R help - Jun 2010 - list() assigning the same value to two items

Hi everybody,

I'd like to have a list with two elements, where both elements have the same
value:

z <- list(a=1, b=1)

If it happens, that I've to change the value, I've to assure that I
change both. This is error prone. Hence, a better way to achieve this is to
define:

tmp <- 1
z <- list(a=tmp, b=tmp)

Now, I'm wondering if it is possible to make this more compact:
z <- list(a=tmp<-1, b=tmp)

Both approaches have the side effect that a variable "tmp" is created.
So is there a way to achieve the same thing without having to define an
additional variable? Something like

z <- list(a=1, b=a)

or even 

z <- list(a=b=1)?

Both commands don't work of course, but I'm just curious whether this is
possible in principle (a rather academic question I've to admit)

Thanks + BR,

Thorn Thaler
Applied Mathematics Group 
Nestl? Research Center
PO Box 44 
CH-1000 Lausanne 26, Switzerland
Phone: + 41 21 785 8220
Fax: + 41 21 785 8556

On 2010-06-21 3:30, Thaler, Thorn, LAUSANNE, Applied Mathematics
wrote:
> Hi everybody,
>
> I'd like to have a list with two elements, where both elements have the
same value:
>
> z<- list(a=1, b=1)
>
> If it happens, that I've to change the value, I've to assure that I
change both. This is error prone. Hence, a better way to achieve this is to
define:
>
> tmp<- 1
> z<- list(a=tmp, b=tmp)
>
> Now, I'm wondering if it is possible to make this more compact:
> z<- list(a=tmp<-1, b=tmp)
>
> Both approaches have the side effect that a variable "tmp" is
created. So is there a way to achieve the same thing without having to define an
additional variable? Something like
>
> z<- list(a=1, b=a)
>
> or even
>
> z<- list(a=b=1)?
>
> Both commands don't work of course, but I'm just curious whether
this is possible in principle (a rather academic question I've to admit)
>
  f <- function(x){ list(a=x, b=x) }
  z <- f(1)
etc.

   -Peter Ehlers

> Thanks + BR,
>
> Thorn Thaler
> Applied Mathematics Group
> Nestl? Research Center
> PO Box 44
> CH-1000 Lausanne 26, Switzerland
> Phone: + 41 21 785 8220
> Fax: + 41 21 785 8556
>

One approach is to use the within function:
> within(list(), {a<-1; b<-a})
$b
[1] 1

$a
[1] 1

Now, whether that is simpler or more compact than other options is for you (or
other users) to decide.

Hope this helps,

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Thaler, Thorn, LAUSANNE, Applied Mathematics
> Sent: Monday, June 21, 2010 3:30 AM
> To: r-help at r-project.org
> Subject: [R] list() assigning the same value to two items
> 
> Hi everybody,
> 
> I'd like to have a list with two elements, where both elements have the
> same value:
> 
> z <- list(a=1, b=1)
> 
> If it happens, that I've to change the value, I've to assure that I
> change both. This is error prone. Hence, a better way to achieve this
> is to define:
> 
> tmp <- 1
> z <- list(a=tmp, b=tmp)
> 
> Now, I'm wondering if it is possible to make this more compact:
> z <- list(a=tmp<-1, b=tmp)
> 
> Both approaches have the side effect that a variable "tmp" is
created.
> So is there a way to achieve the same thing without having to define an
> additional variable? Something like
> 
> z <- list(a=1, b=a)
> 
> or even
> 
> z <- list(a=b=1)?
> 
> Both commands don't work of course, but I'm just curious whether
this
> is possible in principle (a rather academic question I've to admit)
> 
> Thanks + BR,
> 
> Thorn Thaler
> Applied Mathematics Group
> Nestl? Research Center
> PO Box 44
> CH-1000 Lausanne 26, Switzerland
> Phone: + 41 21 785 8220
> Fax: + 41 21 785 8556
> 
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