue is full"
ENDIF

# ✅ Correct
IF NumberOfItems = QueueSize THEN
    OUTPUT "Queue is full"
ENDIF

In a circular queue, TailPointer can be anywhere — even 0 — when the queue is full. Only NumberOfItems reliably indicates fullness.

🔴 Mistake 8: Null / None return from Dequeue​

FUNCTION DeQueue()
    # ❌ Wrong — returns nothing in failure case
    IF NumberOfItems = 0 THEN
        OUTPUT "Empty"
        # Missing RETURN
    ENDIF
    ...

    # ✅ Correct
    IF NumberOfItems = 0 THEN
        OUTPUT "Empty"
        RETURN Null  # or RETURN ""
    ENDIF

If the function has a return type, every branch must return a value.