Lesson 11 of 16 · 50 min · UG / professional
if / elif / else and what counts as true
Late-fee bands must not overlap. == asks; = stores. Empty lists and 0 are the usual ‘no’ in Python; a clerk-facing function should still return an explicit 0 or "ok".
Junction traffic light
if / elif / else choose a path.
You will be able to
- ✓ Write exclusive if/elif/else bands
- ✓ Use == to ask and = to store
- ✓ Know empty/0 as false on a laptop, still return explicit 0 at the edge
Exclusive bands
elif is the next exclusive band. Three separate ifs can double-charge. else is everything left, including on-time. Return 0 for on-time so callers add fees without a special branch. A ternary x if c else y exists on a laptop; an if statement is clearer for clerks.
== asks; = stores. Comparisons < > <= >= != complete the set. Nested ifs stand in for and/or in this sandbox.
Truth tests
On a laptop, empty lists and 0 are false in if x:. Do not rely on that at a kiosk — print "ok" or return 0. Multiway branching is if/elif, not a C switch.
Words that matter
- elif
- Next exclusive band after a failed if.
- ==
- Ask whether two values are equal.
- Truthy
- Laptop: non-zero / non-empty counts as true in if x.
Common mistakes
Avoid: Three separate ifs for overlapping fee bands.
Do this: if / elif / else so a day count hits one band.
Run it step by step
Each box is a real program. Press Run, change a number, Run again — the output must follow your code.
1. Step 1 — a gate
Change age to 17 and Run again.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
2. Step 2 — exclusive bands
4 days should print Mid, not two fees.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Example program — Late fee as a number
On time is 0.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Your turn — Overtime flag
hours = 10. Print OT if hours > 8, else print Regular.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Self-assessment
Check your understanding before you mark the lesson complete.
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