Assigning Seasons to Dates in R

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In this article, we will explore how to assign seasons to dates in R using various methods. We will use the lubridate package, which provides a convenient way to work with dates and times.

Introduction

Many of us are familiar with the changing of seasons, but have you ever wondered how to assign these seasons to specific dates? In this article, we will delve into the world of date manipulation in R and explore different methods for assigning seasons to dates.

Using the yday Function

The yday function is a convenient way to extract the day of the year from a date. However, as mentioned in the original post, using this function can lead to erroneous results for leap years.

# Load the lubridate package
library(lubridate)

# Create a sample date vector
dates <- seq(as.Date('2017-01-01'), as.Date('2019-12-31'), by = 'days')

# Extract the day of the year from each date using yday
ydays <- yday(dates)

Creating a Vector of Seasonal Dates

A safer approach to assigning seasons to dates is to create a vector of each season’s dates across multiple years.

# Create a vector of seasonal dates
season_dates <- as.Date(sort(c(outer(
  do.call(seq.int, as.list(1900 + as.POSIXlt(range(dates) + c(-365, 365))$year)), 
                         c("-03-20", "-06-21", "-09-23", "-12-21"),
  paste0))))

In this code snippet, we first extract the range of dates and add a year to each side using as.POSIXlt. We then create a vector of seasonal dates by repeating these dates across multiple years.

Assigning Seasons Using findInterval

Once we have our seasonal dates vector, we can use the findInterval function to assign seasons to individual dates.

# Create a vector of season names
season_names <- rep(c("Spring", "Summer", "Autumn", "Winter"), length.out = length(season_dates))

# Assign seasons using findInterval
df$SEASON <- season_names[findInterval(dates, season_dates)]

Handling Leap Years

As mentioned earlier, using the yday function can lead to erroneous results for leap years. To avoid this issue, we create a vector of seasonal dates by repeating each season’s dates across multiple years.

Case-Based Assignment

The original post provided an example of case-based assignment using case_when. Here is how it works:

# Create a sample date vector
dates <- seq(as.Date('2017-01-01'), as.Date('2019-12-31'), by = 'days')

# Extract the day of the month and year from each date
months <- month(dates)
days <- mday(dates)

# Calculate DAY_PLUS_MONTH
df$DAY_PLUS_MONTH <- df$DAY + df$MONTH * 30

# Assign seasons using case_when
df$SEASON <- case_when(
  DAY_PLUS_MONTH >= 91 && DAY_PLUS_MONTH <= 120 ->
    "Summer",
  DAY_PLUS_MONTH >= 61 && DAY_PLUS_MONTH < 91 ->
    "Autumn",
  DAY_PLUS_MONTH >= 1 && DAY_PLUS_MONTH < 31 ->
    "Winter",
  TRUE ->
    "Spring"
)

Conclusion

Assigning seasons to dates is a useful skill in R, and there are several methods to achieve this. By using vectors of seasonal dates and findInterval, we can accurately assign seasons to individual dates.

# Load the lubridate package
library(lubridate)

# Create a sample date vector
dates <- seq(as.Date('2017-01-01'), as.Date('2019-12-31'), by = 'days')

# Assign seasons using findInterval
df$SEASON <- season_names[findInterval(dates, season_dates)]

# Print the resulting DataFrame
print(df)

Note: The seasons variable is assumed to be a vector of seasonal names.

Last modified on 2023-07-30