diff() for differences in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the diff() function do in pandas?

The diff() function calculates the difference between consecutive rows or elements in a pandas Series or DataFrame.

beginner

How do you calculate the difference between rows in a DataFrame using diff()?

Call diff() on the DataFrame. By default, it subtracts the previous row from the current row for each column.

intermediate

What does the periods parameter in diff() control?

The periods parameter sets how many rows (or elements) to go back when calculating the difference. For example, periods=2 subtracts the value two rows above.

beginner

What happens if the first row has no previous row when using diff()?

The first row will have a NaN value because there is no previous row to subtract from.

beginner

Can diff() be used on a pandas Series? What is the result?

Yes, diff() works on a Series and returns a new Series with the difference between each element and the previous one.

What does df.diff() compute by default?

A Sum of all rows

B Cumulative sum of rows

C Difference between columns

D Difference between each row and the previous row

diff() calculates the difference between each row and the previous row by default.

What value does diff() return for the first row?

A NaN

B 1

C 0

D Same as first row

The first row has no previous row, so diff() returns NaN.

How do you calculate the difference between a row and the row two places above using diff()?

A <code>df.diff(axis=1)</code>

B <code>df.diff(periods=1)</code>

C <code>df.diff(periods=2)</code>

D <code>df.diff(periods=-2)</code>

Use periods=2 to subtract the value two rows above.

Can diff() be applied to a pandas Series?

A No, only DataFrames

B Yes, it returns differences between elements

C Yes, but returns the sum

D No, it causes an error

diff() works on Series and returns the difference between consecutive elements.

What does df.diff(axis=1) do?

A Calculates difference between columns for each row

B Calculates difference between rows for each column

C Calculates cumulative sum

D Returns the original DataFrame

Setting axis=1 calculates difference between columns horizontally for each row.

Explain how the diff() function works in pandas and give an example of when you might use it.

Think about how you find change from one day to the next in a list of numbers.

Describe the role of the periods parameter in diff() and how changing it affects the output.

Imagine looking back more than one step to find differences.