Function `as.Date` Returns NAs Only in Some Rows

In this article, we’ll delve into the world of data manipulation and date formatting using R. We’ll explore why the as.Date function returns NA values for certain rows of a dataset. The issue arises when dealing with dates stored as strings, but not in a format that can be easily parsed by the as.Date function.

Introduction to Dates in R

In R, dates are represented as character vectors or as objects of class Date. The as.Date function is used to convert character vectors into Date objects. This function takes a string argument and attempts to parse it into a date format specified by the second argument (defaulting to %Y-%m-%d) if available.

Understanding NA Values

NA values are a type of missing value in R, indicating that some data point is unknown or cannot be evaluated. When as.Date encounters an invalid date string, it returns NA, which can be misleading and might lead to incorrect conclusions.

The Problem at Hand

The provided code snippet demonstrates a scenario where the as.Date function returns NA values for certain rows of a dataset. The issue arises when dealing with dates stored as strings, but not in a format that can be easily parsed by the as.Date function.

Code Snippet

# CZECH REPO SAZBA
library(rvest)
library(dplyr)

link <- "https://www.cnb.cz/cs/casto-kladene-dotazy/Jak-se-vyvijela-dvoutydenni-repo-sazba-CNB/"
page <- read_html(link)

date <- page %>% html_nodes('td:nth-child(1)') %>% html_text()
repo <- page %>% html_nodes('td+ td') %>% html_text()

Sazba <- data.frame(cbind(date, repo))
Sazba$repo <- as.numeric(gsub(",", ".", Sazba$repo))
Sazba$date <- gsub(" ", "", Sazba$date)
str(Sazba)
Sazba$date <- as.Date(gsub("[.]", "/", Sazba$date), "%d/%m/%Y")

The Issue with Encoding

The problem lies in the encoding of date values. In some cases, these dates might be stored using different character encodings (e.g., UTF-8, ISO-8859-1) that can lead to unexpected behavior when working with as.Date.

Solution: Using Stringi Package

One solution is to use the stringi package to format the date values. The stringi package provides functions for manipulating strings while considering different character encodings.

# CZECH REPO SAZBA
library(rvest)
library(dplyr)
library(stringi)

link <- "https://www.cnb.cz/cs/casto-kladene-dotazy/Jak-se-vyvijela-dvoutydenni-repo-sazba-CNB/"
page <- read_html(link)

date <- page %>% html_nodes('td:nth-child(1)') %>% html_text()
repo <- page %>% html_nodes('td+ td') %>% html_text()

# Use stringi to escape the date values
date <- stringi::stri_escape_unicode(date)

Sazba <- data.frame(cbind(date, repo))
Sazba$repo <- as.numeric(gsub(",", ".", Sazba$repo))

# Remove any non-ASCII characters before formatting
Sazba$date <- gsub("\\\\u00a0", "", Sazba$date)
str(Sazba)

# Format the date values using stringi
Sazba$date <- stringi::stri_date(Sazba$date, "%d.%m.%Y")

Best Practices

When working with dates in R:

Always consider different character encodings and use functions like stringi::stri_escape_unicode to escape them.
Use the strptime function from base R or the stringi::stri_date function for more control over date parsing.
Avoid relying solely on as.Date when working with dates; instead, consider using both the as.Date and strptime functions.

Conclusion

The problem of as.Date returning NA values in some rows can be resolved by addressing different character encodings of date values. Using the stringi package to format these dates provides a reliable solution. By following best practices for date manipulation in R, you can avoid common pitfalls and work efficiently with dates in your data analysis tasks.

Additional Tips

Always inspect your data using str() or head(), depending on the type of data.
Use dplyr for data manipulation and stringi for string operations.
R has a vast collection of libraries, including lubridate and daytime packages that provide additional date-related functions.

By following these tips and staying up-to-date with new developments in the R ecosystem, you’ll become proficient in working efficiently with dates and improve your data analysis skills.

Last modified on 2025-03-23

Function as.Date Returns NAs Only in Some Rows