Inverting a dictionary in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty dictionary for the inverted result.

Python

inverted = [1]

Drag options to blanks, or click blank then click option'

Adict()

B[]

Cset()

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets creates a list, not a dictionary.

Using set() creates a set, not a dictionary.

We use {} to create an empty dictionary in Python.

2fill in blank

medium

Complete the code to loop through the original dictionary's items.

Python

for key, value in original.[1]():
    inverted[value] = key

Drag options to blanks, or click blank then click option'

Aitems

Bkeys

Cvalues

Diter

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys() only gives keys, not values.

Using values() only gives values, not keys.

The items() method returns key-value pairs from a dictionary.

3fill in blank

hard

Fix the error in the code to invert the dictionary correctly.

Python

inverted = {}
for k, v in data.items():
    inverted[[1]] = k

Drag options to blanks, or click blank then click option'

Av

Bk

Cdata

Dinverted

Attempts:

3 left

💡 Hint

Common Mistakes

Using k as the new key keeps the dictionary the same.

Using data or inverted as keys causes errors.

When inverting, the original value v becomes the new key.

4fill in blank

hard

Fill the four blanks to create a dictionary comprehension that inverts the dictionary.

Python

inverted = { [1]: [2] for [3], [4] in original.items() }

Drag options to blanks, or click blank then click option'

Avalue

Bkey

Ck

Dv

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values incorrectly.

Using variable names inconsistently.

In a dictionary comprehension, the syntax is {new_key: new_value for key, value in dict.items()}. Here, v is the new key and k is the new value.

5fill in blank

hard

Fill all three blanks to invert the dictionary and handle duplicate values by storing keys in a list.

Python

inverted = {}
for [1], [2] in original.items():
    inverted.setdefault([3], []).append([1])

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Ck

Dv

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable names.

Not using setdefault to handle duplicates.

We use setdefault with the value v as key and append the original key k to handle duplicates.

Practice

1.

What does inverting a dictionary mean in Python?

easy

A. Sorting the dictionary by keys

B. Swapping keys and values so values become keys and keys become values

C. Removing duplicate keys from the dictionary

D. Changing all values to uppercase strings

Solution

1. Step 1: Understand dictionary structure

   A dictionary has keys and values paired together.

2. Step 2: Define inverting

   Inverting means swapping each key with its value, so keys become values and values become keys.

3. Final Answer:

   Swapping keys and values so values become keys and keys become values -> Option B

4. Quick Check:

   Inverting = swapping keys and values [OK]

Hint: Invert means swap keys and values in a dictionary [OK]

Common Mistakes:

• Thinking inverting sorts the dictionary
• Confusing inverting with removing duplicates
• Assuming values become uppercase strings

2.

Which of the following is the correct syntax to invert a dictionary d using dictionary comprehension?

easy

A. {k: v for k, v in d}

B. {k: v for v, k in d.items()}

C. {d[v]: d[k] for k, v in d.items()}

D. {v: k for k, v in d.items()}

Solution

1. Step 1: Recall dictionary comprehension syntax

   It uses {new_key: new_value for key, value in dict.items()}.

2. Step 2: Swap keys and values correctly

   To invert, new_key = value and new_value = key, so use {v: k for k, v in d.items()}.

3. Final Answer:

   {v: k for k, v in d.items()} -> Option D

4. Quick Check:

   Correct syntax = {v: k for k, v in d.items()} [OK]

Hint: Use {v: k for k, v in d.items()} to invert dictionary [OK]

Common Mistakes:

• Swapping variables incorrectly in comprehension
• Using d[v] or d[k] inside comprehension wrongly
• Forgetting to call .items() on dictionary

3.

What is the output of this code?

original = {'a': 1, 'b': 2, 'c': 3}
inverted = {v: k for k, v in original.items()}
print(inverted)

medium

A. {1: 'a', 2: 'b', 3: 'c'}

B. {'a': 1, 'b': 2, 'c': 3}

C. {'1': 'a', '2': 'b', '3': 'c'}

D. Error: unhashable type

Solution

1. Step 1: Understand original dictionary

   Keys are 'a', 'b', 'c' and values are 1, 2, 3.

2. Step 2: Invert dictionary using comprehension

   Swapping keys and values gives keys 1, 2, 3 and values 'a', 'b', 'c'.

3. Final Answer:

   {1: 'a', 2: 'b', 3: 'c'} -> Option A

4. Quick Check:

   Inverted dict = {1: 'a', 2: 'b', 3: 'c'} [OK]

Hint: Invert swaps keys and values exactly as pairs [OK]

Common Mistakes:

• Expecting original dictionary output
• Confusing string and integer keys
• Thinking inversion causes error here

4.

What is wrong with this code to invert a dictionary?

d = {'x': 10, 'y': 10}
inverted = {v: k for k, v in d.items()}
print(inverted)

medium

A. It will keep only one key for duplicate values

B. It will invert correctly with no issues

C. It will raise a TypeError

D. It will raise a KeyError

Solution

1. Step 1: Identify duplicate values in dictionary

   Both 'x' and 'y' have value 10, which is duplicated.

2. Step 2: Understand dictionary key uniqueness

   When inverting, keys must be unique, so only one key-value pair with key 10 remains.

3. Final Answer:

   It will keep only one key for duplicate values -> Option A

4. Quick Check:

   Duplicate values cause lost keys in inversion [OK]

Hint: Duplicate values become keys, only last key kept [OK]

Common Mistakes:

• Expecting all keys preserved after inversion
• Thinking it raises an error for duplicates
• Ignoring key uniqueness in dictionaries

5.

Given a dictionary with possible duplicate values, how can you invert it so each value maps to a list of keys that had that value?

original = {'a': 1, 'b': 2, 'c': 1}

Which code correctly inverts it to {1: ['a', 'c'], 2: ['b']}?

hard

A. inverted = {v: [k] for k, v in original.items()}

B. inverted = {v: k for k, v in original.items()}

C. inverted = {}
for k, v in original.items():
inverted.setdefault(v, []).append(k)

D. inverted = {v: k for v, k in original.items()}

Solution

1. Step 1: Understand problem with duplicates

   Simple inversion loses keys when values repeat, so we need lists to hold multiple keys.

2. Step 2: Use setdefault and append to collect keys

   Loop through items, for each value use setdefault to create list if missing, then append key.

3. Step 3: Check code correctness

   inverted = {} for k, v in original.items(): inverted.setdefault(v, []).append(k) uses this approach correctly, building lists of keys per value.

4. Final Answer:

   inverted = {} for k, v in original.items(): inverted.setdefault(v, []).append(k) -> Option C

5. Quick Check:

   Use setdefault + append to group keys by value [OK]

Hint: Use setdefault with append to group keys by value [OK]

Common Mistakes:

• Using simple comprehension losing duplicate keys
• Swapping variables incorrectly in comprehension
• Expecting one-to-one inversion with duplicates