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常见错误

Mistake 1: Forgetting global Inside a Function​

Wrong:

def main():
    MyArray = [0] * 10   # local variable, not accessible elsewhere

Fix:

def main():
    global MyArray
    MyArray = [0] * 10

Why: Without global, Python creates a new local variable. Other functions cannot see it, causing NameError.

Mistake 2: Off-by-One Index Error​

Wrong:

Arr = [0] * 10
for i in range(1, 11):   # index 10 is out of bounds
    Arr[i] = i

Fix:

Arr = [0] * 10
for i in range(10):      # indices 0–9
    Arr[i] = i

Why: Python uses 0-based indexing. range(10) produces 0, 1, ..., 9.

Mistake 3: Not Pre-Allocating Before Index Assignment​

Wrong:

Arr = []
Arr[0] = 5     # IndexError: list assignment index out of range

Fix:

Arr = [0] * 10
Arr[0] = 5     # OK

# OR use append:
Arr = []
Arr.append(5)  # OK

Why: You cannot assign to an index that does not exist. Pre-allocate with [0] * N or use append().

Mistake 4: Shallow Copy in 2D Arrays​

Wrong:

Arr = [[0] * 4] * 3    # 3 references to the same row
Arr[0][1] = 5           # changes ALL rows at column 1

Fix:

Arr = [[0 for j in range(4)] for i in range(3)]
# OR
Arr = [[0] * 4 for i in range(3)]

Why: [row] * 3 creates shallow copies — all rows point to the same list object. Modifying one row "modifies" all rows.

Mistake 5: Wrong Loop Bounds for 2D Arrays​

Wrong:

Arr = [[0 for j in range(4)] for i in range(3)]
for i in range(4):    # should be range(3) for rows
    for j in range(3):  # should be range(4) for columns
        print(Arr[i][j])

Fix:

for i in range(len(Arr)):        # rows = 3
    for j in range(len(Arr[i])): # columns = 4
        print(Arr[i][j])

Why: Use len(Arr) for row count and len(Arr[0]) for column count to avoid hardcoding wrong bounds.

Mistake 6: Not Converting Input Type​

Wrong:

Scores[i] = input("Enter score: ")   # stores string, not integer

Fix:

Scores[i] = int(input("Enter score: "))

Why: input() returns a string. Numeric operations (+, >, etc.) will fail or behave unexpectedly.

Mistake 7: Using = Instead of == in Comparison​

Wrong:

if Arr[i] = SearchValue:   # assignment, not comparison

Fix:

if Arr[i] == SearchValue:

Why: Single = is assignment, which always evaluates as True (truthy). Double == is equality comparison.