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

Frequency counting pattern

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

Traversal

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

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

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

python
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# d[1, 2](/courses/deep/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?