1# WRONG — misses the last element
2items = [10, 20, 30, 40, 50]
3for i in range(len(items) - 1):  # Goes 0..3, skips index 4
4    print(items[i])
5# Prints 10, 20, 30, 40 — misses 50
6
7# CORRECT
8for i in range(len(items)):  # Goes 0..4
9    print(items[i])
10
11# WRONG — fence post error in counting
12def count_between(a, b):
13    return b - a  # For a=1, b=5: returns 4 but there are 5 numbers (1,2,3,4,5)
14
15# CORRECT
16def count_between(a, b):
17    return b - a + 1  # 5 - 1 + 1 = 5

1# WRONG — should be AND, not OR
2age = 25
3if age < 18 or age > 65:  # True for 25? No. But intent was to check working age
4    print("Working age")
5
6# CORRECT — working age is >= 18 AND <= 65
7if age >= 18 and age <= 65:
8    print("Working age")
9
10# WRONG — De Morgan's law mistake
11# "not (A and B)" is NOT the same as "not A and not B"
12if not (is_admin and is_active):  # True when EITHER is false
13    deny_access()
14
15# This is different:
16if not is_admin and not is_active:  # True only when BOTH are false
17    deny_access()

1# WRONG — & has lower precedence than ==
2result = x & 0xFF == 0  # Parsed as: x & (0xFF == 0) → x & False → 0
3# CORRECT
4result = (x & 0xFF) == 0
5
6# WRONG — assignment vs comparison
7x = 5
8if x = 10:  # SyntaxError in Python, but in C: if (x = 10) — always true!
9    pass
10
11# JavaScript pitfall
12if (x == "5") { }  // true — type coercion
13if (x === "5") { }  // false — strict comparison

1# WRONG — integer division truncates
2average = sum(scores) // len(scores)  # 7 // 2 = 3, not 3.5
3
4# CORRECT
5average = sum(scores) / len(scores)  # 7 / 2 = 3.5
6
7# Floating point comparison
8# WRONG
9if 0.1 + 0.2 == 0.3:  # False! 0.1 + 0.2 = 0.30000000000000004
10    print("Equal")
11
12# CORRECT
13if abs((0.1 + 0.2) - 0.3) < 1e-9:
14    print("Equal")

1# WRONG — mutable default argument
2def add_item(item, items=[]):
3    items.append(item)
4    return items
5
6print(add_item("a"))  # ['a']
7print(add_item("b"))  # ['a', 'b'] — not ['b']!
8
9# CORRECT
10def add_item(item, items=None):
11    if items is None:
12        items = []
13    items.append(item)
14    return items
15
16# WRONG — modifying list while iterating
17numbers = [1, 2, 3, 4, 5, 6]
18for n in numbers:
19    if n % 2 == 0:
20        numbers.remove(n)
21print(numbers)  # [1, 3, 5, 6] — 6 survived! Iterator skipped it.
22
23# CORRECT — iterate over a copy or use list comprehension
24numbers = [n for n in numbers if n % 2 != 0]

1# WRONG — variable shadowing
2total = 100
3
4def calculate():
5    total = 0  # This is a LOCAL variable, not the global one
6    for i in range(10):
7        total += i
8    # Global total is still 100
9
10# CORRECT — use nonlocal or return
11def calculate():
12    local_total = 0
13    for i in range(10):
14        local_total += i
15    return local_total
16
17total = calculate()

1# 1. Add print statements at key points
2def binary_search(arr, target):
3    lo, hi = 0, len(arr) - 1
4    while lo <= hi:
5        mid = (lo + hi) // 2
6        print(f"lo={lo}, hi={hi}, mid={mid}, arr[mid]={arr[mid]}")
7        if arr[mid] == target:
8            return mid
9        elif arr[mid] < target:
10            lo = mid + 1
11        else:
12            hi = mid - 1
13    return -1
14
15# 2. Use assert to verify assumptions
16def divide(a, b):
17    assert b != 0, "Divisor must not be zero"
18    result = a / b
19    assert isinstance(result, float), f"Expected float, got {type(result)}"
20    return result
21
22# 3. Test edge cases
23assert binary_search([], 5) == -1        # Empty array
24assert binary_search([5], 5) == 0        # Single element
25assert binary_search([1, 2, 3], 4) == -1 # Not found
26assert binary_search([1, 2, 3], 1) == 0  # First element
27assert binary_search([1, 2, 3], 3) == 2  # Last element