Counter and defaultdict in Action

Counting how often things appear is everywhere in contests. The collections module gives two tools that make tallying painless. 🧮

Plain dicts raise a KeyError when you increment a key that does not exist yet. Special structures remove that friction for you.

A Counter tallies any iterable in one line, giving each item its frequency as the value. Absent keys simply count as zero.

Index a Counter like a dict, but a missing key returns zero instead of crashing. That makes lookups totally safe.

Call most_common to get items sorted from highest count down. Pass a number to limit how many top entries you receive.

You can add or subtract two Counters to combine tallies. It merges matching keys and keeps the totals aligned automatically.

A defaultdict supplies a fresh default for any new key, so you never check existence first. You give it the default factory.

With defaultdict(int), a missing key starts at zero, so you increment directly. No if-in check clutters your loop.

Use defaultdict(list) when each key collects many values. Appending to a brand-new key just starts a fresh empty list.

Reach for Counter when you only count items, and defaultdict when each key needs lists, sets, or custom defaults. 🎯

Two words are anagrams exactly when their Counters are equal. The comparison handles every letter frequency at once.

You want to count occurrences while incrementing missing keys safely.

Counter tallies items instantly and ranks them, while defaultdict skips missing-key checks for counts, lists, or sets. Both kill boilerplate. 🚀