Extract the Last N Characters from String in R

To extract the last N characters from a string in R, you can use the base R functions substring() and substr(), or the str_sub() function from the stringr package. Extracting the last N characters is a common task in data manipulation and text processing. Each of these functions offers unique advantages and can be chosen based on the specific needs of your task. In this article, I will explain multiple methods for efficiently extracting the last N characters from a string using both base R functions and the stringr package.

Key points-

substring() and substr() are native functions in R that can extract parts of a string based on specified positions.
The str_sub() function from the stringr package offers more flexibility and ease of use, especially for operations relative to the end of the string.
Use the nchar() function to calculate the length of the string, which is crucial for determining the starting position for extraction.
To find the starting position for extracting the last N characters, use the formula nchar(string) - n + 1.
The substring() function can be used to extract the last N characters by specifying the starting position derived from the above formula.
The substr() function requires specifying both the start and end positions. For the last N characters, the end position is the length of the string.
The str_sub() function simplifies extracting the last N characters by allowing the use of negative indices to count from the end of the string.
The function call str_sub(string, -n) directly extracts the last N characters by starting from the nth character from the end.

Using substring() Extract Last N Characters

The substring() function in base R extracts the last N characters from a string. Here's how you can use it to extract the last n characters from a string.

First, define a string and specify the number of characters to extract. Use the nchar() function to compute the length of the string, then subtract the nth character from this length (i.e., nchar(string) - nth character). Finally, add 1 to this calculation to get the desired result.


# Extract last chracters of a string using substring()
string <- "AIStudyLabs"
n <- 8
print("Extract the last n characters from the given string:")
substring(string, nchar(string) - n + 1)

From the above example, nchar(string) computes the number of characters in the string AIStudyLabs, which is 15.
nchar(string) - n + 1 calculates the starting position for extracting the last n characters. Here, 15 - 8 + 1 equals 8.
substring(string, 8) extracts the substring from the 8th character to the end of the string. In this case, it extracts Examples.

Yields below output.

Using substr() Extract the Last N Characters in R

Alternatively, you can use the substr() function, another base R function, to extract the last N characters from a string. It takes three parameters: the string, the starting position for extracting the last N characters, and the ending position. It returns the specified substring from the string.


string <- "AIStudyLabs"
n <- 8
print("Given string")
print(string)

print("Extract the last n characters from the given string:")
substr(string, nchar(string) - n + 1, nchar(string))

# Output:
# [1] "Given string"
# [1] "AIStudyLabs"
# [1] "Extract the last n characters from the given string:"
# [1] "Examples"

nchar(string) returns the number of characters in the string AIStudyLabs, which is 15.
nchar(string) - n + 1 it calculates the starting position for the substring extraction. Given that nchar(string) is 15 and n is 8, the calculation is 15 - 8 + 1, which equals 8. So, the starting position is the 8th character of the string.
substr(string, 8, 15): This function extracts a substring from string starting at position 8 and ending at position 15. Which is Examples.

Using stringr::str_sub() in R

The str_sub() function from the stringr package offers a straightforward and reliable method for managing string operations. It allows to extract a substring starting from the nth character to the end of the string. To specify the substring from the end, you can place a negative sign before the nth character.


# install stringr package
install.packages("stringr")
library(stringr)
string <- "AIStudyLabs"
n <- 8
print("Given string")
print(string)
print("Extract the last n characters from the given string:")
str_sub(string, -n)

# Output:
# [1] "Given string"
# [1] "AIStudyLabs"
# [1] "Extract the last n characters from the given string:"
# [1] "Examples"

From the above code, str_sub(string, -n) extracts the substring starting from the nth character from the end of the string. Given that n is 8, it retrieves the last 8 characters of AIStudyLabs, resulting in Examples.

Conclusion

In this article, I have explained that extracting the last N characters from a string in R can be efficiently achieved using various functions, including the base R functions substring() and substr(), as well as the str_sub() function from the stringr package.

Happy learning!!