# [R] Remove empty list from list

Mon Aug 28 19:43:50 CEST 2006

```On this day 28/08/2006 19:20, Muhammad Subianto wrote:
> Dear all,
> I am still working with "list".
> If I have an empty list how can I remove from list data.
> Here is a toy example:
> x <- list(matrix(1:20, 5, 4),matrix(1:20, 5, 4),matrix(1:20, 5,
> 4),matrix(1:20, 5, 4),matrix(1:20, 5, 4))
> y <- list(c(1, -1, -1, 1, 1),c(1, 1, -1, -1, -1),c(1, 1, 1, 1, 1),c(1,
> 1, -1, 1, -1),c(-1, -1, -1, -1, -1))
> ## Thanks to Gabor Grothendieck for this trick.
> ## SIMPLIFY? SIMPLIFY >< simplify
> xy.list <- mapply(cbind, x, y, SIMPLIFY=FALSE)
>
> point.class <- t(cbind(c(10,20,15,4,-1),c(21,10,15,34,-1),c(11,13,6,3,1),c(7,5,5,2,1),c(8,9,5,12,-1)))
> class.diffsame <- points.neighb(as.matrix(point.class), xy.list, 5)
> pd.class <- points.diff(class.diffsame,xy.list)
>
> nc.test <- vector("list",length(pd.class))
> for (i in 1:length(pd.class)) {
>      nc.test[[i]] <- pd.class[[i]]\$point.diff
> }
> nc.test
>> nc.test
> [[1]]
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    1    6   11   16    1
> [2,]    4    9   14   19    1
> [3,]    5   10   15   20    1
>
> [[2]]
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    1    6   11   16    1
> [2,]    2    7   12   17    1
>
> [[3]]
>      [,1] [,2] [,3] [,4] [,5]
>
> [[4]]
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    3    8   13   18   -1
> [2,]    5   10   15   20   -1
>
> [[5]]
>      [,1] [,2] [,3] [,4] [,5]
>
> I want to remove these:
> nc.test[[3]]
> nc.test[[5]]
> Because my list data have more 1000 lists are there any simple way to do this?
>
>
>
> points.neighb <- function(p.class, list.nc, class.col) {
>    ntuples <- nrow(p.class)
>    instvec <- vector("list",length=ntuples)
>    for (i in 1:ntuples) {
>         # Thanks to Petr Pikal for this trick
>         instvec[[i]]\$class.diff <- (p.class[i,class.col] -
> list.nc[[i]][,class.col])!=0
>         instvec[[i]]\$class.same <- (p.class[i,class.col] -
> list.nc[[i]][,class.col])==0
>    }
>    instvec
> }
>
> points.diff <- function(p.class, list.nc) {
>    ntuples <- length(list.nc)
>    instvec <- vector("list",ntuples)
>    for (i in 1:ntuples) {
>         instvec[[i]]\$point.diff <- list.nc[[i]][p.class[[i]]\$class.diff,]
>         instvec[[i]]\$point.same <- list.nc[[i]][p.class[[i]]\$class.same,]
>    }
>    instvec
> }
>

Dear
Jim Holtman and Gabor Grothendieck,
Thank you both very much for the suggestions!
These is exactly what I was looking for.

## delete null/empty entries in a list
delete.NULLs  <-  function(x.list){
x.list[unlist(lapply(x.list, length) != 0)]
}

> delete.NULLs  <-  function(x.list){
+     x.list[unlist(lapply(x.list, length) != 0)]
+ }
> delete.NULLs  <-  function(x.list){
+     x.list[unlist(lapply(x.list, length) != 0)]
+ }
>
> delete.NULLs(nc.test)
[[1]]
[,1] [,2] [,3] [,4] [,5]
[1,]    1    6   11   16    1
[2,]    4    9   14   19    1
[3,]    5   10   15   20    1

[[2]]
[,1] [,2] [,3] [,4] [,5]
[1,]    1    6   11   16    1
[2,]    2    7   12   17    1

[[3]]
[,1] [,2] [,3] [,4] [,5]
[1,]    3    8   13   18   -1
[2,]    5   10   15   20   -1

>

```