Lesson 12 of 21 · 52 min · UG / professional

Recursion, tiny functions, map and filter

A loop is usually clearer than recursion for campus totals. lambda is a nameless one-line def. map and filter are loops with extra syntax — you will still write the loop.

Roll-number search

Scan a list; sort a copy.

You will be able to

✓ Prefer a loop for campus totals; know recursion as a function that calls itself
✓ See lambda as a nameless one-line def you can replace with a tiny named function
✓ Write map and filter as ordinary loops (the laptop names are extra syntax)

A loop is usually clearer than recursion for hostel totals

Summing hall counts 1 through n is a while with i = i + 1. Recursion is the same job as a function that calls itself with a smaller n until a stop test. Both can be correct. Recursion on a campus total is how internships hit a recursion limit and then “fix” it by raising the limit. Use a loop unless the data is actually a tree (a folder of folders, a JSON of nested fees).

When you do recurse, the first line is the stop: if n <= 0: return 0. Then return n + sum_upto(n - 1). Practise a tiny countdown in this box so the call is not a mystery when a later course walks a menu tree.

lambda, map, filter are loops with extra hats

lambda amount: amount * 118 / 100 is a nameless one-line def. The moment you need a name, an if, or a docstring, write def add_gst. map(add_gst, bases) on a laptop walks bases and collects results — you already wrote that as a new list plus append. filter(passed, marks) keeps items that pass a test — also a loop. You will still write the loop here. The laptop names are not faster; they are shorter.

A lambda inside map is how 1 a.m. code becomes unreadable. Name the function. Keep map/filter for when the team already uses them, not as a personality.

Laptop: recursion, lambda, map, filter

def sum_upto(n):
    if n <= 0:
        return 0
    return n + sum_upto(n - 1)

add_gst = lambda amount: amount * 118 / 100
print(list(map(add_gst, [100, 200])))
print(list(filter(lambda m: m >= 40, [32, 41, 55])))

Words that matter

Recursion: A function that calls itself with a smaller problem, plus a stop test.
lambda: A nameless one-line function — replace with def when it grows.
map / filter: Laptop names for a transform loop and a keepers loop.

Common mistakes