Dear valentin,
Thanks for a comprehensive background....
THanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Valentin Petzel <valen...@petzel.at>
Sent: Friday, January 13, 2023 4:48 AM
To: akshay kulkarni <akshay...@hotmail.com>
Cc: R help Mailing list <r-help@r-project.org>
Subject: Re: [R] return value of {....}
Hello Akshay,
R is quite inspired by LISP, where this is a common thing. It is not in fact
that {...} returned something, rather any expression evalulates to some value,
and for a compound statement that is the last evaluated expression.
{...} might be seen as similar to LISPs (begin ...).
Now this is a very different thing compared to {...} in something like C, even
if it looks or behaves similarly. But in R {...} is in fact an expression and
thus has evaluate to some value. This also comes with some nice benefits.
You do not need to use {...} for anything that is a single statement. But you
can in each possible place use {...} to turn multiple statements into one.
Now think about a statement like this
f <- function(n) {
x <- runif(n)
x**2
}
Then we can do
y <- f(10)
Now, you suggested way would look like this:
f <- function(n) {
x <- runif(n)
y <- x**2
}
And we'd need to do something like:
f(10)
y <- somehow_get_last_env_of_f$y
So having a compound statement evaluate to a value clearly has a benefit.
Best Regards,
Valentin
09.01.2023 18:05:58 akshay kulkarni <akshay...@hotmail.com>:
Dear Valentin,
But why should {....} "return" a value? It could just
as well evaluate all the expressions and store the resulting objects in
whatever environment the interpreter chooses, and then it would be left to the
user to manipulate any object he chooses. Don't you think returning the last,
or any value, is redundant? We are living in the 21st century world, and the
R-core team might,I suppose, have a definite reason for"returning" the last
value. Any comments?
Thanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Valentin Petzel <valen...@petzel.at>
Sent: Monday, January 9, 2023 9:18 PM
To: akshay kulkarni <akshay...@hotmail.com>
Cc: R help Mailing list <r-help@r-project.org>
Subject: Re: [R] return value of {....}
Hello Akshai,
I think you are confusing {...} with local({...}). This one will evaluate the
expression in a separate environment, returning the last expression.
{...} simply evaluates multiple expressions as one and returns the result of
the last line, but it still evaluates each expression.
Assignment returns the assigned value, so we can chain assignments like this
a <- 1 + (b <- 2)
conveniently.
So when is {...} useful? Well, anyplace where you want to execute complex stuff
in a function argument. E.g. you might do:
data %>% group_by(x) %>% summarise(y = {if(x[1] > 10) sum(y) else mean(y)})
Regards,
Valentin Petzel
09.01.2023 15:47:53 akshay kulkarni <akshay...@hotmail.com>:
> Dear members,
> I have the following code:
>
>> TB <- {x <- 3;y <- 5}
>> TB
> [1] 5
>
> It is consistent with the documentation: For {, the result of the last
> expression evaluated. This has the visibility of the last evaluation.
>
> But both x AND y are created, but the "return value" is y. How can this be
> advantageous for solving practical problems? Specifically, consider the
> following code:
>
> F <- function(X) { expr; expr2; { expr5; expr7}; expr8;expr10}
>
> Both expr5 and expr7 are created, and are accessible by the code outside of
> the nested braces right? But the "return value" of the nested braces is
> expr7. So doesn't this mean that only expr7 should be accessible? Please help
> me entangle this (of course the return value of F is expr10, and all the
> other objects created by the preceding expressions are deleted. But expr5 is
> not, after the control passes outside of the nested braces!)
>
> Thanking you,
> Yours sincerely,
> AKSHAY M KULKARNI
>
> [[alternative HTML version deleted]]
>
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