R Notebook_String Manipulation

Julius Schmid

STRING MANIPULATION

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Although R is a statistical language with numeric vectors and matrices playing a central role, character strings are surprisingly important as well. Ranging from birth dates stored in medical research data files to textmining applications, character data arises quite frequently in R programs. Accordingly, R has a number of string-manipulation utilities, many of which will be introduced in this chapter.

grep()

The call grep(pattern,x) searches for a specified substring pattern in a vector x of strings. If x has n elements—that is, it contains n strings—then grep(pattern,x) will return a vector of length up to n. Each element of this vector will be the index in x at which a match of pattern as a substring of x[i]) was found.

Here’s an example of using grep:

Our pattern is the string “Pole”, and we are looking for the word or pole within the strings “Equator”, “North Pole”, and “South Pole”.

grep("Pole",c("Equator","North Pole","South Pole"))

[1] 2 3

Since the pattern is only contained in the latter two, the grep() function returns the entry indices 2 and 3.

Note that the grep() function is case-sensitive! See what happens if our pattern is “pole” instead of “Pole”:

grep("pole",c("Equator","North Pole","South Pole"))

integer(0)

grep() returns no indices, which means that the exact case-sensitive pattern is could not be found within the strings above.

In the first case, the string “Pole” was found in elements 2 and 3 of the second argument, hence the output (2,3). In the second case, string “pole” was not found anywhere, so an empty vector was returned.

nchar()

The call nchar(x) finds the length of a string x. Here’s an example:

nchar("South Pole")

[1] 10

The string “South Pole” was found to have 10 characters. C programmers, take note: There is no NULL character terminating R strings. Also note that the results of nchar() will be unpredictable if x is not in character mode. For instance, nchar(NA) turns out to be 2, and nchar(factor(“abc”)) is 1. For more consistent results on nonstring objects, use Hadley Wickham’s stringr package on CRAN.

paste()

The call paste(…) concatenates several strings, returning the result in one long string. Here are some examples:

The arguments in the paste() function are the strings we want to concatenate, together with an optional argument sep which determines the connector of the strings. The default connector is just a space.

paste("North","Pole")

[1] "North Pole"

paste("North","Pole",sep="")

[1] "NorthPole"

paste("North","Pole",sep=".")

[1] "North.Pole"

paste("North","and","South","Poles")

[1] "North and South Poles"

As you can see, the optional argument sep can be used to put something other than a space between the pieces being spliced together. If you specify sep as an empty string, the pieces won’t have any character between them.

Concatenate “Final” and “Exam” using the steps shown above.

We call exactly the same functions as above, just alternating the strings, but leaving the connectors the same:

# Enter Answer Here
paste("Final","Exam")

[1] "Final Exam"

paste("Final","Exam",sep="")

[1] "FinalExam"

paste("Final","Exam",sep=".")

[1] "Final.Exam"

paste("Final","and","Midterm","Exams")

[1] "Final and Midterm Exams"

Depending on the optional argument in the sep parameter, we get different outputs.

substr()

The call substr(x,start,stop) returns the substring in the given character position range start:stop in the given string x. Here’s an example:

substring("Equator",3,5)

[1] "uat"

We start with letter number 3 (u) and return everything until letter number 5 (t). Hence, the output is “uat”.

strsplit()

The call strsplit(x,split) splits a string x into an R list of substrings based on another string split in x. Here’s an example:

strsplit("6-16-2011",split="-")

[[1]]
[1] "6"    "16"   "2011"

The strsplit() function split the date into the three components month (June), day (16) and year (2011).

Use the function above to split “11-28-2022”

#Enter Answer Here
strsplit("11-28-2022",split="-")

[[1]]
[1] "11"   "28"   "2022"

Again, the strsplit() function split the date into the three components month (November), day (11) and year (2022).