Dictionary operations: Class 12 practice
Dictionaries map keys to values. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Lists
The idea behind dictionary operations
Dictionaries map keys to values. d[k] = v adds or updates; d.get(k, default) avoids errors for missing keys; len(d) counts keys; iterating over d gives keys.
Step-by-step method
- Track the dictionary after each statement.
- Updating an existing key replaces the value.
- get() returns the default if the key is missing.
Worked example 1 (Basic)
d = {'a': 13, 'b': 19, 'c': 16} d['b'] = 31 print(d['b'], len(d))
- 'b' is updated to 31
- Still 3 keys
- Answer: 31 3
Worked example 2 (Standard)
d = {'a': 17, 'b': 1, 'c': 19} print(d.get('z', 40) + d['a'])
- 'z' missing → 40
- 40 + 17 = 57
- Answer: 57
Worked example 3 (Challenge)
d = {'a': 6, 'b': 5, 'c': 12, 'd': 19} s = 0 for k in d: s += d[k] print(s)
- Iterating gives keys; add each value
- = 42
- Answer: 42
Common mistakes and misconceptions
- Thinking duplicate keys are kept
- Expecting d[k] on a missing key to return None
Practice questions
- (Basic) d = {'a': 5, 'b': 14, 'c': 18} d['b'] = 24 print(d['b'], len(d))
- (Basic) d = {'a': 12, 'b': 16, 'c': 13} d['b'] = 28 print(d['b'], len(d))
- (Standard) d = {'a': 8, 'b': 9, 'c': 4} print(d.get('z', 32) + d['a'])
- (Standard) d = {'a': 4, 'b': 1, 'c': 12} print(d.get('z', 40) + d['a'])
- (Challenge) d = {'a': 19, 'b': 16, 'c': 2, 'd': 16} s = 0 for k in d: s += d[k] print(s)
- (Challenge) d = {'a': 19, 'b': 16, 'c': 8, 'd': 12} s = 0 for k in d: s += d[k] print(s)
Answer key with full working
- Q1: 'b' is updated to 24 → Still 3 keys → Answer: 24 3
- Q2: 'b' is updated to 28 → Still 3 keys → Answer: 28 3
- Q3: 'z' missing → 32 → 32 + 8 = 40 → Answer: 40
- Q4: 'z' missing → 40 → 40 + 4 = 44 → Answer: 44
- Q5: Iterating gives keys; add each value → = 53 → Answer: 53
- Q6: Iterating gives keys; add each value → = 55 → Answer: 55
Syllabus note
This skill is listed under "Python Revision Tour" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is dictionary operations in?
"Python Revision Tour", Class 12 Computer Science (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.