how to interpret the code below , how R treat and evaluate it and get the result "foo",it is hard to understand for me
`foo}\`` <- "foo"
glue("{
{
'}\\'' # { and } in comments, single quotes
\"}\\\"\" # or double quotes are ignored
`foo}\\`` # as are { in backticks
}
}")
#> foo
You're not the only one to find this example difficult to read.
First, a variable with a funny name is created, and assigned the value "foo":
`foo}\`` <- "foo"
`foo}\``
[1] "foo"
Then glue should evaluate an R expression contained in the first level of curly braces, and itself contained in curly braces:
{
'}\\''
\"}\\\"\" # or double quotes are ignored
`foo}\\`` # as are { in backticks
}
As it's a string, backslashes are used to be able to write special characters.
When R parses each line of the expression, it converts the special characters to their value, and removes the comments after #.
The first line of the expression becomes
parse(text = "'}\\''")
expression('}\'')
> eval(expression('}\''))
[1] "}'"
The second line of the expression becomes:
> parse(text='\"}\\\"\"')
expression("}\"")
> eval(expression("}\""))
[1] "}\""
And finally the last line:
> parse(text = "`foo}\\``")
expression(`foo}\``)
> eval(expression(`foo}\``))
[1] "foo"
To sum up, we have :
{
"}'"
"}\""
"foo"
}
[1] "foo"
