PCEP: Control Flow: Conditional Blocks and Loops

What this domain covers

Control flow is the largest PCEP domain. You must trace if/elif/else, while, for over range(), and break/continue, including nested and loop-else constructs.

Core rules

• Only the first true branch of an if/elif chain runs. Conditions are tested top to bottom; else is optional.
• range(stop) starts at 0; range(start, stop, step) excludes stop. Negative steps count down: list(range(5, 0, -2)) == [5, 3, 1].
• while checks its condition before every iteration; if initially false, the body never runs.
• break exits the nearest loop immediately. continue skips to the next iteration.
• A loop's else runs only when the loop ends normally, that is, without break.
• pass is a no-op placeholder. The conditional expression a if cond else b is an expression, not a statement.

Worked example

for i in range(1, 4): if i == 2: continue; print(i) prints 1 then 3. range(1, 4) yields 1, 2, 3; when i is 2, continue jumps to the next value, so print is skipped only for 2. Replacing continue with break would print only 1, because the loop would terminate at i == 2.

Traps

• while x > 0: with no change to x inside creates an infinite loop.
• Indentation defines blocks; a wrongly indented print may sit outside the loop.
• for i in range(3): finishes with i == 2; the loop variable persists afterwards.
• if x = 1: is invalid. Chained comparisons like 0 < x < 10 mean 0 < x and x < 10.
• Appending to a list while iterating over it can skip elements.