replace() for value substitution in Pandas - Step-by-Step Execution

Concept Flow - replace() for value substitution

Start with DataFrame

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Call replace() with mapping

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Check each cell value

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If value matches key in mapping?

No→Keep original value

Yes↓

Replace with mapped value

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Return new DataFrame with substitutions

The replace() method scans the DataFrame values and substitutes matching values with new ones based on a given mapping.

Execution Sample

Pandas

import pandas as pd

df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 2, 6]})
df_replaced = df.replace({2: 20, 4: 40})
print(df_replaced)

This code replaces all 2s with 20 and all 4s with 40 in the DataFrame.

Execution Table

Step	Cell Checked	Original Value	Match in Mapping?	Action	Resulting Value
1	Row 0, Col A	1	No	Keep original	1
2	Row 0, Col B	4	Yes	Replace with 40	40
3	Row 1, Col A	2	Yes	Replace with 20	20
4	Row 1, Col B	2	Yes	Replace with 20	20
5	Row 2, Col A	3	No	Keep original	3
6	Row 2, Col B	6	No	Keep original	6
7	End	-	-	-	Replacement complete

💡 All cells checked; replacements done where mapping matched.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
df	[[1,4],[2,2],[3,6]]	[[1,40],[2,2],[3,6]]	[[1,40],[20,20],[3,6]]	[[1,40],[20,20],[3,6]]	[[1,40],[20,20],[3,6]]
df_replaced	Empty	[[1,40],[2,2],[3,6]]	[[1,40],[20,20],[3,6]]	[[1,40],[20,20],[3,6]]	[[1,40],[20,20],[3,6]]

Key Moments - 2 Insights

Why does replace() only change some values and not others?

Because replace() only substitutes values that exactly match keys in the mapping, as shown in execution_table rows 2, 3, and 4 where only 2 and 4 are replaced.

Does replace() change the original DataFrame?

No, replace() returns a new DataFrame with substitutions, leaving the original unchanged, as seen in variable_tracker where 'df' stays the same while 'df_replaced' changes.

Visual Quiz - 3 Questions

Look at the execution_table, what is the resulting value at Row 1, Column B after replacement?

A 2

B 20

C 40

D 6

💡 Hint

Check execution_table row 4 for Row 1, Col B value after replacement.

At which step does the first replacement happen according to the execution_table?

A Step 3

B Step 1

C Step 2

D Step 5

💡 Hint

Look at the 'Match in Mapping?' column to find the first 'Yes'.

If we add {3: 30} to the mapping, how would the final value at Row 2, Column A change?

A It becomes 30

B It becomes 20

C It stays 3

D It becomes 40

💡 Hint

Refer to variable_tracker for current value and think about how adding 3:30 affects replacements.

Concept Snapshot

replace(mapping) substitutes values in DataFrame
Only exact matches in mapping keys are replaced
Returns a new DataFrame, original stays unchanged
Mapping is a dict: {old_value: new_value}
Works element-wise across all columns

Full Transcript

The replace() method in pandas scans each cell in a DataFrame and replaces values that match keys in a given mapping dictionary with their corresponding new values. It returns a new DataFrame with these substitutions, leaving the original DataFrame unchanged. For example, if the mapping is {2: 20, 4: 40}, all 2s become 20 and all 4s become 40. The process checks each cell one by one, replacing only when a match is found. This method is useful for cleaning or updating data values quickly and safely.