diff() for differences in Pandas - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of diff() with default parameters

What is the output of the following code snippet using pandas diff() with default settings?

Pandas

import pandas as pd
s = pd.Series([10, 15, 20, 25])
result = s.diff()
print(result)

A

0    NaN
1    10.0
2    15.0
3    20.0
dtype: float64

B

0    NaN
1    5.0
2    5.0
3    5.0
dtype: float64

C

0    10.0
1    5.0
2    5.0
3    5.0
dtype: float64

D

0    NaN
1    15.0
2    20.0
3    25.0
dtype: float64

💡 Hint

Remember diff() calculates the difference between the current and previous element by default.

The diff() function by default calculates the difference between the current value and the previous value (periods=1). The first element has no previous value, so it is NaN.

❓ data_output

intermediate

Effect of periods parameter in diff()

Given the DataFrame below, what is the output of df['A'].diff(periods=2)?

Pandas

import pandas as pd
df = pd.DataFrame({'A': [3, 8, 15, 24, 35]})
result = df['A'].diff(periods=2)
print(result)

A

0     NaN
1     NaN
2    12.0
3    16.0
4    20.0
dtype: float64

B

0     NaN
1     NaN
2    15.0
3    24.0
4    35.0
dtype: float64

C

0     NaN
1     5.0
2     7.0
3     9.0
4    11.0
dtype: float64

D

0     NaN
1     NaN
2     8.0
3    15.0
4    24.0
dtype: float64

💡 Hint

The periods parameter defines how many rows back to subtract from the current row.

diff(periods=2) subtracts the value two rows before from the current row. The first two rows have no value two rows before, so they are NaN.

🔧 Debug

advanced

Identify the error in diff() usage

What error will the following code produce and why?

Pandas

import pandas as pd
s = pd.Series([1, 2, 3])
result = s.diff(periods='two')
print(result)

A No error, outputs the difference with periods=2

B TypeError: unsupported operand type(s) for -: 'int' and 'str'

C SyntaxError: invalid syntax

D ValueError: periods must be an integer

💡 Hint

Check the type of the periods argument.

The periods parameter must be an integer. Passing a string causes a ValueError.

🚀 Application

advanced

Using diff() to find daily sales changes

You have a DataFrame with daily sales data. Which code snippet correctly adds a new column 'daily_change' showing the difference in sales from the previous day?

Pandas

import pandas as pd
df = pd.DataFrame({'date': ['2024-01-01', '2024-01-02', '2024-01-03'], 'sales': [100, 150, 130]})

A df['daily_change'] = df['sales'].diff(axis=1)

B df['daily_change'] = df['sales'].diff(periods=0)

C df['daily_change'] = df['sales'].diff()

D df['daily_change'] = df['sales'].diff(periods=-1)

💡 Hint

Default diff() compares current row with previous row.

diff() with default periods=1 calculates difference from previous row, which is what daily change means.

🧠 Conceptual

expert

Understanding diff() behavior with non-numeric data

What happens when you apply diff() on a pandas Series containing strings?

Pandas

import pandas as pd
s = pd.Series(['a', 'b', 'c'])
result = s.diff()

A Raises TypeError because subtraction is not supported for strings

B Returns the original Series unchanged

C Returns the difference in ASCII values between consecutive characters

D Returns a Series of NaN values

💡 Hint

diff() uses subtraction internally.

diff() tries to subtract consecutive values. Strings cannot be subtracted, so it raises a TypeError.