R编程: Creating a list of paired elements

Question

我有一个元素列表说:

l <- c("x","ya1","xb3","yb3","ab","xc3","y","xa1","yd4")

在这个列表中，我想列出匹配的 x,y 对，即

(("xa1" "ya1") ("xb3" "yb3") ("x" "y"))

本质上，我需要捕获 X 元素、Y 元素，然后将它们配对:我知道如何进行 X、Y 提取部分:

xelems <- grep("^x", l, perl=TRUE, value=TRUE)
yelems <- grep("^y", l, perl=TRUE, value=TRUE)

当

1. xElem == yElem # if xElem and yElem are one char long, i.e. 'x' and 'y'    
2. substr(xElem,1,nchar(xElem)) == substr(yElem,1,nchar(yElem))

没有顺序，即匹配的 xElem 和 yElem 可以放在任何位置。

但是我对下一部分不是很确定。我比较熟悉 SKILL programming language (SKILL 是 LISP 的衍生物)我是这样写的:

procedure( get_xy_pairs(inputList "l")
  let(( yElem (xyPairs nil) xList yList)
    xList=setof(i inputList rexMatchp("^x" i))
    yList=setof(i inputList rexMatchp("^y" i))
    when(xList && yList
      unless(length(xList)==length(yList)
    warn("xList and yList mismatch : %d vs %d\n" length(xList) length(yList))
      )
      foreach(xElem xList
        if(xElem=="x"
          then yElem="y"
          else yElem=strcat("y" substring(xElem 2 strlen(xElem)))
        )
        if(member(yElem yList)
          then xyPairs=cons(list(xElem yElem) xyPairs)
          else warn("x element %s has no matching y element \n" xElem)
        )
      )
    )
    xyPairs
  )
)

当在 l 上运行时，这将返回

get_xy_pairs(l)
*WARNING* x element xc3 has no matching y element 
(("xa1" "ya1") ("xb3" "yb3") ("x" "y"))

由于我对 R 还是个新手，如果你们能提供帮助，我将不胜感激。此外，我知道 R 的人倾向于避免 for 循环而更喜欢 lapply ？

Answer 1

也许这样的事情会奏效。 (仅在您的示例数据上进行了测试。)

## Remove any item not starting with x or y
l2 <- l[grepl("^x|^y", l)]

## Split into a list of items starting with x
##   and items starting with y
L <- split(l2, grepl("^x", l2))

## Give "names" to the "starting with y" group
names(L[[1]]) <- gsub("^y", "x", L[[1]])

## Use match to match the names in the y group with
##   the values from the x group. This results in a
##   nice named vector with the pairs you want
Matches <- L[[1]][match(L[[2]], names(L[[1]]), nomatch=0)]
Matches
#     x   xb3   xa1 
#   "y" "yb3" "ya1" 

作为data.frame:

MatchesDF <- data.frame(x = names(Matches), y = unname(Matches))
MatchesDF
#     x   y
# 1   x   y
# 2 xb3 yb3
# 3 xa1 ya1