Counting missing values in Pandas - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Counting missing values in a DataFrame column

What is the output of this code that counts missing values in the 'Age' column?

Pandas

import pandas as pd

data = {'Name': ['Alice', 'Bob', 'Charlie', 'David'],
        'Age': [25, None, 30, None]}
df = pd.DataFrame(data)

missing_count = df['Age'].isna().sum()
print(missing_count)

A 2

B 0

C 4

D None

💡 Hint

Use isna() to find missing values and sum() to count them.

The 'Age' column has two missing values (None). isna() marks them as True, sum() counts True as 1 each, so the result is 2.

❓ data_output

intermediate

Count missing values per column in a DataFrame

What is the output of this code that counts missing values for each column?

Pandas

import pandas as pd

data = {'A': [1, None, 3], 'B': [None, None, 6], 'C': [7, 8, 9]}
df = pd.DataFrame(data)

missing_per_column = df.isna().sum()
print(missing_per_column)

A

A    2
B    1
C    0
dtype: int64

B

A    0
B    0
C    0
dtype: int64

C

A    1
B    2
C    0
dtype: int64

D

A    1
B    1
C    1
dtype: int64

💡 Hint

Use isna() on the whole DataFrame and sum() to count per column.

Column 'A' has 1 missing, 'B' has 2 missing, 'C' has none. So the output shows counts per column.

❓ visualization

advanced

Visualizing missing values count per column

Which option produces a bar chart showing the count of missing values per column?

Pandas

import pandas as pd
import matplotlib.pyplot as plt

data = {'X': [1, None, 3, None], 'Y': [None, 2, None, 4], 'Z': [5, 6, 7, 8]}
df = pd.DataFrame(data)

missing_counts = df.isna().sum()

# Which code below plots the bar chart correctly?

A

df.plot(kind='bar')
plt.show()

B

missing_counts.plot(kind='bar')
plt.show()

C

missing_counts.plot.bar()
plt.show()

D

df.isna().plot(kind='bar')
plt.show()

💡 Hint

Plot the Series of missing counts, not the whole DataFrame.

missing_counts is a Series with counts per column. Using plot(kind='bar') on it creates the correct bar chart.

🧠 Conceptual

advanced

Understanding the difference between isna() and notna()

If df is a DataFrame, what does df.notna().sum() return?

A Boolean DataFrame showing missing values

B Total number of rows in the DataFrame

C Count of missing values per column

D Count of non-missing values per column

💡 Hint

notna() is the opposite of isna().

notna() returns True for non-missing values. sum() counts True as 1, so it counts non-missing values per column.

🔧 Debug

expert

Why does this code give an error when counting missing values?

What error does this code raise and why? import pandas as pd data = {'A': [1, 2, None], 'B': [None, 4, 5]} df = pd.DataFrame(data) missing = df.isna.sum() print(missing)

A AttributeError: 'method' object has no attribute 'sum' because isna is missing parentheses

B TypeError: unsupported operand type(s) for +: 'method' and 'method' because sum is called incorrectly

C KeyError because 'isna' is not a column

D No error, prints the count of missing values

💡 Hint

Check if isna is called as a method.

isna is a method and must be called with parentheses: isna(). Without parentheses, isna is a method object, which has no sum attribute.

Practice

1. What does the pandas method df.isna().sum() do when applied to a DataFrame df?

easy

A. Counts the total number of rows in the DataFrame

B. Counts the number of missing values in each column of the DataFrame

C. Replaces missing values with zero in the DataFrame

D. Returns the data types of each column in the DataFrame

Solution

1. Step 1: Understand df.isna()

   This method returns a DataFrame of the same shape as df with True where values are missing and False otherwise.

2. Step 2: Apply .sum() on the boolean DataFrame

   Summing True values (which count as 1) per column gives the count of missing values in each column.

3. Final Answer:

   Counts the number of missing values in each column of the DataFrame -> Option B

4. Quick Check:

   df.isna().sum() counts missing per column [OK]

Hint: Use isna() then sum() to count missing per column [OK]

Common Mistakes:

• Thinking sum() counts rows instead of missing values
• Confusing isna() with fillna()
• Assuming it replaces missing values

2. Which of the following is the correct syntax to count all missing values in a pandas DataFrame df?

easy

A. df.isna().count()

B. df.count().sum()

C. df.isna().sum().sum()

D. df.isnull().count()

Solution

1. Step 1: Use df.isna() or df.isnull() to find missing values

   Both return a boolean DataFrame marking missing values as True.

2. Step 2: Use .sum().sum() to count all missing values

   The first sum() sums missing per column, the second sums those counts to get total missing in the DataFrame.

3. Final Answer:

   df.isna().sum().sum() -> Option C

4. Quick Check:

   Total missing = df.isna().sum().sum() [OK]

Hint: Double sum() after isna() counts all missing values [OK]

Common Mistakes:

• Using count() which counts non-missing values
• Using isna().count() which counts all rows, not missing
• Confusing isna() with isnull().count()

3. Given the DataFrame df below:

import pandas as pd
data = {'A': [1, None, 3], 'B': [None, None, 6], 'C': [7, 8, 9]}
df = pd.DataFrame(data)

What is the output of df.isna().sum()?

medium

A. A 1
B 1
C 1
dtype: int64

B. A 0
B 0
C 0
dtype: int64

C. A 2
B 1
C 0
dtype: int64

D. A 1
B 2
C 0
dtype: int64

Solution

1. Step 1: Identify missing values in each column

   Column A has one None (missing), column B has two None values, column C has none.

2. Step 2: Apply df.isna().sum()

   This counts missing per column: A=1, B=2, C=0.

3. Final Answer:

   A 1 B 2 C 0 dtype: int64 -> Option D

4. Quick Check:

   Count missing per column matches output [OK]

Hint: Count None or NaN per column to verify sums [OK]

Common Mistakes:

• Mixing counts of missing and non-missing
• Miscounting missing in column B
• Assuming None counts as zero

4. You run the code below but get an error:

import pandas as pd
data = {'X': [1, 2, None], 'Y': [None, 5, 6]}
df = pd.DataFrame(data)
missing = df.isna.sum()

What is the error and how to fix it?

medium

A. Missing parentheses after isna; fix with df.isna().sum()

B. DataFrame missing columns; add columns before counting

C. sum() is not a pandas method; use count() instead

D. None values cause error; replace with zeros first

Solution

1. Step 1: Identify the error in method call

   The code uses df.isna.sum() missing parentheses after isna, which is a method.

2. Step 2: Correct the syntax

   Use df.isna().sum() to call the method and then sum missing values.

3. Final Answer:

   Missing parentheses after isna; fix with df.isna().sum() -> Option A

4. Quick Check:

   Method calls need parentheses [OK]

Hint: Remember to add () after isna to call the method [OK]

Common Mistakes:

• Forgetting parentheses on isna()
• Using sum() instead of count() incorrectly
• Thinking None causes errors in counting

5. You have a DataFrame df with columns ['age', 'income', 'score']. You want to find the percentage of missing values in each column. Which code snippet correctly calculates this?

hard

A. df.isna().sum() / len(df) * 100

B. df.isna().count() / len(df) * 100

C. df.count() / df.isna().sum() * 100

D. df.isna().sum() * 100

Solution

1. Step 1: Count missing values per column

   Use df.isna().sum() to get missing counts per column.

2. Step 2: Calculate percentage of missing values

   Divide missing counts by total rows len(df) and multiply by 100 to get percentage.

3. Final Answer:

   df.isna().sum() / len(df) * 100 -> Option A

4. Quick Check:

   Missing % = (missing count / total rows) * 100 [OK]

Hint: Divide missing count by total rows, multiply by 100 [OK]

Common Mistakes:

• Using count() which counts non-missing values
• Multiplying missing count by 100 without dividing
• Dividing count by missing count incorrectly