Clash Royale CLAN TAG#URR8PPP

R remove objects from a list with if else statement

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.

5 Answers
5

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.

Filter

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

Filter

> 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 #


?Filter

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.

for

for

repeat

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

for

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.

for (i in 4)

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.

i

subscript out of bounds

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

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.

Sapply

for

mylist

(i in length(mylist))

(i in 1:length(mylist))

For a few special cases, other methods mentioned above didn't work, although while could work. I did find that if you use Rcoster's approach while without adding an evaluation of the returned list, it would still return the wrong answer -- since every time when the condition i<=length(mylist) got evaluated, the length of mylist can change, due to the deletion of the mylist[-i]. Therefore, the while evaluation could stop before it reaches all unwanted elements in the list.

while

while

i<=length(mylist)

mylist

mylist[-i]

while

For example, in my case, I wanted to make sure all length of mylist[[i]]<=4. The list is very large (2284879 elements to start with). When the first time I ran the while evaluation, the program stopped before taking out all mylist[[i]]>4 because the length of mylist got smaller. I had to use table(lengths(mylist)) to judge whether all unwanted lists were taken out. If not, then run the while loop again. Luckily it only took two runs. But I thought it is necessary to point it out, in case someone else came here and try to use this while approach.

mylist[[i]]<=4

while

mylist[[i]]>4

mylist

table(lengths(mylist))

while

while

PS. for loop should be usable too, if some evaluation added to judge whether re-run is needed.

for

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