PCEP Control Flow, Loops, Lists and Logic Practice Question
A company needs to filter a list of temperatures in Celsius to only those above 0, then convert to Fahrenheit (multiply by 9/5 and add 32). Which code snippet correctly accomplishes this using a list comprehension?
⚠ Common exam trap
Python Institute often tests the distinction between the filter `if` (placed after the `for` clause) and the conditional expression `if-else` (placed in the expression part), and the trap here is that candidates mistakenly add an `else` to a filter-only comprehension, expecting it to work like a ternary operator.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
[t*9/5+32 for t in temps if t>0]
It uses the standard list comprehension syntax: `[expression for item in iterable if condition]`. Here, `t*9/5+32` is the expression that converts Celsius to Fahrenheit, `for t in temps` iterates over the list, and `if t>0` filters out temperatures at or below zero. This produces a new list containing only the Fahrenheit equivalents of positive Celsius temperatures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
[t*9/5+32 for t in temps if t>0]
Why this is correct
Correct list comprehension with expression and filter.
- ✗
[t for t in temps if t>0 then t*9/5+32]
Why it's wrong here
Python list comprehensions have no `then` keyword; the syntax is `[expression for item in iterable if condition]`. Writing `if t>0 then t*9/5+32` raises a SyntaxError. The `then` construct belongs to languages such as shell or SQL CASE expressions, not Python.
- ✗
[t*9/5+32 for t in temps if t>0 else 0]
Why it's wrong here
A conditional expression cannot follow the comprehension's `if` filter; `if t>0 else 0` is invalid syntax there, so the snippet raises a SyntaxError. The `else` clause belongs to a ternary in the output expression, which would be written `[t*9/5+32 if t>0 else 0 for t in temps]` — but that keeps every element, not filtering.
- ✗
[t*9/5+32 for t in temps if t>0 else t]
Why it's wrong here
Placing `else t` after the `if t>0` filter is a syntax error; a comprehension's `if` clause takes no `else`. The ternary form `[t*9/5+32 if t>0 else t for t in temps]` is valid but returns unconverted non-positive values, so it fails the filter requirement.
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