(a) Write a Python expression to check if the key, 'RNo' is present in D1.

OR

(b) Write a Python expression to check if any key in D1 has a value 12.

Q: (a) Write a Python expression to check if the key, 'RNo' is present in D1. OR (b) Write a Python expression to check if any key in D1 has a value 12.

Both parts test the `in` membership operator on a dictionary — once on keys (default) and once on values. **(a) Check if key 'RNo' is present in D1:** ```python 'RNo' in D1 ``` This returns `True` if 'RNo' is a key of D1, else `False`. **(b) Check if any key has value 12:** ```python 12 in D1.values() ``` Since `in` on a dictionary by default searches only keys, values must be checked explicitly using `D1.values()`.

Python Dictionaries

Dictionary Membership Operators

Official Answer

Both parts test the in membership operator on a dictionary — once on keys (default) and once on values.

(a) Check if key 'RNo' is present in D1:

``python


'RNo' in D1

This returns True if 'RNo' is a key of D1, else False.




(b) Check if any key has value 12:

`python


12 in D1.values()

Since in on a dictionary by default searches only keys, values must be checked explicitly using D1.values()`.

dictionarymembership operatorinkeysvalues()hashingPython dictionary

Marking Scheme

11 mark: correct expression `'RNo' in D1` for part (a), OR correct expression `12 in D1.values()` for part (b); any logically equivalent syntactically correct alternative is accepted.

Hint

Remember: `in` on a dictionary checks keys by default; use `.values()` to check values.

Quick Oral Answer

The `in` operator checks dictionary keys by default in O(1) time via hashing; to check values I must explicitly call `.values()`, which is an O(n) linear search.

Analysis & Explanation

This question checks whether a student understands that a Python dictionary is a key-value hash map and that the in operator behaves differently depending on what part of the dictionary is being searched.

Concept

key in D1 searches only the keys, using Python's underlying hash table, so it runs in O(1) average time.
To search values, D1.values() must be called explicitly to get a view of all values, and then in performs a linear O(n) scan over that view.

Exam trap

Students often write 12 in D1 for part (b), which actually checks whether 12 is a key, not a value — this is the single most common error examiners report for this question type.

Real-world relevance

This pattern is used constantly in real programs, e.g. checking if a roll number/ID already exists before inserting a new record, or checking if any student scored a particular mark.

Common Mistakes

1Writing `12 in D1` instead of `12 in D1.values()` for part (b) — this silently checks keys instead of values and gives a wrong logical result without raising an error.
2Confusing `D1.keys()` and `D1.values()` — since both return view objects, students sometimes swap them.

Interesting Facts

Key lookup in a Python dictionary using `in` runs in average O(1) time because dictionaries are implemented as hash tables internally.

Value lookup using `.values()` is O(n) because there is no hash index built on values — Python must scan every entry linearly.

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Frequently Asked Questions

Does 'in' check keys or values by default in a Python dictionary?

By default, the `in` operator on a dictionary checks only the keys, not the values. To check values you must explicitly use `D1.values()`.

What is the time complexity of key lookup vs value lookup in a dictionary?

Key lookup via `in` is O(1) on average due to hashing, while value lookup via `D1.values()` is O(n) since it requires a linear scan of all values.