R help - Nov 2005 - force apply() to return a list

Hi

I have a function f() that I want to apply() to a matrix.  I want the
answer to be a list.

However, sometimes all the vectors returned by f() are the same length
and apply() returns a matrix.  I do not want this.

Hi

How do I force apply() to consistently return a list?
Toy example follows.

R> f <- function(x){1:max(x[1],4)}
R> a1 <- cbind(1:5,5:1)
R> apply(a1,1,f)
[[1]]
[1] 1 2 3 4

[[2]]
[1] 1 2 3 4

[[3]]
[1] 1 2 3 4

[[4]]
[1] 1 2 3 4

[[5]]
[1] 1 2 3 4 5

list returned: desired behaviour.  Now try the
same but with a different matrix from a1:


R> a2 <- cbind(1:3,3:1)
R> apply(a2,1,f)
      [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    2
[3,]    3    3    3
[4,]    4    4    4


matrix returned: this is undesired behaviour (in my application, I
pass the list to do.call()).  How do I force apply() to behave
consistently and return a list, irrespectively of the length of
vectors returned by f()?



--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743

a solution is the following

f <- function(x){list(1:max(x[1], 4))}
a2 <- cbind(1:3, 3:1)
out <- lapply(apply(a2, 1, f), "[[", 1)


I hope it helps.

Best,
Dimitris


----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://www.med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm



----- Original Message ----- 
From: "Robin Hankin" <r.hankin at noc.soton.ac.uk>
To: "RHelp" <r-help at stat.math.ethz.ch>
Sent: Monday, November 21, 2005 9:16 AM
Subject: [R] force apply() to return a list

>
> Hi
>
> I have a function f() that I want to apply() to a matrix.  I want 
> the
> answer to be a list.
>
> However, sometimes all the vectors returned by f() are the same 
> length
> and apply() returns a matrix.  I do not want this.
>
> Hi
>
> How do I force apply() to consistently return a list?
> Toy example follows.
>
> R> f <- function(x){1:max(x[1],4)}
> R> a1 <- cbind(1:5,5:1)
> R> apply(a1,1,f)
> [[1]]
> [1] 1 2 3 4
>
> [[2]]
> [1] 1 2 3 4
>
> [[3]]
> [1] 1 2 3 4
>
> [[4]]
> [1] 1 2 3 4
>
> [[5]]
> [1] 1 2 3 4 5
>
> list returned: desired behaviour.  Now try the
> same but with a different matrix from a1:
>
>
> R> a2 <- cbind(1:3,3:1)
> R> apply(a2,1,f)
>      [,1] [,2] [,3]
> [1,]    1    1    1
> [2,]    2    2    2
> [3,]    3    3    3
> [4,]    4    4    4
>
>
> matrix returned: this is undesired behaviour (in my application, I
> pass the list to do.call()).  How do I force apply() to behave
> consistently and return a list, irrespectively of the length of
> vectors returned by f()?
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
>  tel  023-8059-7743
>
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