map() for element-wise mapping in Data Analysis Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map() with a lambda function

What is the output of this Python code using map()?

Data Analysis Python

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

A [2, 3, 4, 5]

B [1, 4, 9, 16]

C [1, 8, 27, 64]

D [1, 2, 3, 4]

💡 Hint

Remember, map() applies the function to each element in the list.

The lambda function squares each number. So 1²=1, 2²=4, 3²=9, 4²=16.

❓ data_output

intermediate

Mapping a dictionary to a list

What is the output of this code that uses map() on a dictionary's keys?

Data Analysis Python

data = {'a': 1, 'b': 2, 'c': 3}
result = list(map(str.upper, data))
print(result)

A ['a', 'b', 'c']

B ['1', '2', '3']

C ['A', 'B', 'C']

D ['A', 'B', 'C', 'D']

💡 Hint

Mapping applies to dictionary keys by default.

The map() applies str.upper to each key: 'a' → 'A', 'b' → 'B', 'c' → 'C'.

🔧 Debug

advanced

Identify the error in map() usage

What error does this code raise when run?

Data Analysis Python

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

A NameError: name 'y' is not defined

B TypeError: unsupported operand type(s) for +: 'int' and 'function'

C SyntaxError: invalid syntax

D No error, output is [2, 3, 4]

💡 Hint

Check if all variables inside the lambda are defined.

The variable 'y' is not defined anywhere, so Python raises a NameError.

🚀 Application

advanced

Using map() to convert string numbers to integers

Given a list of strings representing numbers, which option correctly converts them to integers using map()?

Data Analysis Python

str_numbers = ['10', '20', '30', '40']

A list(map(float, str_numbers))

B list(map(str, str_numbers))

C list(map(lambda x: x + 1, str_numbers))

D list(map(int, str_numbers))

💡 Hint

You want to convert strings to integers, not to strings or floats.

Using int converts each string to an integer. Other options either keep strings or convert to float.

🧠 Conceptual

expert

Behavior of map() with multiple iterables

What is the output of this code using map() with two lists?

Data Analysis Python

a = [1, 2, 3]
b = [4, 5, 6]
result = list(map(lambda x, y: x + y, a, b))
print(result)

A [5, 7, 9]

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

C [4, 10, 18]

D TypeError: <lambda>() missing 1 required positional argument

💡 Hint

When multiple iterables are passed, map applies the function to pairs of elements.

The lambda adds elements from both lists pairwise: 1+4=5, 2+5=7, 3+6=9.

Practice

1. What does the map() function do when applied to a list or a DataFrame column in Python?

easy

A. It applies a function or dictionary to each element, changing them one by one.

B. It sorts the list or column in ascending order.

C. It removes duplicate elements from the list or column.

D. It combines two lists or columns into one.

Solution

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

   The map() function is used to apply a transformation to each element of a list or column.

2. Step 2: Identify what map() changes

   It changes each element individually using a function or dictionary mapping.

3. Final Answer:

   It applies a function or dictionary to each element, changing them one by one. -> Option A

4. Quick Check:

   map() changes elements [OK]

Hint: Remember: map changes each item individually [OK]

Common Mistakes:

• Thinking map sorts data
• Confusing map with removing duplicates
• Assuming map merges lists

2. Which of the following is the correct syntax to use map() with a dictionary to replace values in a list colors = ['red', 'blue', 'green'] with numbers?

easy

A. mapped = list(map(lambda x: {'red':1, 'blue':2}[x], colors))

B. mapped = list(map({'red':1, 'blue':2}, colors))

C. mapped = list(map(colors.map({'red':1, 'blue':2})))

D. mapped = list(map(colors, {'red':1, 'blue':2}))

Solution

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

   The first argument must be a function, the second is the iterable (list).

2. Step 2: Identify which option uses a function properly

   Only mapped = list(map(lambda x: {'red':1, 'blue':2}[x], colors)) uses a lambda function to map dictionary values to list elements correctly.

3. Final Answer:

   mapped = list(map(lambda x: {'red':1, 'blue':2}[x], colors)) -> Option A

4. Quick Check:

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

Hint: map(func, iterable) always has function first [OK]

Common Mistakes:

• Swapping function and iterable arguments
• Trying to pass dictionary directly as function
• Using map() without a function

3. What is the output of this code?

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

medium

A. [1, 4, 9, 16]

B. [1, 2, 3, 4]

C. [0, 2, 4, 6]

D. [2, 4, 6, 8]

Solution

1. Step 1: Understand the lambda function

   The lambda function multiplies each element by 2.

2. Step 2: Apply the function to each element in the list

   1*2=2, 2*2=4, 3*2=6, 4*2=8, so the result is [2, 4, 6, 8].

3. Final Answer:

   [2, 4, 6, 8] -> Option D

4. Quick Check:

   Each number doubled = [2,4,6,8] [OK]

Hint: Multiply each element by 2 mentally [OK]

Common Mistakes:

• Confusing map with filter
• Not applying function to all elements
• Mistaking squares for doubles

4. The code below throws an error. What is the problem?

colors = ['red', 'blue', 'green']
mapping = {'red': 1, 'blue': 2}
result = list(map(mapping, colors))
print(result)

medium

A. The map() function requires three arguments, but only two were given.

B. The list colors contains an invalid value 'green'.

C. You cannot pass a dictionary directly as the function argument in map().

D. The print statement is missing parentheses.

Solution

1. Step 1: Check the map() function arguments

   map() expects a function as the first argument, but a dictionary was passed instead.

2. Step 2: Understand why this causes an error

   A dictionary is not callable, so map() cannot use it as a function.

3. Final Answer:

   You cannot pass a dictionary directly as the function argument in map(). -> Option C

4. Quick Check:

   map() needs a function, not a dict [OK]

Hint: map() first argument must be callable (a function) [OK]

Common Mistakes:

• Thinking dictionary can be used directly as function
• Blaming list contents for error
• Ignoring function argument rules

5. You have a list of fruit names: fruits = ['apple', 'banana', 'cherry', 'date']. You want to replace each fruit with its color using this dictionary: colors = {'apple': 'red', 'banana': 'yellow', 'cherry': 'red'}. Using map(), how can you replace fruits not in the dictionary with the string 'unknown'?

hard

A. Use list(map(colors[x], fruits))

B. Use list(map(lambda x: colors.get(x, 'unknown'), fruits))

C. Use list(map(lambda x: colors[x], fruits))

D. Use list(map(colors.get, fruits))

Solution

1. Step 1: Understand the need for default value

   Since 'date' is not in the dictionary, we need a default value to avoid errors.

2. Step 2: Use dictionary's get() method with default in a lambda

   The lambda function calls colors.get(x, 'unknown') to return the color or 'unknown'.

3. Final Answer:

   list(map(lambda x: colors.get(x, 'unknown'), fruits)) -> Option B

4. Quick Check:

   Use get() with default for missing keys [OK]

Hint: Use dict.get(key, default) inside map lambda [OK]

Common Mistakes:

• Not handling missing keys causing KeyError
• Passing dictionary directly without function
• Using map without default value