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_e4 at 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 <valentin at petzel.at>
> *Sent:* Monday, January 9, 2023 9:18 PM
> *To:* akshay kulkarni <akshay_e4 at hotmail.com>
> *Cc:* R help Mailing list <r-help at 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_e4 at 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]]
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
[[alternative HTML version deleted]]
> 09.01.2023 18:05:58 akshay kulkarni <akshay_e4 at hotmail.com>: > > We are living in the 21st century world, and the R-core team might,I suppose, have a definite reason ... >Maybe compatibility reasons with S and R-versions from the 20st century? But maybe, you would have expected some reason even then. best regards, Heinz
R's
{ expr1; expr2; expr3}
acts much like C's
( expr1, expr2, expr3)
E.g.,
$ cat a.c
#include <stdio.h>
int main(int argc, char* argv[])
{
double y = 10 ;
double x = (printf("Starting... "), y = y + 100, y * 20);
printf("Done: x=%g, y=%g\n", x, y);
return 0;
}
$ gcc -Wall a.c
$ ./a.out
Starting... Done: x=2200, y=110
I don't like that syntax (e.g., commas between expressions instead of the
usual semicolons after expressions). Perhaps John Chambers et all didn't
either.
-Bill
On Fri, Jan 13, 2023 at 2:28 AM Valentin Petzel <valentin at petzel.at>
wrote:
> 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_e4 at 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 <valentin at petzel.at>
> > *Sent:* Monday, January 9, 2023 9:18 PM
> > *To:* akshay kulkarni <akshay_e4 at hotmail.com>
> > *Cc:* R help Mailing list <r-help at 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_e4 at 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]]
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more,
see
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]
Dear valentin,
Thanks for a comprehensive background....
THanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Valentin Petzel <valentin at petzel.at>
Sent: Friday, January 13, 2023 4:48 AM
To: akshay kulkarni <akshay_e4 at hotmail.com>
Cc: R help Mailing list <r-help at 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_e4 at 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 <valentin at petzel.at>
Sent: Monday, January 9, 2023 9:18 PM
To: akshay kulkarni <akshay_e4 at hotmail.com>
Cc: R help Mailing list <r-help at 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_e4 at 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]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
[[alternative HTML version deleted]]
I wonder if the real confusino is not R's scope rules?
(begin .) is not Lisp, it's Scheme (a major Lisp dialect),
and in Scheme, (begin (define x ...) (define y ...) ...)
declares variables x and y that are local to the (begin ...)
form, just like Algol 68. That's weirdness 1. Javascript
had a similar weirdness, when the ECMAscript process eventually
addressed. But the real weirdness in R is not just that the
existence of variables is indifferent to the presence of curly
braces, it's that it's *dynamic*. In
f <- function (...) {
... use x ...
x <- ...
... use x ...
}
the two occurrences of "use x" refer to DIFFERENT variables.
The first occurrence refers to the x that exists outside the
function. It has to: the local variable does not exist yet.
The assignment *creates* the variable, so the second
occurrence of "use x" refers to the inner variable.
Here's an actual example.> x <- 137
> f <- function () {
+ a <- x
+ x <- 42
+ b <- x
+ list(a=a, b=b)
+ }> f()
$a
[1] 137
$b
[1] 42
Many years ago I set out to write a compiler for R, and this was
the issue that finally sank my attempt. It's not whether the
occurrence of "use x" is *lexically* before the creation of x.
It's when the assignment is *executed* that makes the difference.
Different paths of execution through a function may result in it
arriving at its return point with different sets of local variables.
R is the only language I routinely use that does this.
So rule 1: whether an identifier in an R function refers to an
outer variable or a local variable depends on whether an assignment
creating that local variable has been executed yet.
And rule 2: the scope of a local variable is the whole function.
If the following transcript not only makes sense to you, but is
exactly what you expect, congratulations, you understand local
variables in R.
> x <- 0
> g <- function () {
+ n <- 10
+ r <- numeric(n)
+ for (i in 1:n) {
+ if (i == 6) x <- 100
+ r[i] <- x + i
+ }
+ r
+ }> g()
[1] 1 2 3 4 5 106 107 108 109 110
On Fri, 13 Jan 2023 at 23:28, Valentin Petzel <valentin at petzel.at>
wrote:
> 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_e4 at 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 <valentin at petzel.at>
> > *Sent:* Monday, January 9, 2023 9:18 PM
> > *To:* akshay kulkarni <akshay_e4 at hotmail.com>
> > *Cc:* R help Mailing list <r-help at 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_e4 at 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]]
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more,
see
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]