相关疑难解决方法(0)

在Clojure中,使用惰性序列构造函数很容易创建无限序列.例如,

(def N (iterate inc 0))

返回一个N等价于无限序列的数据对象

(0 1 2 3 ...)

评估该值会N导致无限循环.评估(take 20 N)返回前20个数字.由于序列是惰性的,因此inc只有在您要求时才会迭代该函数.由于Clojure是同质的,因此递归地存储惰性序列.

在R中,是否可以做类似的事情？你能提出一些样本R代码,它产生的数据对象N相当于完全无限的自然数序列吗？评估完整对象N应该会产生一个循环,但类似的东西head(N)应该只返回前导数字.

注意:我对懒惰序列而不是自然数本身更感兴趣.

编辑:以下是Clojure的来源lazy-seq:

(defmacro lazy-seq
"Takes a body of expressions that returns an ISeq or nil, and yields
a Seqable object that will invoke the body only the first time seq
is called, and will cache the result and return it on all subsequent
seq calls. See also …

我正在尝试运行一些R代码,因为内存而崩溃.我得到的错误是:

Error in sendMaster(try(lapply(X = S, FUN = FUN, ...), silent = TRUE)) : 
  long vectors not supported yet: memory.c:3100

产生麻烦的功能如下:

StationUserX <- function(userNDX){
  lat1 = deg2rad(geolocation$latitude[userNDX])
  long1 = deg2rad(geolocation$longitude[userNDX])
  session_user_id = as.character(geolocation$session_user_id[userNDX])
  #Find closest station
  Distance2Stations <- unlist(lapply(stationNDXs, Distance2StationX, lat1, long1))
  # Return index for closest station and distance to closest station
  stations_userX = data.frame(session_user_id = session_user_id, 
                              station = ghcndstations$ID[stationNDXs], 
                              Distance2Station = Distance2Stations)    
  stations_userX = stations_userX[with(stations_userX, order(Distance2Station)), ]
  stations_userX = stations_userX[1:100,] #only the 100 closest stations...
  row.names(stations_userX)<-NULL
  return(stations_userX)
} …