Programming in Python · Week 3 — Nested conditionals

Week map

Logical operator notation

• A and B → both must be true → x > 0 and x < 10 true only inside (0,10).
• A or B → at least one true → day == "Sat" or day == "Sun for weekend.
• not A → negation → not (x == 5) same as x != 5 for values.
• Short-circuit: False and f() does not call f(); True or g() skips g().

Truth tables (mini)

De Morgan’s laws

• not (A and B) ≡ not A or not B
• not (A or B) ≡ not A and not B

Nested if structure

python
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x = 6
y = 2
if x > 5:
    if y > 5:
        print("both")
    else:
        print("outer only")
else:
    print("neither")

Pattern families

Easy — Compound predicate

• Evaluate and/or on two compares.
• Predict result of not on simple inequality.
• Short-circuit: second operand may not run.

Medium — Nested branch trace

• Two-level nesting with prints in each block.
• Identify which else runs.
• Convert word problem “if A and B then … else if C …” to code shape.

Hard — Decision table to code

• Mutually exclusive cases as if / elif chain.
• Negated compound conditions via de Morgan.
• Equivalent rewrites: nested if vs single compound predicate.

Worked mini-examples

python
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temp = 22
humid = 80
if temp > 18 and humid < 70:
    comfort = "dry warm"
else:
    comfort = "other"
# comfort = "other" (humid fails)

python
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d = "Sat"
if d == "Fri" or d == "Sat" or d == "Sun":
    kind = "off"
else:
    kind = "work"
# kind = "off"

python
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age = 16
licensed = False
# not (age >= 18 and licensed)
if not (age >= 18 and licensed):
    can_drive = False
else:
    can_drive = True
# can_drive False

python
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a, b = 3, 3
if a > 0:
    if b > 0:
        sign = "++"
    else:
        sign = "+-"
else:
    sign = "-"
# sign = "++"

Traps

• and confused with “between” — need two inequalities: low <= x and x <= high.
• Dangling else paired with wrong if after editing indentation.
• or is inclusive — both true still satisfies or.
• Negating without parentheses: not x > 5 parses as (not x) > 5 — wrong; use not (x > 5).
• Nested structure duplicated work — two separate if x>0 and if y>0 differ from if x>0 and y>0.

Diagnostic (try yourself)

1. What is result?

python
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n = 7
if n % 2 == 0 and n > 5:
    result = "A"
elif n % 2 == 1 and n > 5:
    result = "B"
else:
    result = "C"

1. Rewrite without not ( ... and ... ) using de Morgan:

python
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if not (x < 0 or x > 100):
    ok = True

1. What prints?

python
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p, q = 2, 10
if p > 0:
    if q < 5:
        print(1)
    else:
        print(2)
else:
    print(3)

1. For integers a and b, write one if condition that is true exactly when both are positive.
2. if True or expensive(): — is expensive() called? Why?