[R] Remove empty list from list - remove only one row and make as matrix

[Muhammad Subianto]

Dear all,
After I work around, I found in my list of data with only one row which 
need to remove or make it as matrix.
Here I write again my other 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

# delete null/empty entries in a list
dff <- delete.NULLs(nc.test)
dff; str(dff)
 > dff
[[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]  1  6 11 16  1

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

 >

lapply(dff, nrow)
 > lapply(dff, nrow)
[[1]]
[1] 3

[[2]]
NULL

[[3]]
[1] 2

 >

#I can use
#dff[unlist(lapply(dff, nrow) == 1)] #2,3, etc

I have two questions here:
a. I need to remove dff[[2]]
b. How to make it as matrix (in list). I mean the result something like

[[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

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


Best, Muhammad Subianto


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
}

# Thanks to Jim Holtman for this trick
delete.NULLs  <-  function(x.list){
     x.list[unlist(lapply(x.list, length) != 0)]
}