Lesson 7 of 21 · 50 min · UG / professional

Comprehensions — the short spelling of a filter loop

Keep heat days; double a list of quantities. The long form is a new list plus append. The one-liner on a laptop is a comprehension. Debug the long form at 1 a.m.

Weather station

Filter a table of readings.

You will be able to

✓ Build a filtered list with a loop and append (the long form)
✓ Build a mapped list — new values, same count — the same way
✓ Recognise a comprehension as the laptop one-liner for that loop

Keep heat days; double the quantities

The DHT logger wrote 28, 31, 34. Keep temperatures at or above 32. That is a filter: a new list, an if, append. Canteen packs of 2, 4, 1 become 4, 8, 2 for a double-shift indent. That is a map: every input produces one output, append each. Neither loop should overwrite the source — the logger file is evidence.

On a laptop, [t for t in temps if t >= 32] and [q * 2 for q in qty] are the short spellings. They are not a new language. Debug the long form at 1 a.m. when the one-liner hid a wrong cutoff. This sandbox forbids the short spelling so you cannot hide.

Filter and map are different jobs

A filter’s result is often shorter. A map’s result is the same length. If you mix them — keep heat days and also convert to Fahrenheit in one confused loop — name two lists or two functions. Comprehensions can nest on a laptop; nested confusion is still confusion.

When the “map” is a GST function you already wrote, call it inside the loop. The comprehension on a laptop would call it too. The function is the reusable part; the loop is only the walk.

Laptop: the same two jobs as one-liners

temps = [28, 31, 34, 29]
hot = [t for t in temps if t >= 32]
qty = [2, 4, 1]
doubled = [q * 2 for q in qty]
print(hot)
print(doubled)

Words that matter

Filter: A new list of items that pass a test — often shorter.
Map: A new list of transformed items — usually the same length.
Comprehension: Laptop short spelling of a filter or map loop.

Common mistakes