map() function in Python - Mini Project: Build & Apply

Using the map() function in Python

📖 Scenario: You work at a fruit store and have a list of fruit prices in dollars. You want to quickly calculate the prices in cents to help with a new pricing system.

🎯 Goal: Learn how to use the map() function to apply a calculation to each item in a list.

📋 What You'll Learn

Create a list of fruit prices in dollars

Create a function to convert dollars to cents

Use the map() function with the conversion function and the list

Print the resulting list of prices in cents

💡 Why This Matters

🌍 Real World

Stores and shops often need to convert prices between units like dollars and cents for calculations and display.

💼 Career

Understanding map() helps automate repetitive tasks on lists, which is common in data processing and software development.

1

Create the list of fruit prices in dollars

Create a list called prices_dollars with these exact values: 0.5, 1.2, 0.75, 2.0, 1.5

Python

# Create the list prices_dollars with the given values
# Your code here

Hint

Use square brackets [] to create a list and separate values with commas.

2

Create a function to convert dollars to cents

Define a function called dollars_to_cents that takes one parameter price and returns price * 100

Python

prices_dollars = [0.5, 1.2, 0.75, 2.0, 1.5]
# Define the function dollars_to_cents(price) below
# Your code here

Hint

Use def to define the function and multiply the input by 100.

3

Use map() to convert all prices to cents

Create a variable called prices_cents that uses map() with the function dollars_to_cents and the list prices_dollars. Convert the result to a list.

Python

prices_dollars = [0.5, 1.2, 0.75, 2.0, 1.5]
def dollars_to_cents(price):
    return price * 100
# Use map() to create prices_cents list
# Your code here

Hint

Use list(map(function, list)) to apply the function to each item and get a list back.

4

Print the list of prices in cents

Write a print() statement to display the variable prices_cents

Python

prices_dollars = [0.5, 1.2, 0.75, 2.0, 1.5]
def dollars_to_cents(price):
    return price * 100
prices_cents = list(map(dollars_to_cents, prices_dollars))
# Print the prices_cents list
# Your code here

Hint

Use print(prices_cents) to show the final list.

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