R remove objects from a list with if else statement

Question

I have a list of data frames, and would like to remove those with less than 2 rows off from mylist:

a<-data.frame(x=c(1:4),y=c("m", "n", "o", "p"))
b<-data.frame(x=c(2:6),y=c("q", "w", "e", "r", "t"))
c<-data.frame(x=c(6,7),y=c("j","k"),z=c("$","#"))
d<-data.frame(x="9",y="q",z="+")
mylist<-list(a,b,c,d)

for (i in length(mylist)){
if (nrow(mylist[[i]])<=2){
mylist<-mylist[-i]
}
else{
mylist<-myslit
}}

However it only seemed to remove data.frame d. Data frame c is still in "mylist" after running the for loop.

Answer 1

You can do this more easily using an apply loop:

row_lt2 <- which(sapply(mylist, nrow) < 2)
mylist[-row_lt2]
[[1]]
  x y
1 1 m
2 2 n
3 3 o
4 4 p

[[2]]
  x y
1 2 q
2 3 w
3 4 e
4 5 r
5 6 t

[[3]]
  x y z
1 6 j $
2 7 k #

Notice I use negative indexing to remove items instead of selecting them.

Answer 2

To add to the other answers: this is exactly the type of thing the higher-order Filter function is made for:

> Filter(function(x) {nrow(x) >= 2}, mylist)
[[1]]
  x y
1 1 m
2 2 n
3 3 o
4 4 p

[[2]]
  x y
1 2 q
2 3 w
3 4 e
4 5 r
5 6 t

[[3]]
  x y z
1 6 j $
2 7 k #

Answer 3

You can't do this procedure using for because the indices change. Using for, after removing line 2, you will examine line 3, but you need examine line 2 again (because the line 2 isn't more the same line as before). Change it to repeat or while.

a<-data.frame(x=c(1:4),y=c("m", "n", "o", "p"))
b<-data.frame(x=c(2:6),y=c("q", "w", "e", "r", "t"))
c<-data.frame(x=c(6,7),y=c("j","k"),z=c("$","#"))
d<-data.frame(x="9",y="q",z="+")
mylist<-list(a,b,c,d)

i <- 1
while (i <= length(mylist)) {
 if (nrow(mylist[[i]])<=2){
  mylist<-mylist[-i]
 }
 else{
  i <- i+1
 }
}

Or just use @Paul solution... :P

Answer 4

Paul has provided an answer already, but your mistake has not been pointed out.

Your code has two problems. First, you need to supply a range to your loop:

for (i in 1:length(mylist))

or for (i in seq_along(length(mylist)))

Without this, your initialization looked like for (i in 4) after evaluation, meaning that only one iteration was run, removing element 4 and not even looking at all previous elements.

However, if you fix that problem, another one emerges. Namely, your list no longer has 4 elements after removing element 3. It only has 3 elements, while your i index will go up until 4, resulting in subscript out of bounds error.

Therefore one can only suggest the approach using apply, as described by @Paul.

Also, opposed to the assertion otherwise, it is possible to achieve the same using for loop, only your approach needs to be slightly different:

for (i in 1:length(mylist)) {
    if (nrow(mylist[[i]])>2)
    {
        mylist2[i]<-mylist[i]
    }
}  
print(mylist2)

Here you select list elements that are greater than 2, and assign them to a new list. Sapply will be more speedy though.