试图附加一个列表,但有些内容中断

我正在尝试创建一个空列表,该列表将包含尽可能多的元素 num.of.walkers . 然后我尝试在每个创建的元素上附加一个新的子列表(新子列表的长度对应于 a .

当我在R里闲逛时,一切都很顺利:

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])

然后我试着写一个函数。这是我软弱的尝试,导致了一个错误。有人能插嘴我做错了什么吗?

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