R: What do you call the :: and ::: operators and how do they differ? [duplicate]

问题

I'm wondering how the functions of the :: and ::: operators differ in R.

However, I can't figure out what these operators are called and so a google or SO search has not proven helpful. I also get an error when I try ?:: in R.

So...

What are the :: and ::: operators called?
How do :: and ::: differ? (i.e., what exactly does each do)?

回答1:

It turns out there is a unique way to access help info for operators such as these colons: add quotations marks around the operator. [E.g., ?'::' or help(":::")].

Also, instead of quotation marks, back-ticks (i.e, ` ) also work.

Double Colon Operator and Triple Colon Operator

The answer to the question can be found on the help page for "Double Colon and Triple Colon Operators" (see here).

For a package pkg, pkg::name returns the value of the exported variable name in namespace pkg, whereas pkg:::name returns the value of the internal variable name. The package namespace will be loaded if it was not loaded before the call, but the package will not be attached to the search path.

The difference can be seen by examining the code of each:

> `::`
function (pkg, name) 
{
    pkg <- as.character(substitute(pkg))
    name <- as.character(substitute(name))
    getExportedValue(pkg, name)
}
<bytecode: 0x00000000136e2ae8>
<environment: namespace:base>

> `:::`
function (pkg, name) 
{
    pkg <- as.character(substitute(pkg))
    name <- as.character(substitute(name))
    get(name, envir = asNamespace(pkg), inherits = FALSE)
}
<bytecode: 0x0000000013482f50>
<environment: namespace:base>

:: calls getExportedValue(pkg, name), returning the value of the exported variable name in the package's namespace.

::: calls get(name, envir = asNamespace(pkg), inherits = FALSE), searching for the object name in the Namespace environment of the package, and returning the value of the internal variable name.

So, what exactly is a namespace?

This site does a good job of explaining the concept of namespaces in R. Importantly:

As the name suggests, namespaces provide “spaces” for “names”. They provide a context for looking up the value of an object associated with a name.

来源：https://stackoverflow.com/questions/41582136/r-what-do-you-call-the-and-operators-and-how-do-they-differ

标签

operators

terminology