# Mistake 1: Creating arrays in loops
# BAD:
arr = np.array([])
for i in range(1000):
    arr = np.append(arr, i)  # Slow!

# GOOD:
arr = np.arange(1000)       # Fast!

# Mistake 2: Not using vectorization
# BAD:
result = np.zeros(len(arr))
for i in range(len(arr)):
    result[i] = arr[i] * 2

# GOOD:
result = arr * 2