pct_change() for percentage change in Pandas - Step-by-Step Execution

Concept Flow - pct_change() for percentage change

Start with DataFrame

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Select column or series

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Calculate difference between current and previous value

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Divide difference by previous value

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Return new series with percentage changes

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Handle NaN for first element

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Output percentage change series

pct_change() calculates the percent difference between each value and the previous one in a column or series, returning a new series with these percentage changes.

Execution Sample

Pandas

import pandas as pd
s = pd.Series([100, 120, 90, 150])
result = s.pct_change()
print(result)

This code calculates the percentage change between each number and the previous number in the series.

Execution Table

Step	Current Value	Previous Value	Difference	Percentage Change	Output Value
1	100	NaN	NaN	NaN	NaN
2	120	100	20	0.20	0.20
3	90	120	-30	-0.25	-0.25
4	150	90	60	0.6667	0.6667
5	End	-	-	-	Calculation complete

💡 Reached end of series; no more values to compare.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
s	[100, 120, 90, 150]	[100, 120, 90, 150]	[100, 120, 90, 150]	[100, 120, 90, 150]	[100, 120, 90, 150]
result	[NaN, NaN, NaN, NaN]	[NaN, 0.20, NaN, NaN]	[NaN, 0.20, -0.25, NaN]	[NaN, 0.20, -0.25, 0.6667]	[NaN, 0.20, -0.25, 0.6667]

Key Moments - 3 Insights

Why is the first output value NaN?

The first value has no previous value to compare to, so pct_change() returns NaN for it, as shown in step 1 of the execution_table.

How is the percentage change calculated for step 3?

At step 3, difference is 90 - 120 = -30. Percentage change is -30 divided by 120 = -0.25, as shown in the execution_table.

What does a negative percentage change mean?

A negative percentage change means the current value is less than the previous value, indicating a decrease, as seen at step 3 with -0.25.

Visual Quiz - 3 Questions

Look at the execution_table, what is the percentage change at step 4?

A -0.25

B 0.25

C 0.6667

D NaN

💡 Hint

Check the 'Percentage Change' column at step 4 in the execution_table.

At which step does the percentage change become negative?

A Step 2

B Step 3

C Step 4

D Never

💡 Hint

Look for the first negative value in the 'Percentage Change' column in the execution_table.

If the first value in the series was 0 instead of 100, what would be the percentage change at step 2?

A Infinite or error

B NaN

C 0.20

D -0.20

💡 Hint

Recall that percentage change divides by the previous value; dividing by zero causes an error or infinite result.

Concept Snapshot

pct_change() calculates the percent difference between each value and the previous one in a series or DataFrame column.
Syntax: series.pct_change()
Returns a new series with NaN for the first element (no previous value).
Useful to see relative changes over time or between rows.
Handles positive and negative changes as decimals (e.g., 0.20 = 20%).

Full Transcript

The pct_change() function in pandas calculates the percentage change between each value and the previous value in a series or DataFrame column. It starts by taking the first value, which has no previous value, so it returns NaN for that position. Then for each next value, it subtracts the previous value from the current value, divides the difference by the previous value, and returns this as the percentage change. For example, if the current value is 120 and the previous is 100, the difference is 20, and the percentage change is 20 divided by 100, which is 0.20 or 20%. Negative values mean the current value is less than the previous, showing a decrease. This function helps to understand how data changes step-by-step in relative terms.