Subsetting elements of a vector in R - r

Question

I have been trying to create a list with the coordinates of a vector which have the same values.
Example: for a vector Points = (2, 2, 3, 6, 3, 6, 3), coordinates that have the same value are (1,2), (3, 5, 7) and (4,6). So the output should be something like
[[1]]
1 2
[[2]]
3 5 7
[[3]]
4 6
Just to contextualize the necessity of this function: I'm trying to simulate random walks with multiple starting points. Once any given chains hit each other, the one with smallest index will be update. The vector Points is the position of all the walks at some time step t. I need to verify, at each time step, which chains have the same value, in order to update just the one with smallest index. In this particular example, just chains 1, 3 and 4 would be updated at instant t+1.

Answer 1

Maybe something like this:
X = c(2, 3, 4 )
Y = c(4, 3 ,5)
Z = c(8, 8 , 9, 6)
W = c(3, 3, 7,4)
V = c(9, 9, 9,3)
findeev<-function(vi){ #takes a list of vector(s) to extract from `allv`
nms=paste("v",vi,sep="")
ne=max(sapply(allv[vi],length)) #max number of elements
ine=1:ne
# Computes sd along the positions of all vetors
rest=apply(cbind(sapply(allv[vi],function(vt)vt[ine])),1,sd)
# determine if some vectors have the same value across a position
resc=which(rest==0)
if(length(resc)){ret=allv[vi];ret$pos=resc
names(ret)[-length(ret)]<-nms
ret} else invisible()
}
allv=list(X,Y,Z,W,V)
findeev(c(3,5)) #exmple
sol=list();j=1 # This contains all pairs
for(i in (length(allv)-1):2){
# compare all combinations of vector using `findeev`
res<-apply(t(combn(1:length(allv),i )),1,findeev)
if(is.null(res))next #not found continues
# eliminate NAs form list and assign that to sol[[j]]
sol[[j]]<-Filter(Negate(function(x) is.null(unlist(x))), res)
j=j+1
}
sol
You get:
> findeev(c(3,5))
$v3
[1] 8 8 9 6
$v5
[1] 9 9 9 3
$pos
[1] 3
> sol
[[1]]
[[1]][[1]]
[[1]][[1]]$v1
[1] 2 3 4
[[1]][[1]]$v2
[1] 4 3 5
[[1]][[1]]$v4
[1] 3 3 7 4
[[1]][[1]]$pos
[1] 2
[[2]]
[[2]][[1]]
[[2]][[1]]$v1
[1] 2 3 4
[[2]][[1]]$v2
[1] 4 3 5
[[2]][[1]]$pos
[1] 2
[[2]][[2]]
[[2]][[2]]$v1
[1] 2 3 4
[[2]][[2]]$v4
[1] 3 3 7 4
[[2]][[2]]$pos
[1] 2
[[2]][[3]]
[[2]][[3]]$v2
[1] 4 3 5
[[2]][[3]]$v4
[1] 3 3 7 4
[[2]][[3]]$pos
[1] 2
[[2]][[4]]
[[2]][[4]]$v3
[1] 8 8 9 6
[[2]][[4]]$v5
[1] 9 9 9 3
[[2]][[4]]$pos
[1] 3