Programming in Python · Week 2 — Conditionals

Week map

Conditional notation

• if cond: → conditional block → runs body only when cond is True.
• elif cond: → else-if → tested only if all prior branches failed.
• else: → fallback → runs when every test above was false.
• cond → predicate → expression with boolean result → x >= 10, name == "Ada".

Comparison operators

Truthiness

• Falsy: 0, 0.0, False, None, "", [], {}, ().
• Truthy: most other values, including nonzero numbers and non-empty strings.

python
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score = 0
if score:
    print("played")
else:
    print("skipped")

Branch selection rules

1. Evaluate if condition. If true, run its block and skip all elif and else.
2. Else evaluate first elif. If true, run it and skip rest.
3. If none true, run else if present; if no else, no block runs.

python
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n = 14
if n < 10:
    label = "low"
elif n < 20:
    label = "mid"
else:
    label = "high"
# label is "mid"

Pattern families

Easy — Which branch runs?

• Single if / else with numeric compare.
• Predict print output from one predicate.
• Identify when else runs.

Medium — elif chains and boundaries

• Inclusive vs exclusive thresholds (<= vs <).
• Overlapping conditions — only first true branch executes.
• Assign variable in branches; trace final value.

Hard — Truthiness and edge inputs

• Empty string vs "0" string (truthy).
• if x: vs if x == True (usually prefer explicit compare for clarity).
• Multiple variables set in different branches — know which path ran.

Worked mini-examples

python
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x = 25
if x > 30:
    print("A")
elif x > 20:
    print("B")
elif x > 10:
    print("C")
else:
    print("D")

python
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k = 3
if k > 5:
    t = 1
# no else
# t may not exist — NameError if referenced

python
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age = 18
if age >= 18:
    status = "adult"
else:
    status = "minor"
# status = "adult" (18 counts)

python
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name = ""
if name:
    print("hi")
else:
    print("empty")
# prints empty

Traps

• elif still evaluated mentally when if was true — it is skipped entirely.
• Off-by-one on inclusive bounds.
• Assigning in one branch but using variable assuming another branch ran.
• == True with integers: only 1 equals True oddly; prefer if flag: or if flag is True rarely.
• Dangling logic: two separate if statements both may run — different from if / elif.

Diagnostic (try yourself)

1. What is printed?

python
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v = 12
if v > 15:
    print("X")
elif v > 8:
    print("Y")
else:
    print("Z")

1. After this code, what is w?

python
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w = 5
if w < 3:
    w = 10
elif w < 7:
    w = 20
else:
    w = 30

1. What is printed?

python
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flag = 0
if flag:
    print("on")
else:
    print("off")

1. Rewrite using only if and else (no elif): assign "pass" when score >= 40, else "fail". Variable score already exists.
2. Two separate if statements both test x > 0. First prints "A", second prints "B". If x = 5, what prints? If x = -1, what prints?