Why custom functions matter in Pandas - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple function that doubles a number.

Pandas

def double(x):
    return x [1] 2

result = double(5)
print(result)

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' will add 2 instead of doubling.

The function multiplies the input by 2 to double it.

2fill in blank

medium

Complete the code to apply a custom function to a pandas DataFrame column.

Pandas

import pandas as pd

def add_five(n):
    return n + 5

df = pd.DataFrame({'num': [1, 2, 3]})
df['new_num'] = df['num'].[1](add_five)
print(df)

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

Aapplymap

Bapply

Cmap

Dtransform

Attempts:

3 left

💡 Hint

Common Mistakes

Using applymap which works on DataFrames, not Series.

The apply method applies a function to each element in the Series.

3fill in blank

hard

Fix the error in the custom function that should return the square of a number.

Pandas

def square(x):
    return x [1] x

print(square(4))

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

A*

B+

C-

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' will add the number to itself, not square it.

To get the square, multiply the number by itself using '*'.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths only if length is greater than 3.

Pandas

words = ['apple', 'bat', 'car', 'door']
lengths = {word: [1] for word in words if len(word) [2] 3}
print(lengths)

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

Alen(word)

B<

C>

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will select words with length less than 3.

The dictionary maps each word to its length if the length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths if length is greater than 3.

Pandas

words = ['apple', 'bat', 'car', 'door']
result = { [1]: [2] for word in words if len(word) [3] 3 }
print(result)

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

Aword.upper()

Blen(word)

C>

Dword.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.lower() instead of uppercase.

The dictionary comprehension maps each word in uppercase to its length if the length is greater than 3.

Practice

1. Why do we use custom functions when working with pandas DataFrames?

easy

A. To reuse code and make data changes clear and simple

B. To make the DataFrame larger automatically

C. To delete all data quickly

D. To change the pandas version

Solution

1. Step 1: Understand the purpose of custom functions

   Custom functions let us write a piece of code once and use it many times, which saves effort.

2. Step 2: Connect to pandas usage

   In pandas, custom functions help apply the same change or calculation to many rows or columns easily and clearly.

3. Final Answer:

   To reuse code and make data changes clear and simple -> Option A

4. Quick Check:

   Custom functions improve code reuse and clarity [OK]

Hint: Custom functions simplify repeated data changes [OK]

Common Mistakes:

• Thinking custom functions automatically change DataFrame size
• Believing custom functions delete data
• Confusing custom functions with pandas version control

2. Which of the following is the correct way to define a simple custom function in Python to double a number?

easy

A. function double(x) { return x * 2 }

B. def double x: return x * 2

C. def double(x): return x * 2

D. double = x => x * 2

Solution

1. Step 1: Identify Python function syntax

   Python functions start with 'def', followed by the function name and parentheses with parameters.

2. Step 2: Check each option

   def double(x): return x * 2 uses correct Python syntax. function double(x) { return x * 2 } is JavaScript syntax. def double x: return x * 2 misses parentheses. double = x => x * 2 is JavaScript arrow function syntax.

3. Final Answer:

   def double(x): return x * 2 -> Option C

4. Quick Check:

   Python function syntax = def name(params): [OK]

Hint: Python functions start with 'def' and use parentheses [OK]

Common Mistakes:

• Using JavaScript syntax in Python
• Omitting parentheses in function definition
• Confusing arrow functions with Python syntax

3. Given the code below, what will be the output of df['new']?

import pandas as pd
def add_five(x):
    return x + 5
df = pd.DataFrame({'num': [1, 2, 3]})
df['new'] = df['num'].apply(add_five)
print(df['new'])

medium

A. [1, 2, 3]

B. [6, 7, 8]

C. [5, 6, 7]

D. Error: add_five is not defined

Solution

1. Step 1: Understand the function add_five

   The function adds 5 to the input number.

2. Step 2: Apply the function to each element in 'num'

   Applying add_five to [1, 2, 3] results in [6, 7, 8].

3. Final Answer:

   [6, 7, 8] -> Option B

4. Quick Check:

   1+5=6, 2+5=7, 3+5=8 [OK]

Hint: Apply adds function result to each element [OK]

Common Mistakes:

• Thinking apply changes original column without assignment
• Confusing function name or missing definition
• Expecting original values without addition

4. What is wrong with this code snippet?

import pandas as pd
def multiply_by_two(x):
    x * 2
df = pd.DataFrame({'val': [4, 5, 6]})
df['result'] = df['val'].apply(multiply_by_two)
print(df['result'])

medium

A. The DataFrame is not imported correctly

B. The DataFrame is missing a column

C. apply() cannot be used with custom functions

D. The function multiply_by_two does not return a value

Solution

1. Step 1: Check the function multiply_by_two

   The function calculates x * 2 but does not return the result.

2. Step 2: Understand apply behavior

   apply expects the function to return a value to create the new column. Without return, it returns None for each row.

3. Final Answer:

   The function multiply_by_two does not return a value -> Option D

4. Quick Check:

   Functions must return values for apply to work [OK]

Hint: Always return a value in your custom function [OK]

Common Mistakes:

• Forgetting the return statement in functions
• Assuming apply works without returned values
• Thinking DataFrame import affects apply

5. You have a DataFrame with a column 'score' containing numbers. You want to create a new column 'grade' that assigns 'Pass' if the score is 50 or more, and 'Fail' otherwise. Which custom function and pandas code correctly achieve this?

hard

A. def grade_func(x): return 'Pass' if x >= 50 else 'Fail'
df['grade'] = df['score'].apply(grade_func)

B. def grade_func(x): if x >= 50: 'Pass' else: 'Fail'
df['grade'] = df['score'].apply(grade_func)

C. def grade_func(x): return x >= 50 or 'Pass' and 'Fail'
df['grade'] = df['score'].apply(grade_func)

D. def grade_func(x): return 'Pass' if x > 50 else 'Fail'
df['grade'] = df['score'].apply(grade_func)

Solution

1. Step 1: Check the function logic for grading

   def grade_func(x): return 'Pass' if x >= 50 else 'Fail' df['grade'] = df['score'].apply(grade_func) correctly returns 'Pass' if x is 50 or more, else 'Fail'. def grade_func(x): if x >= 50: 'Pass' else: 'Fail' df['grade'] = df['score'].apply(grade_func) misses return statement. def grade_func(x): return x >= 50 or 'Pass' and 'Fail' df['grade'] = df['score'].apply(grade_func) uses incorrect logical operators, resulting in True or 'Fail'. def grade_func(x): return 'Pass' if x > 50 else 'Fail' df['grade'] = df['score'].apply(grade_func) uses > 50, which excludes 50.

2. Step 2: Confirm pandas apply usage

   All options use apply correctly, but only def grade_func(x): return 'Pass' if x >= 50 else 'Fail' df['grade'] = df['score'].apply(grade_func) has correct function logic and syntax.

3. Final Answer:

   def grade_func(x): return 'Pass' if x >= 50 else 'Fail' df['grade'] = df['score'].apply(grade_func) -> Option A

4. Quick Check:

   Use return with correct condition for grading [OK]

Hint: Use return and correct condition in custom function [OK]

Common Mistakes:

• Forgetting return in function
• Using wrong comparison operator
• Confusing logical expressions without return