R help - Feb 2024 - [EXT] Initializing vector and matrices

Hi Steven,
It's not entirely clear what you actually want to achieve in the end.

As soon as you "know" x1, and assuming that the different
"xi" do not
differ in length in the real application, you know the length of the 
target vector.
Instead of the loop, you can use 'Reduce' without having to initialize a
starting vector.

# generate sample vectors, put them in a list

xi<-lapply(1:5, \(x)sample(5))

# look at xi
xi

# sum over xi
Reduce("+",xi)

this works also for matrices

# generate sample matrices, put them in a list
Xi<-lapply(1:3, \(x)matrix(sample(16), nrow=4))

# look at them
Xi

# sum over Xi
Reduce("+",Xi)

Hope that helps

Eik


Am 28.02.2024 um 09:56 schrieb Steven Yen:
> Is there as way to initialize a vector (matrix) with an unknown length 
> (dimension)? NULL does not seem to work. The lines below work with a 
> vector of length 4 and a matrix of 4 x 4. What if I do not know 
> initially the length/dimension of the vector/matrix?
> 
> All I want is to add up (accumulate)? the vector and matrix as I go 
> through the loop.
> 
> Or, are there other ways to accumulate such vectors and matrices?
> 
>  > x<-rep(0,4)? # this works but I like to leave the length open
>  >? for (i in 1:3){
> +? x1<-1:4
> +? x<-x+x1
> + }
>  > x
> [1]? 3? 6? 9 12
> 
>  > y = 0*matrix(1:16, nrow = 4, ncol = 4); # this works but I like to 
> leave the dimension open
>  ???? [,1] [,2] [,3] [,4]
> [1,]??? 0??? 0??? 0??? 0
> [2,]??? 0??? 0??? 0??? 0
> [3,]??? 0??? 0??? 0??? 0
> [4,]??? 0??? 0??? 0??? 0
>  > for (i in 1:3){
> +?? y1<-matrix(17:32, nrow = 4, ncol = 4)
> +?? y<-y+y1
> + }
>  > y
>  ???? [,1] [,2] [,3] [,4]
> [1,]?? 51?? 63?? 75?? 87
> [2,]?? 54?? 66?? 78?? 90
> [3,]?? 57?? 69?? 81?? 93
> [4,]?? 60?? 72?? 84?? 96
>  >
> 
> ______________________________________________
> 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.
-- 
Eik Vettorazzi

Universit?tsklinikum Hamburg-Eppendorf
Institut f?r Medizinische Biometrie und Epidemiologie

Christoph-Probst-Weg 1
4. Obergeschoss, Raum 04.1.021.1

20246 Hamburg

Telefon: +49 (0) 40 7410 - 58243
Fax:     +49 (0) 40 7410 - 57790

Web: www.uke.de/imbe

Webex: https://webteaching-uke.webex.com/meet/e.vettorazzi


--

_____________________________________________________________________

Universit?tsklinikum Hamburg-Eppendorf; K?rperschaft des ?ffentlichen Rechts;
Gerichtsstand: Hamburg | www.uke.de
Vorstandsmitglieder: Prof. Dr. Christian Gerloff (Vorsitzender), Joachim Pr?l?,
Prof. Dr. Blanche Schwappach-Pignataro, Matthias Waldmann (komm.)
_____________________________________________________________________

SAVE PAPER - THINK BEFORE PRINTING

Hello Eik:

Thanks. I do not need to sample. Essentially, I have a do loop which 
produces 24 vectors of length of some length (say k=300) and 24 matrices 
of 300x300. Then, I simply need to? take the averages of these 24 
vectors and matrices:

x=(x1+x2+...+x24)/k

y=(y1+y2+...+y24)/k

I am just looking for ways to do this in a do loop, which requires 
initialization (to 0's) of x and y. My struggle is not knowning length 
of x until x1 is produced in the first of the loop. Thanks.

Steven

On 2/28/2024 6:22 PM, Eik Vettorazzi wrote:
> Hi Steven,
> It's not entirely clear what you actually want to achieve in the end.
>
> As soon as you "know" x1, and assuming that the different
"xi" do not
> differ in length in the real application, you know the length of the 
> target vector.
> Instead of the loop, you can use 'Reduce' without having to
initialize
> a starting vector.
>
> # generate sample vectors, put them in a list
>
> xi<-lapply(1:5, \(x)sample(5))
>
> # look at xi
> xi
>
> # sum over xi
> Reduce("+",xi)
>
> this works also for matrices
>
> # generate sample matrices, put them in a list
> Xi<-lapply(1:3, \(x)matrix(sample(16), nrow=4))
>
> # look at them
> Xi
>
> # sum over Xi
> Reduce("+",Xi)
>
> Hope that helps
>
> Eik
>
>
> Am 28.02.2024 um 09:56 schrieb Steven Yen:
>> Is there as way to initialize a vector (matrix) with an unknown 
>> length (dimension)? NULL does not seem to work. The lines below work 
>> with a vector of length 4 and a matrix of 4 x 4. What if I do not 
>> know initially the length/dimension of the vector/matrix?
>>
>> All I want is to add up (accumulate)? the vector and matrix as I go 
>> through the loop.
>>
>> Or, are there other ways to accumulate such vectors and matrices?
>>
>> ?> x<-rep(0,4)? # this works but I like to leave the length open
>> ?>? for (i in 1:3){
>> +? x1<-1:4
>> +? x<-x+x1
>> + }
>> ?> x
>> [1]? 3? 6? 9 12
>>
>> ?> y = 0*matrix(1:16, nrow = 4, ncol = 4); # this works but I like
to
>> leave the dimension open
>> ????? [,1] [,2] [,3] [,4]
>> [1,]??? 0??? 0??? 0??? 0
>> [2,]??? 0??? 0??? 0??? 0
>> [3,]??? 0??? 0??? 0??? 0
>> [4,]??? 0??? 0??? 0??? 0
>> ?> for (i in 1:3){
>> +?? y1<-matrix(17:32, nrow = 4, ncol = 4)
>> +?? y<-y+y1
>> + }
>> ?> y
>> ????? [,1] [,2] [,3] [,4]
>> [1,]?? 51?? 63?? 75?? 87
>> [2,]?? 54?? 66?? 78?? 90
>> [3,]?? 57?? 69?? 81?? 93
>> [4,]?? 60?? 72?? 84?? 96
>> ?>
>>
>> ______________________________________________
>> 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.
>