Python Dictionary Class 11: Key-Value Pairs, Methods, Traversal and Character Frequency

Understand Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11)

s = "banana"
freq = {}
for ch in s:
    if ch in freq:
        freq[ch] += 1
    else:
        freq[ch] = 1
print(freq)   # {'b': 1, 'a': 3, 'n': 2}

Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11): key facts

Access

• •d[k] gives KeyError if missing
• •d.get(k) gives None
• •d.get(k, default) gives default
• •k in d tests keys

in never tests values.

Change

• •d[k] = v adds or replaces
• •del d[k] removes
• •d.pop(k) removes and returns
• •d.update(e) merges

Duplicate keys are not allowed.

Views

• •keys()
• •values()
• •items() give (key, value) pairs
• •wrap in list() to print

for k, v in d.items() unpacks pairs.

Other methods

• •dict.fromkeys(keys, v)
• •setdefault(k, v)
• •copy()
• •len(d) pairs

e = d is an alias, not a copy.

How to answer Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11) questions

1. 1Write the dictionary with its pairs in the order of insertion.
2. 2For d[k] = v check whether k is already a key: replace the value if it is, add a pair at the end if not.
3. 3For a loop write the current key, value and any counter on each pass.
4. 4For get(), pop() and setdefault() note both the value returned and the change made to the dictionary.
5. 5Print the dictionary with braces, quotes on string keys and a colon after each key.

Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11) in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11)

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Keys are the index

  A dictionary is looked up by key, like a word in a dictionary; the value is the meaning.

  Use it when: Access and in-tests.

• in looks at keys

  "x" in d asks about keys only.

  Use it when: Membership questions.

• get is gentle

  d[k] shouts KeyError; d.get(k) quietly returns None or your default.

  Use it when: Counting and lookup programs.

Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11): solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

d = {"Maths": 90, "Sci": 85}
d["Eng"] = 78
d["Sci"] = 88
del d["Maths"]
print(d, len(d))

d = {"x": 1, "y": 2, "z": 3}
print(list(d.keys()), list(d.values()))
print(list(d.items()))
print(d.get("w"), d.get("w", 0))
d.update({"y": 20, "w": 4})
print(d)
print(d.pop("x"), d)

s = "banana"
freq = {}
for ch in s:
    if ch in freq:
        freq[ch] += 1
    else:
        freq[ch] = 1
print(freq)
d = dict.fromkeys(["a", "b"], 0)
print(d.setdefault("c", 5), d.setdefault("a", 9))
print(d)

prices = {"pen": 10, "book": 120, "bag": 450}
total = 0
for k, v in prices.items():
    total += v
    if v > 100:
        print(k)
print(total)

Where students lose marks in Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11)

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using a list as a key

  Not knowing that keys must be immutable.

  Wrong working

  d = {[1, 2]: "pair"} # TypeError: unhashable type

  Right working

  d = {(1, 2): "pair"}

  ✓Do this: Use a tuple, number or string as the key.

  Usually costs: 1 mark

• ✗Mistake: Expecting in to check values

  Treating a dictionary like a list.

  Wrong working

  d = {"a": 1}; 1 in d is True

  Right working

  1 in d is False; 1 in d.values() is True

  ✓Do this: in tests keys.

  Usually costs: 1 mark

• ✗Mistake: Reading a missing key with square brackets

  Not planning for absent keys.

  Wrong working

  freq[ch] += 1 for a new character raises KeyError

  Right working

  freq[ch] = freq.get(ch, 0) + 1

  ✓Do this: Use get() or test with in first.

  Usually costs: 1 to 2 marks

Practice questions on Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11)

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which of these can be used as a key in a dictionary?
   (a) [1, 2]
   (b) (1, 2)
   (c) {1, 2}
   (d) {"a": 1}

   Hint

   Keys must be immutable.

   Answer

   (b) (1, 2)

2. Q2 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): For a dictionary d, d["k"] raises an error when "k" is absent, but d.get("k") does not.
   Reason (R): get() returns None, or a given default, for a missing key.
   (a) Both A and R are true, and R is the correct explanation of A.
   (b) Both A and R are true, but R is not the correct explanation of A.
   (c) A is true, but R is false.
   (d) A is false, but R is true.

   Hint

   KeyError.

   Answer

   (a) Both A and R are true, and R is the correct explanation of A.

3. Q3 · 4 marks · Short answerHarder board level

   Give the output:

   d = {1: "a", 2: "b"}
   d[3] = "c"
   d[1] = "z"
   print(d, len(d), 2 in d, "b" in d)
   print(list(d.keys()))

   (4 marks)

   Hint

   Keys 1, 2, 3 only.

   Answer

   d[3] adds a pair; d[1] replaces the value. d is {1: 'z', 2: 'b', 3: 'c'}, length 3; 2 is a key: True; 'b' is a value, not a key: False. Output: {1: 'z', 2: 'b', 3: 'c'} 3 True False, then [1, 2, 3]

4. Q4 · 4 marks · Short answerHarder board level

   Write a program that finds the student with the highest marks in marks = {"Ali": 72, "Bea": 91, "Cy": 85} and prints the name and marks. Give the output. (4 marks)

   Hint

   Loop over items with a best-so-far variable.

   Answer

   best = 0; name = ""; for k, v in marks.items(): if v > best: best = v; name = k; then print(name, best). Ali sets best 72, Bea replaces it with 91, Cy (85) does not. Output: Bea 91

5. Q5 · 5 marks · Short answerHOTS / competency

   Give the output of the program and explain the three lines:

   d = {"a": 1, "b": 2}
   e = d
   e["c"] = 3
   f = d.copy()
   f["d"] = 4
   print(d)
   print(f)
   print(d is e, {"a": 1, "a": 2})

   (5 marks)

   Hint

   Alias versus copy.

   Answer

   e = d is an alias, so e['c'] = 3 changes d too: {'a': 1, 'b': 2, 'c': 3}. f = d.copy() is a separate dictionary, so f['d'] = 4 changes only f: {'a': 1, 'b': 2, 'c': 3, 'd': 4}. d is e is True. In {'a': 1, 'a': 2} the later value wins: {'a': 2}. Last line output: True {'a': 2}

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Can keys repeat in a dictionary?Show answer

   No; keys are unique, and a repeated key keeps the last value.

2. 2.What does get() return for a missing key?Show answer

   None, or the default you give.

3. 3.What does the in operator test in a dictionary?Show answer

   The keys.

4. 4.Which method gives (key, value) pairs?Show answer

   items().

5. 5.What does setdefault(k, v) do if k exists?Show answer

   Returns the existing value without changing it.

Dictionaries: Keys, Values, get, items, update and Counting Characters (CBSE 11): frequently asked questions