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常见错误

1. Outer loop uses range(n) instead of range(n - 1)​

In bubble sort, i only needs to go up to n-2 because n-1 passes would be redundant. Using range(n) works but wastes one pass — in a 4-mark question, this loses accuracy.

# Wrong
for i in range(n):       # ❌ extra pass
# Correct
for i in range(n - 1):   # ✅ n-1 passes

2. Inner loop bound wrong in bubble sort​

Using range(n - 1) for the inner loop ignores that the last i elements are already sorted.

# Wrong — compares already-sorted elements
for j in range(n - 1):
# Correct — skips sorted tail
for j in range(n - 1 - i):

3. Comparison direction reversed​

Always check: do we want ascending (use >) or descending (use <)? Default exam assumption is ascending.

# Wrong (sorts descending)
if arr[j] < arr[j + 1]:
# Correct (sorts ascending)
if arr[j] > arr[j + 1]:

4. Forgetting to decrement j in insertion sort shift loop​

Without j -= 1, the while loop runs forever.

# Wrong — infinite loop
while j >= 0 and arr[j] > key:
    arr[j + 1] = arr[j]
# Correct
while j >= 0 and arr[j] > key:
    arr[j + 1] = arr[j]
    j -= 1

5. Off-by-one in insertion sort outer loop​

Starting i at 0 means arr[0] gets treated as unsorted, causing index errors.

# Wrong — i should start at 1
for i in range(0, n):
# Correct
for i in range(1, n):

6. Recursive insertion sort missing base case or wrong parameter​

Without a base case, recursion never terminates.

# Wrong — no base case, will cause RecursionError
def insertionSortRec(arr, n):
    insertionSortRec(arr, n - 1)
    # ... insert logic ...

# Correct
def insertionSortRec(arr, n):
    if n <= 1:          # ✅ base case
        return
    insertionSortRec(arr, n - 1)
    # ... insert logic ...

7. Function returns None instead of the sorted array​

If the question specifies a function (not a procedure), you must return arr.

# Wrong — no return
def bubbleSort(arr):
    # ... sorting logic ...

# Correct
def bubbleSort(arr):
    # ... sorting logic ...
    return arr

8. Confusing i and j in bubble sort nested loops​

Using i for the inner loop and j for the outer loop is not wrong per se, but examiners expect i for outer and j for inner. Mixing them may lead to confusion when tracing.