Data Manipulation in R: Stacking Rows Based on Count

In this article, we will explore a common data manipulation problem in R. The task is to stack rows from one dataframe based on the count value in another dataframe. We’ll break down the solution step-by-step and discuss the underlying concepts.

Introduction

When working with data, it’s not uncommon to encounter scenarios where you need to manipulate or transform your data in some way. In this case, we’re dealing with two dataframes: df1 and df2. The goal is to take rows from df2 that correspond to the count value in df1 and stack them together into a new dataframe.

Background

To understand the problem, let’s first discuss what each of our dataframes looks like:

df1

df2

We want to take the rows from df2 that correspond to the value ‘A’ in df1, and then repeat those rows three times (since the count is 3). We do the same for the value ‘B’, but only repeat it twice.

Solution

The R solution involves several steps:

Step 1: Find the index of the ‘Value’ column in df2

To get the sequence of values from df2 based on the count value in df1, we need to find the index of the ‘Value’ column in df2. This can be done using the match() function, which returns the indices of all occurrences of a vector in another vector.

# Find the index of the 'Value' column in df2
index <- match(df1$Value, df2$Value)

Step 2: Get the sequence of values from df2 based on count

Using the match() function, we can get the indices of all occurrences of df1$Value in df2$Value. Then, we add a sequence of numbers (from 0 to df1$Count - 1) to these indices. This will give us the sequence of values from df2 that correspond to each count value in df1.

# Get the sequence of values from df2 based on count
sequence <- seq(0, df1$Count - 1)

# Repeat this step for all elements in index
repeat_sequence <- rep(sequence, df1$Count)

Step 3: Stack the list output into a two column dataframe

We can use the stack() function to stack our repeated sequence into a single column. We then convert this into a data.frame using setNames().

# Stack the sequence of values from df2
out <- stack(setNames(Map(function(x, y) df2$Value[match(x, df2$Value) + 
                            repeat_sequence], df1$Value, df1$Count), df1$Item))

# Get the SubItem for each repeated element
out$SubItem <- rep(df1$SubItem,  df1$Count)

Output

After executing this code, we get:

Conclusion

In this article, we’ve explored a common data manipulation problem in R. By using the match() function to find the index of the ‘Value’ column in df2 and then getting the sequence of values from df2 based on count, we can stack rows together into a new dataframe. We hope that this explanation has helped clarify the steps involved in solving this type of problem.

Last modified on 2023-05-10