trying to append a list, but something breaks - list

Question

I'm trying to create an empty list which will have as many elements as there are num.of.walkers. I then try to append, to each created element, a new sub-list (length of new sub-list corresponds to a value in a.
When I fiddle around in R everything goes smooth:
list.of.dist[[1]] <- vector("list", a[1])
list.of.dist[[2]] <- vector("list", a[2])
list.of.dist[[3]] <- vector("list", a[3])
list.of.dist[[4]] <- vector("list", a[4])
I then try to write a function. Here is my feeble attempt that results in an error. Can someone chip in what am I doing wrong?
countNumberOfWalks <- function(walk.df) {
list.of.walkers <- sort(unique(walk.df$label))
num.of.walkers <- length(unique(walk.df$label))
#Pre-allocate objects for further manipulation
list.of.dist <- vector("list", num.of.walkers)
a <- c()
# Count the number of walks per walker.
for (i in list.of.walkers) {
a[i] <- nrow(walk.df[walk.df$label == i,])
}
a <- as.vector(a)
# Add a sublist (length = number of walks) for each walker.
for (i in i:num.of.walkers) {
list.of.dist[[i]] <- vector("list", a[i])
}
return(list.of.dist)
}
> num.of.walks.per.walker <- countNumberOfWalks(walk.df)
Error in vector("list", a[i]) : vector size cannot be NA

Answer 1

Assuming 'walk.df' is something like:
walk.df <- data.frame(label=sample(1:10,100,T),var2=1:100)
then:
countNumberOfWalks <- function(walk.df) {
list.of.walkers <- sort(unique(walk.df$label))
num.of.walkers <- length(unique(walk.df$label))
list.of.dist <- vector("list", num.of.walkers)
for (i in 1:num.of.walkers) {
list.of.dist[[i]] <- vector("list",
nrow(walk.df[walk.df$label == list.of.walkers[i],]))}
return(list.of.dist)
}
Will achieve what you're after.