map() function in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map() with lambda and list

What is the output of this Python code?

Python

result = list(map(lambda x: x**2, [1, 2, 3, 4]))
print(result)

A [1, 4, 9, 16]

B [1, 8, 27, 64]

C [2, 4, 6, 8]

D [1, 2, 3, 4]

💡 Hint

Think about what the lambda function does to each element.

The lambda function squares each number. So 1**2=1, 2**2=4, 3**2=9, 4**2=16.

❓ Predict Output

intermediate

map() with multiple iterables

What is the output of this code snippet?

Python

def add(x, y):
    return x + y

result = list(map(add, [1, 2, 3], [4, 5, 6]))
print(result)

A [5, 7, 9]

B [1, 2, 3, 4, 5, 6]

C [4, 5, 6]

D [1, 6, 9]

💡 Hint

map applies the function to pairs of elements from both lists.

The add function sums pairs: 1+4=5, 2+5=7, 3+6=9.

❓ Predict Output

advanced

map() with filter and string conversion

What is the output of this code?

Python

numbers = [10, 15, 20, 25]
result = list(map(str, filter(lambda x: x % 20 == 0, numbers)))
print(result)

A ['10', '15', '20', '25']

B ['10', '20']

C ['20']

D ['20', '25']

💡 Hint

filter keeps only numbers divisible by 20, then map converts them to strings.

Only 20 is divisible by 20, so filter returns [20]. map converts it to ['20'].

❓ Predict Output

advanced

map() with None as function argument

What is the output of this code?

Python

result = list(map(None, [1, 2, 3]))
print(result)

A []

B [1, 2, 3]

C [None, None, None]

D TypeError

💡 Hint

In Python 3, map() requires a callable function, None is not callable.

Passing None as the function causes a TypeError because None is not callable.

🧠 Conceptual

expert

Behavior of map() with infinite iterator

Consider this code: import itertools result = map(lambda x: x*2, itertools.count(1)) output = [] for i, val in enumerate(result): if i == 5: break output.append(val) print(output) What is the output printed?

A [0, 2, 4, 6, 8]

B [2, 4, 6, 8, 10]

C [1, 2, 3, 4, 5]

D Infinite loop, no output

💡 Hint

itertools.count(1) generates numbers starting at 1 endlessly; map doubles them; loop stops after 5 items.

The loop collects first 5 doubled numbers: 1*2=2, 2*2=4, 3*2=6, 4*2=8, 5*2=10.

Practice

1. What does the map() function do in Python?

easy

A. Combines two lists into a dictionary

B. Creates a new list by filtering items from an iterable

C. Sorts the items of a list in ascending order

D. Applies a given function to each item of an iterable and returns a map object

Solution

1. Step 1: Understand the purpose of map()

   The map() function takes a function and an iterable, then applies the function to each item.

2. Step 2: Identify the return type

   It returns a map object, which can be converted to a list to see the results.

3. Final Answer:

   Applies a given function to each item of an iterable and returns a map object -> Option D

4. Quick Check:

   map() applies function to items [OK]

Hint: Remember: map applies function to each item [OK]

Common Mistakes:

• Confusing map() with filter()
• Thinking map() returns a list directly
• Assuming map() sorts or combines lists

2. Which of the following is the correct syntax to use map() to double each number in the list [1, 2, 3]?

easy

A. map(lambda x: x * 2, [1, 2, 3])

B. map([1, 2, 3], lambda x: x * 2)

C. map(x * 2, [1, 2, 3])

D. map(lambda x: x * 2)

Solution

1. Step 1: Check the order of arguments in map()

   The first argument must be a function, the second an iterable.

2. Step 2: Verify the function syntax

   Using a lambda function like lambda x: x * 2 is correct to double each item.

3. Final Answer:

   map(lambda x: x * 2, [1, 2, 3]) -> Option A

4. Quick Check:

   Function first, iterable second [OK]

Hint: Function goes first, iterable second in map() [OK]

Common Mistakes:

• Swapping function and iterable arguments
• Omitting the iterable argument
• Passing an expression instead of a function

3. What is the output of the following code?

nums = [1, 2, 3, 4]
squared = map(lambda x: x**2, nums)
print(list(squared))

medium

A. [1, 4, 9, 16]

B. [2, 4, 6, 8]

C. [1, 2, 3, 4]

D. Error: map object cannot be converted to list

Solution

1. Step 1: Understand the lambda function

   The lambda function squares each number: x**2.

2. Step 2: Apply map and convert to list

   Mapping squares over [1, 2, 3, 4] gives [1, 4, 9, 16]. Converting map object to list shows these values.

3. Final Answer:

   [1, 4, 9, 16] -> Option A

4. Quick Check:

   Squares of 1 to 4 = [1, 4, 9, 16] [OK]

Hint: Convert map to list to see results [OK]

Common Mistakes:

• Forgetting to convert map object to list
• Confusing square with double
• Expecting map to print directly

4. What is wrong with this code snippet?

nums = [1, 2, 3]
result = map(lambda x: x + 1)
print(list(result))

medium

A. Lambda function syntax is incorrect

B. Missing iterable argument in map()

C. Cannot convert map object to list

D. Using print inside map is invalid

Solution

1. Step 1: Check map() arguments

   The map function requires two arguments: a function and an iterable. Here, the iterable is missing.

2. Step 2: Identify the error cause

   Without the iterable, map() cannot apply the function, causing a TypeError.

3. Final Answer:

   Missing iterable argument in map() -> Option B

4. Quick Check:

   map needs function and iterable [OK]

Hint: Always provide both function and iterable to map() [OK]

Common Mistakes:

• Forgetting the iterable argument
• Assuming map works with one argument
• Misunderstanding lambda function usage

5. You have two lists: names = ['alice', 'bob', 'carol'] and ages = [25, 30, 22]. Using map(), how can you create a list of strings like ['alice is 25', 'bob is 30', 'carol is 22']?

hard

A. list(map(lambda x: x[0] + ' is ' + x[1], names, ages))

B. map(lambda n, a: n + ' is ' + a, names, ages)

C. list(map(lambda n, a: f'{n} is {a}', names, ages))

D. list(map(lambda n: n + ' is ' + ages, names))

Solution

1. Step 1: Understand map with multiple iterables

   map can take multiple iterables and pass corresponding items to the function.

2. Step 2: Use lambda with two parameters

   The lambda takes name and age, then formats the string using f-string for clarity.

3. Step 3: Convert map object to list

   Wrapping with list() shows the final list of formatted strings.

4. Final Answer:

   list(map(lambda n, a: f'{n} is {a}', names, ages)) -> Option C

5. Quick Check:

   map with multiple iterables and f-string [OK]

Hint: Use lambda with multiple args and list() to see results [OK]

Common Mistakes:

• Passing only one iterable when two are needed
• Concatenating string and int without conversion
• Not converting map object to list