R中字符串和字符有什么区别？

Question

I'm not pretty familiar with R, but anyhow I'm writing a R wrapper for ac library. I come across this problem. How do I decide if the input argument is a string? In details,should I write like this:

dyn.load("hello.so")
do_process <- function(str) {
        if(!is.character(str))
            stop("not a character or string");
    result <- .Call("hello", as.character(str))
    return result
}

or this:

dyn.load("hello.so")
do_process <- function(str) {
        if(!is.string(str))
            stop("not a character or string");
    result <- .Call("hello", as.string(str))
    return result
}

Thanks.

Answer 1

is.string is a function from the xtable package. In the details section of the help page it explicitly says "These functions are private functions used by print.xtable. They are not intended to be used elsewhere."

As such, I would avoid using those functions.

In R there is no string data type. Instead it is called character and you can use is.character to do the check you're describing.

Also, as I mentioned in my comment, avoid using important base functions as variable names. Specifically str is used to view the structure of an object.

Answer 2

In R, there's no fundamental distinction between a string and a character. A "string" is just a character variable that contains one or more characters.

One thing you should be aware of, however, is the distinction between a scalar character variable, and a vector . A character vector is a set of strings stored as a single object. Most R functions that work on character input are vectorised, ie they will return the appropriate value for each element in such a vector.

For example:

# a string containing one character
x <- "a"
nchar(x)
# 1

# a string containing multiple characters
x <- "foo"
nchar(x)
# 3

# a character vector: two strings, each containing three characters
x <- c("foo", "bar")

# length() returns the no. of elements in a vector, NOT the length of each string
length(x)
# 2

# nchar() returns the length of each string in a vector
nchar(x)
# 3 3