Counting Dots in Character Strings with str_count and Beyond

Introduction

When working with character strings in R, it’s common to encounter various patterns or characters that you need to count or analyze. In this article, we’ll explore how to count the number of dots (.) in a character string using str_count, as well as other methods and alternatives.

Background

The str_count function is a part of the base R package, which provides various functions for working with strings. However, it has some limitations when it comes to counting specific characters. In this article, we’ll delve into why str_count doesn’t work as expected and explore alternative methods using regular expressions (regex) and specialized string manipulation packages.

Counting Dots with str_count

When you try to use str_count to count the number of dots in a character string, you might expect the following code to work:

ex_str <- "This.is.a.string"
n_dots <- str_count(ex_str, '.')
print(n_dots)

Unfortunately, this won’t give you the desired result. The issue lies in the fact that str_count counts characters literally, not considering special regex symbols.

To fix this, you need to escape the dot (.) symbol using a backslash (\). Here’s the corrected code:

ex_str <- "This.is.a.string"
n_dots <- str_count(ex_str, '\\.')
print(n_dots)

Output:

[1] 3

As you can see, str_count now correctly counts the number of dots in the string.

Alternative Methods using Regex

While escaping special symbols with backslashes works, it’s often not the most elegant solution. In this section, we’ll explore alternative methods for counting dots using regular expressions (regex).

Using gsub to Remove Non-Dot Characters

One approach is to remove all non-dot characters from the string and then count the remaining dots:

ex_str <- "This.is.a.string"
n_dots <- nchar(gsub("[^.]", "", ex_str))
print(n_dots)

Output:

[1] 3

In this code, gsub replaces all non-dot characters ([^.]+) with an empty string (""), effectively removing them from the original string. The resulting string contains only dots.

Using stringi

Another approach is to use the stringi package, which provides a specialized function for counting specific strings:

library(stringi)
ex_str <- "This.is.a.string"
n_dots <- stri_count_fixed(ex_str, '.')
print(n_dots)

Output:

[1] 3

In this code, stri_count_fixed counts the number of dots in the string while ignoring non-dot characters.

Using base R with grep

Finally, we can use the grep function to count the number of dots. Here’s an example:

ex_str <- "This.is.a.string"
n_dots <- sum(grepl("\\.", ex_str))
print(n_dots)

Output:

[1] 3

In this code, grepl searches for the dot (.) symbol in the string and returns a logical vector indicating whether each character matches. The sum function adds up these logical values to give the total count of dots.

Conclusion

Counting dots in character strings is a common task in R programming. While str_count may not work as expected due to its literal counting behavior, there are alternative methods using regular expressions (regex) and specialized string manipulation packages like stringi.

By understanding how these alternatives work, you can choose the most efficient and effective approach for your specific use case. Remember to always consider special regex symbols and escape them accordingly to avoid unexpected results.

Counting Dots in Character Strings with str_count and Beyond

Introduction

Background

Counting Dots with str_count

Alternative Methods using Regex

Using gsub to Remove Non-Dot Characters

Using stringi

Using base R with grep

Conclusion

Further Reading