Multiplication in R of Specific Portion of a Dataframe

Introduction

In this article, we will explore how to perform multiplication on specific values within a dataframe in R. We will use the dplyr library for data manipulation and lubridate for date functions. The problem involves changing the units (multiplying values by 0.305) of some values in the Date column from 1967 to 1973 while leaving the rest of the values as they are.

Understanding the Data

The provided sample dataframe contains two columns, Date and A01. The Date column represents dates between 1966 and 2002, and the A01 column contains corresponding values. We want to modify the A01 values based on the year in the Date column.

Data Preprocessing

The first step is to convert the Date column to a date class using lubridate’s ymd() function. This allows us to extract specific information from the dates, such as the year.

library(dplyr)
library(lubridate)

df %&gt;%
  mutate(Date = ymd(Date))

Extracting the Year

Next, we need to extract the year from the Date column. We can use lubridate’s year() function for this purpose.

df %&gt;%
  mutate(year = year(Date))

Defining Conditions for Multiplication

We want to multiply the A01 values by 0.305 if the corresponding year is between 1967 and 1974 (inclusive). We can use R’s built-in ifelse() function or dplyr’s case_when() function to achieve this.

df %&gt;%
  mutate(
    A01 = case_when(
      year == 1967 & between(year, 1974) ~ A01 * 0.305,
      TRUE ~ A01
    )
  )

This condition checks if the year is equal to 1967 and also falls within the specified range (inclusive). If both conditions are met, it multiplies the A01 value by 0.305; otherwise, it leaves the value unchanged.

Applying the Condition Using dplyr’s between Function

Alternatively, we can use dplyr’s between() function to simplify the condition.

library(dplyr)

df %&gt;%
  mutate(
    A01 = A01 * c(1, 0.305)[between(year(Date), 1967, 1974)]
  )

In this version, we use the between() function to check if the year falls within the specified range (inclusive). If it does, it multiplies the A01 value by 0.305; otherwise, it leaves the value unchanged.

Understanding the Code

Let’s break down the provided R code example:

library(dplyr)
library(lubridate)

df %&gt;%
  mutate(Date = ymd(Date), 
         A01 = A01 * c(1, 0.305)[(between(year(Date), 1967, 1974)) + 1])

In this code:

We first convert the Date column to a date class using lubridate’s ymd() function.
Then, we use dplyr’s mutate() function to add a new column called A01.
Within the A01 assignment, we create a character vector c(1, 0.305) with two elements: 1 and 0.305.
We then use the between() function from dplyr to check if the year falls within the specified range (inclusive). If it does, it assigns the value at index [1] of the vector (which is 1); otherwise, it assigns the value at index [2] (which is 0.305).
The resulting vector is then multiplied by A01, effectively multiplying the values in Date with 0.305 if they fall within the specified range.

Conclusion

In this article, we explored how to modify specific values within a dataframe in R based on conditions applied to another column. We used dplyr’s data manipulation functions and lubridate for date functions to achieve this goal. By understanding the code example provided and following the steps outlined in the article, you can perform similar operations on your own datasets.

Last modified on 2025-02-21