Conditional Statements
A program normally reads top to bottom like a recipe. A conditional is a fork in the road: the program walks up, reads the signpost — a yes/no question like "is age >= 18?" — and takes exactly ONE of the paths. if is the fork, elif adds more prongs ("if not that, then is it this?"), and else is the path for "none of the above". The crucial rule: in an if/elif/else chain, the FIRST true signpost wins and every later prong is skipped without even being read. Order your questions from most specific to most general, or the general one will greedily catch everything first.
The fork in the road
step 1 / 6A program is a road. A conditional is a fork: each diamond asks one yes/no question and the walker takes exactly one branch.score = 73
What counts as false — the full list
| Python (falsy) | JavaScript (falsy) |
|---|---|
| False, None | false, null, undefined |
| 0, 0.0, 0j | 0, -0, 0n, NaN |
| "" (empty string) | "" (empty string) |
| [], (), {}, set() | — (empty arrays/objects are TRUTHY in JS!) |
Python conditional forms
- if / elif / else — first true branch wins, everything after is skipped unread.
- Ternary reads middle-out: grade = "pass" if score >= 50 else "retry".
- match/case (3.10+) is the multiway switch: match cmd: case "quit": ... case _: ...
- if items: is the idiom for "non-empty"; if x is None: for "unset" — because 0 and "" are falsy but often valid values!
- Guard clauses keep the happy path flat: if not user: return early.
score = 73
if score >= 80: # most specific first!
grade = "great"
elif score >= 50:
grade = "pass"
else:
grade = "retry"
print(grade) # passscore = 90. Chain: if score >= 50: print("pass") / elif score >= 80: print("great") / else: .... What prints?
grade(score) returns "great" for 80+, "pass" for 50-79, "retry" below 50. Mind the chain order!