Melt for wide-to-long reshaping in Data Analysis Python

Introduction

Melt helps change data from wide format to long format. This makes it easier to analyze and plot data.

You have survey data with one row per person and many columns for answers, and want one row per answer.

You want to combine multiple columns of monthly sales into one column with month names and sales values.

You need to prepare data for a chart that requires long format, like a line plot over time.

You want to stack several related columns into two columns: one for variable names and one for values.

Syntax

Data Analysis Python

pd.melt(frame, id_vars=None, value_vars=None, var_name=None, value_name='value')

frame is your original DataFrame.

id_vars are columns to keep as identifiers (not melted).

Examples

Melt all columns into two columns: variable and value.

Data Analysis Python

pd.melt(df)

Keep 'Name' column fixed, melt the rest.

Data Analysis Python

pd.melt(df, id_vars=['Name'])

Melt only 'Math' and 'Science' columns, rename columns to 'Subject' and 'Score'.

Data Analysis Python

pd.melt(df, id_vars=['Name'], value_vars=['Math', 'Science'], var_name='Subject', value_name='Score')

Sample Program

This code creates a small table with student names and scores in Math and Science. Then it melts the scores into a long format with columns 'Name', 'Subject', and 'Score'.

Data Analysis Python

import pandas as pd

data = {
    'Name': ['Alice', 'Bob'],
    'Math': [85, 90],
    'Science': [92, 88]
}
df = pd.DataFrame(data)

melted = pd.melt(df, id_vars=['Name'], value_vars=['Math', 'Science'], var_name='Subject', value_name='Score')
print(melted)

Important Notes

If you don't specify id_vars, all columns are melted into variable and value columns.

Melted data is useful for plotting libraries like seaborn that expect long format.

Summary

Melt changes wide data to long data by stacking columns.

Use id_vars to keep columns fixed.

Rename variable and value columns with var_name and value_name.

Practice

1. What does the melt function do in data analysis?

easy

A. It merges two dataframes horizontally.

B. It converts wide data format to long data format by stacking columns.

C. It filters rows based on a condition.

D. It sorts data rows based on column values.

Solution

1. Step 1: Understand data formats

   Wide data has many columns representing variables; long data stacks these variables into fewer columns.

2. Step 2: Role of melt function

   The melt function stacks columns into rows, converting wide data to long format.

3. Final Answer:

   It converts wide data format to long data format by stacking columns. -> Option B

4. Quick Check:

   melt = wide to long [OK]

Hint: Melt stacks columns into rows to reshape wide data [OK]

Common Mistakes:

• Confusing melt with sorting or filtering
• Thinking melt merges dataframes
• Assuming melt changes row order only

2. Which of the following is the correct syntax to melt a dataframe df keeping the column 'ID' fixed?

easy

A. pd.melt(df, id_vars='ID')

B. pd.melt(df, fixed='ID')

C. df.melt(id='ID')

D. pd.melt(df, keep='ID')

Solution

1. Step 1: Identify correct parameter for fixed columns

   The parameter to keep columns fixed during melt is id_vars.

2. Step 2: Check syntax correctness

   Only pd.melt(df, id_vars='ID') uses the correct parameter and function call.

3. Final Answer:

   pd.melt(df, id_vars='ID') -> Option A

4. Quick Check:

   Use id_vars to keep columns fixed [OK]

Hint: Use id_vars to keep columns fixed in melt [OK]

Common Mistakes:

• Using wrong parameter names like fixed or keep
• Calling melt as a dataframe method without pd
• Passing id instead of id_vars

3. Given the dataframe:

import pandas as pd
df = pd.DataFrame({
  'ID': [1, 2],
  'Math': [90, 80],
  'Science': [85, 95]
})

result = pd.melt(df, id_vars='ID', var_name='Subject', value_name='Score')
print(result)

What is the output?

medium

A. A dataframe with columns ID, Subject but no Score column.

B. A dataframe with columns ID, Math, Science unchanged.

C. A dataframe with columns ID, Subject, Score and 4 rows stacking Math and Science scores.

D. An error because var_name and value_name are invalid parameters.

Solution

1. Step 1: Understand melt with id_vars and renaming

   Keeping 'ID' fixed, melt stacks 'Math' and 'Science' into one column named 'Subject' and their values into 'Score'.

2. Step 2: Predict output rows and columns

   There are 2 rows originally and 2 subject columns, so output has 4 rows with columns 'ID', 'Subject', 'Score'.

3. Final Answer:

   A dataframe with columns ID, Subject, Score and 4 rows stacking Math and Science scores. -> Option C

4. Quick Check:

   melt stacks columns into rows with renamed columns [OK]

Hint: Melt stacks columns into rows, increasing row count [OK]

Common Mistakes:

• Expecting original wide format output
• Thinking var_name/value_name cause errors
• Ignoring increase in row count after melt

4. What is wrong with this code snippet?

import pandas as pd
df = pd.DataFrame({'ID':[1,2],'A':[10,20],'B':[30,40]})
result = pd.melt(df, id='ID', var_name='Variable', value_name='Value')
print(result)

medium

A. The parameter 'id' should be 'id_vars' in pd.melt.

B. The dataframe is missing columns to melt.

C. The melt function cannot rename columns.

D. The print statement is incorrect.

Solution

1. Step 1: Check parameter names in melt

   The correct parameter to specify fixed columns is id_vars, not id.

2. Step 2: Identify error cause

   Using id='ID' causes a TypeError because melt does not recognize 'id'.

3. Final Answer:

   The parameter 'id' should be 'id_vars' in pd.melt. -> Option A

4. Quick Check:

   Use id_vars, not id, to fix columns [OK]

Hint: Use id_vars, not id, to keep columns fixed in melt [OK]

Common Mistakes:

• Using 'id' instead of 'id_vars'
• Thinking melt cannot rename columns
• Assuming print causes error

5. You have a dataframe:

import pandas as pd
df = pd.DataFrame({
  'Student': ['Alice', 'Bob'],
  'Math_2023': [88, 92],
  'Science_2023': [95, 85],
  'Math_2024': [90, 94],
  'Science_2024': [93, 89]
})

You want to reshape it to long format with columns: Student, Year, Subject, and Score. Which approach is correct?

hard

A. Use melt with var_name=['Subject', 'Year'] to split automatically.

B. Use melt with id_vars=['Student', 'Year'] directly.

C. Use melt without id_vars and rename columns to 'Year' and 'Subject'.

D. Use melt with id_vars=['Student'], then split the variable column into 'Subject' and 'Year'.

Solution

1. Step 1: Melt keeping 'Student' fixed

   Use id_vars=['Student'] to keep student names fixed and stack the year-subject columns.

2. Step 2: Split the combined variable column

   The variable column contains strings like 'Math_2023'; split this into 'Subject' and 'Year' columns manually after melt.

3. Final Answer:

   Use melt with id_vars=['Student'], then split the variable column into 'Subject' and 'Year'. -> Option D

4. Quick Check:

   Melt then split combined columns for multi-level info [OK]

Hint: Melt first, then split combined variable names into parts [OK]

Common Mistakes:

• Trying to split variable names automatically in melt
• Using id_vars with non-existing columns
• Not splitting combined variable names after melt