Exporting to CSV in Data Analysis Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this CSV export code?

Consider the following Python code that exports a DataFrame to a CSV file. What will be the content of the CSV file?

Data Analysis Python

import pandas as pd

data = {'Name': ['Alice', 'Bob'], 'Age': [25, 30]}
df = pd.DataFrame(data)
df.to_csv('output.csv', index=False)

A "Name,Age\nAlice,25\nBob,30\n"

B "0,Name,Age\n0,Alice,25\n1,Bob,30\n"

C "Name,Age\n0,Alice\n1,Bob\n"

D "Name,Age\nAlice,25\nBob,30"

💡 Hint

Check the effect of index=False in to_csv method.

The parameter index=False tells pandas not to write row numbers to the CSV. So the CSV contains only the column headers and the data rows.

❓ data_output

intermediate

How many rows will the exported CSV contain?

Given this DataFrame export code, how many data rows (excluding header) will the CSV file have?

Data Analysis Python

import pandas as pd

data = {'City': ['NY', 'LA', 'Chicago'], 'Population': [8000000, 4000000, 2700000]}
df = pd.DataFrame(data)
df.to_csv('cities.csv', index=False)

A 1

B 4

C 2

D 3

💡 Hint

Count the number of rows in the DataFrame.

The DataFrame has 3 rows, so the CSV will have 3 data rows plus 1 header row.

🔧 Debug

advanced

Why does this CSV export include row numbers?

This code exports a DataFrame to CSV but the CSV includes row numbers as the first column. Why?

Data Analysis Python

import pandas as pd

data = {'Product': ['Pen', 'Pencil'], 'Price': [1.5, 0.5]}
df = pd.DataFrame(data)
df.to_csv('products.csv')

A Because index=True by default, so row numbers are included.

B Because the DataFrame has a custom index that is exported.

C Because the to_csv method requires index=False to include row numbers.

D Because the CSV format always includes row numbers.

💡 Hint

Check the default value of the index parameter in to_csv.

By default, to_csv includes the DataFrame index as the first column. To exclude it, you must set index=False.

🚀 Application

advanced

How to export only specific columns to CSV?

You have a DataFrame with columns 'A', 'B', 'C'. You want to export only columns 'A' and 'C' to a CSV file. Which code does this correctly?

Data Analysis Python

import pandas as pd

data = {'A': [1,2], 'B': [3,4], 'C': [5,6]}
df = pd.DataFrame(data)

A df.to_csv('subset.csv', columns=['A', 'C'], index=False)

B df[['A', 'C']].to_csv('subset.csv', index=False)

C df.to_csv('subset.csv', index=False).filter(['A', 'C'])

D df.to_csv('subset.csv', index=False).loc[:, ['A', 'C']]

💡 Hint

Check the columns parameter in to_csv method.

The to_csv method accepts a columns parameter to select columns to export. Option A uses this correctly. Option B also works by selecting columns before exporting.

🧠 Conceptual

expert

What happens if you export a DataFrame with missing values to CSV?

You export a DataFrame containing some missing values (NaN) to CSV using to_csv with default settings. How are missing values represented in the CSV?

A Missing values are replaced by zeros in the CSV.

B Missing values are saved as empty fields in the CSV.

C Missing values cause an error during export.

D Missing values are saved as the string 'NaN' in the CSV.

💡 Hint

Check the default na_rep parameter in to_csv.

By default, missing values are written as empty fields in the CSV file.