041. Elif Statement

RAM Manager: Memory Pressure Levels

Aryan’s RAM manager needs to classify memory usage into alert levels:

0-50%   → "normal"
51-75%  → "moderate"
76-90%  → "high"
91-100% → "critical"

His first attempt uses separate if statements. This works for the current ranges because of accidental ordering, BUT the broken version computes the percentage incorrectly — it divides by 100 instead of total_ram. So mem_pressure(4096, 8192) gives "normal" (pct ≈ 0.5%) instead of "moderate" (pct = 50%).

The fix requires both: correcting the percentage calculation AND converting the separate if chain to if/elif/else to make branches truly mutually exclusive.

💡 Fun fact: Python has no switch/case statement before version 3.10 — by design. Guido van Rossum, Python’s creator, argued that if/elif chains were clear enough and a switch would just add redundancy. Python 3.10 finally added match/case in 2021, but only after a decade of community debate. Most real-world Python code still uses if/elif for simple dispatch.

⚠️ Watch out: The classic mistake is using separate if statements instead of elif for mutually exclusive ranges. A score of 95 satisfies score >= 90, score >= 80, score >= 70, and score >= 60 — all four conditions are true. With separate if blocks each one runs, and the final value of grade will be "D" because the last matching assignment wins.

🤔 Think about it: The broken code uses a mutable `grade` variable that gets overwritten on each matching `if`. The fixed code returns immediately from the first matching `elif`. Both work if refactored correctly — but which approach is safer when you add a new grade tier later, and why?

Learning objectives

Use elif for mutually exclusive conditions
Understand that elif only runs when all prior conditions were False
Choose elif vs multiple if statements correctly

Key concepts

elif
conditional chains
mutually exclusive

Try it

Concept detail

if/elif/else creates mutually exclusive branches — AT MOST ONE block runs.

The critical difference:

Separate if — ALL run independently

if score >= 90: grade = “A” # True for 95 → grade = “A” if score >= 80: grade = “B” # True for 95 → grade = “B” (overwrites!) if score >= 70: grade = “C” # True for 95 → grade = “C” (overwrites!) if score >= 60: grade = “D” # True for 95 → grade = “D” (overwrites!)

result: “D” for score 95 — completely wrong

elif — stops after first match

if score >= 90: return “A” # True for 95 → returns “A”, done elif score >= 80: return “B” # never reached elif score >= 70: return “C” # never reached

When to use separate if vs elif: Use elif when branches are mutually exclusive (one range per branch, classify something) Use separate if when multiple conditions can be true simultaneously (checking multiple flags)

Python has no switch/case statement before 3.10. Use if/elif chains. Python 3.10+ has ‘match/case’ (structural pattern matching) for more complex dispatch.

Solution

def letter_grade(score):
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

Tests

def test_grade_a():
    assert letter_grade(95) == "A"
    assert letter_grade(90) == "A"

def test_grade_b():
    assert letter_grade(85) == "B"
    assert letter_grade(80) == "B"

def test_grade_b_not_overwritten():
    # Broken code: score=85 sets grade="B" then grade="C" then grade="D"
    # Because all if conditions >= 80, >= 70, >= 60 are True for 85
    result = letter_grade(85)
    assert result == "B", f"Got {repr(result)} — separate ifs overwrite grade for 85"

def test_grade_c():
    assert letter_grade(75) == "C"

def test_grade_d():
    assert letter_grade(65) == "D"
    assert letter_grade(60) == "D"

def test_grade_f():
    assert letter_grade(59) == "F"
    assert letter_grade(0) == "F"