Week 7 — collections continued

Quiz 2 scope: Weeks 1–8 per IITM May 2026 foundation courses. Source baseline: IITM BS admissions important-dates calendar · May 2026 cycle. Times on assessments are operational conventions — verify hall ticket.

Week map

Classify → Represent → Execute → Trap-check

• Recognize: Ask: How do you count occurrences with a dict?
• Procedure: Insert key-value pairs. Overwriting key updates value. Keys must be immutable (str, int, tuple of immutables).
• Variations / traps: Watch for: KeyError when key never inserted—use get or in.

Formula chain (compressed)

dict key→value → in keys → frequency via d[k]=d.get(k,0)+1.

1. Dict literal — {k: v, ...} — map keys to values
2. Lookup — d[key] — KeyError if missing
3. get — d.get(key, default) — safe lookup
4. Frequency — d[x] = d.get(x,0) + 1 — count occurrences
5. Iteration — for k in d: / d.items() — keys vs key-value pairs

Deep study

Programming in Python · Week 7 — Dictionaries and frequency

Week map

Dictionary notation

• d = {"a": 1, "b": 2} → mapping from keys to values → keys must be hashable (immutable types).
• d[key] → lookup value → raises KeyError if key missing.
• d[key] = value → insert or overwrite.
• key in d → membership test → True if key exists.
• len(d) → number of key-value pairs.

python
Run Inline
scores = {"Ana": 88, "Ben": 92}
scores["Ana"]      # 88
scores["Cal"] = 75 # add new pair

Safe access

• d.get(key) → returns None if missing (or default: d.get(key, 0)).
• d.get(key, 0) → frequency pattern starter — return 0 when key unseen.

python
Run Inline
counts = {}
word = "aba"
for ch in word:
    counts[ch] = counts.get(ch, 0) + 1
# counts = {'a': 2, 'b': 1}

Frequency counting pattern

python
Run Inline
freq = {}
for item in data:
    freq[item] = freq.get(item, 0) + 1

Traversal

python
Run Inline
for key in d:
    print(key, d[key])

for key, val in d.items():
    print(key, val)

Pattern families

Easy — Lookup and update

Medium — Frequency count

Hard — Nested dict and grouping

Worked mini-examples

python
Run Inline
capitals = {}
capitals["India"] = "New Delhi"
capitals["France"] = "Paris"

python
Run Inline
votes = ["yes", "no", "yes", "yes", "no"]
tally = {}
for v in votes:
    tally[v] = tally.get(v, 0) + 1
# {'yes': 3, 'no': 2}

python
Run Inline
freq = {"a": 3, "b": 7, "c": 2}
best_key = None
best_val = -1
for k, v in freq.items():
    if v > best_val:
        best_val = v
        best_key = k
# best_key = 'b'

python
Run Inline
d = {"x": 1}
d["x"] = 10   # overwrite, not duplicate key
# len(d) still 1

python
Run Inline
# d[1, 2](/courses/may26-python/notes/1%2C%202) = 5  # TypeError — list not hashable
d[(1, 2)] = 5    # OK — tuple is hashable

Traps

• Using d[key] when key may be absent — use get or check in first.
• Lists as keys — not allowed.
• Assuming dict preserves insertion order in very old Python mental models (3.7+ preserves order, but logic should not depend on order unless stated).
• freq[key] += 1 on missing key raises error — initialize first.
• Confusing .keys() with values when iterating.

Diagnostic (try yourself)

1. What does {"x": 1, "y": 2}["y"] return? What happens for ["z"]?
2. Count letters in "hello" using get(key, 0) + 1 pattern. What is count of 'l'?
3. Write one line to check if "admin" is a key in dict users.
4. After d = {}; d["a"] = 1; d["a"] = 2, what is d and len(d)?
5. Why does counts[word] = counts[word] + 1 fail on first sighting of word in an empty dict?

ChatGPT prep archive

• Chatty reference archive

Archived import for extra depth — complements the notes above, not official IITM material.

Core concepts

• Dict maps hashable keys to values; literal {k: v} or dict().
• Lookup: d[k] raises KeyError if missing; d.get(k, default) safe.
• Frequency: loop items, counts[x] = counts.get(x, 0) + 1.
• Iteration: d.keys(), d.values(), d.items() for pairs.

Notation & vocabulary

Pattern families

Easy — Build small dict

Medium — Frequency table

Hard — Group or invert

Drill these on the pattern atlas — filter to week 7.

Traps

• KeyError when key never inserted—use get or in.
• Lists as dict keys (unhashable).
• Iterating dict gives keys only by default in older mental models—use items for pairs.
• Assuming sorted order of keys without sorting.

Retrieval prompts

1. How do you count occurrences with a dict?
2. Difference between d[k] and d.get(k)?
3. Why can’t a list be a dict key?

Practice loop

1. Read Deep study (if present) or core concepts once.
2. Recite the formula chain without looking.
3. Open one easy pattern on the interactive atlas for week 7.
4. Attempt without solutions; mark studied after an honest try.
5. Say one trap aloud before closing the tab.

Related

• Full weeks 1–8 prep guide
• Pattern atlas note
• Formula chain desk