Creating a DataFrame with Day-by-Day Columns Using Pandas

Introduction

In this article, we will explore how to create a new DataFrame with day-by-day columns from an existing DataFrame. This can be useful in various scenarios where you need to track changes or cumulative values over time.

We will use the pandas library in Python, which is widely used for data manipulation and analysis.

Background

The problem statement provides us with a DataFrame containing information about items, their start dates, due dates, and values. We want to create a new DataFrame where each row represents a day from the start date to the due date, and the value of each day is the cumulative sum of the original value.

For example, if we have an item with a start date of January 1st, 2020, and a due date of February 29th, 2020, we want to create a new DataFrame where each row represents a day from January 1st, 2020, to February 29th, 2020. The value of each day is the cumulative sum of the original value.

Approach

Our approach involves several steps:

1. Create a date range: We will use the pd.date_range function to create a date range from the start date to the due date.
2. Explode the date range: We will use the explode method to create a new row for each day in the date range.
3. Group by item and day: We will group the resulting DataFrame by item and day using the groupby method.
4. Calculate cumulative sum: We will calculate the cumulative sum of the values using the sum method.

Code

import pandas as pd

# Create a sample DataFrame
data = {
    'Item_name': ['Item 1', 'Item 2'],
    'Start_date': ['2020-01-01', '2020-02-01'],
    'Due_date': ['2020-01-31', '2020-03-01']
}
df = pd.DataFrame(data)

# Convert date columns to datetime format
df['Start_date'] = pd.to_datetime(df['Start_date'])
df['Due_date'] = pd.to_datetime(df['Due_date'])

# Create a date range from start date to due date
date_range = [pd.date_range(s, d, freq='D') for s, d in zip(df.Start_date, df.Due_date)]

# Explode the date range into separate rows
df_exploded = df.set_index(['Item_name', 'Value']).assign(date_range=date_range).explode('date_range')

# Reset index to create a new DataFrame with item and day as columns
df_reset = df_exploded.reset_index()

# Group by item and day, calculate cumulative sum of values
result_df = (df_reset.groupby(['Item_name', 'date_range'])['Value']
             .sum()
             .unstack())

print(result_df)

Explanation

Let’s break down the code step by step:

1. We create a sample DataFrame with Item_name, Start_date, and Due_date columns.
2. We convert the date columns to datetime format using pd.to_datetime.
3. We create a date range from the start date to the due date using pd.date_range. The freq='D' parameter specifies that we want a daily frequency.
4. We explode the date range into separate rows using the explode method.
5. We reset the index of the DataFrame to create a new DataFrame with Item_name and date_range as columns.
6. We group the resulting DataFrame by Item_name and date_range, calculate the cumulative sum of values using the sum method, and unstack the result.

Example Use Cases

This technique can be applied to various scenarios where you need to track changes or cumulative values over time. Some examples include:

• Stock market analysis: You can create a DataFrame with stock prices for different dates and calculate the daily return on investment (ROI) by exploding the date range.
• Customer behavior analysis: You can create a DataFrame with customer data, including purchase history, and calculate the cumulative sum of sales over time using this technique.
• Financial forecasting: You can use this technique to forecast future values based on historical data.

Conclusion

In conclusion, we have demonstrated how to create a new DataFrame with day-by-day columns from an existing DataFrame using pandas in Python. This technique can be applied to various scenarios where you need to track changes or cumulative values over time.

Last modified on 2025-05-02