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：我知道如何做X，Y提取部分：

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

时，X元素与Y元素配对

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可以放在任何地方。

但我对下一部分不太确定。我更熟悉SKILL programming language（SKILL是LISP派生词），这就是我写它的方式：

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
  )
)

在l上运行时，会返回

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

由于我还是 R 的新手，如果你们大家能提供帮助，我将不胜感激。此外，我确实理解 R 人们倾向于避免循环而更多地变成lapply？

Answer 1

也许这样的事情会起作用。（仅对您的样本数据进行了测试。）

## 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

Answer 2

我会将元组存储在一个列表中，即：

xypairs
[[1]]
[1] "x"    "y"

[[2]]
[2] "xb3"  "yb3"

您可以使用match和substring简化您的程序。

xends <- substring(xelems, 2)
yends <- substring(yelems, 2)
ypaired <- match(xends, yends)  # Indices of yelems that match xelems

# Now we need to handle the no-matches:
xsorted <- c(xelems, rep(NA, sum(is.na(ypaired))))
ysorted <- yelems[ypaired]
ysorted <- c(ysorted, yelems[!(yelems %in% ysorted)])

# Now we create the list of tuples:
xypairs <- lapply(1:length(ysorted), function(i) {
  c(xsorted[i], ysorted[i])
})

结果：

xypairs
[[1]]
[1] "x" "y"

[[2]]
[1] "xb3" "yb3"

[[3]]
[1] "xc3" NA   

[[4]]
[1] "xa1" "ya1"

[[5]]
[1] NA    "yd4"

R编程：创建配对元素列表

2 个答案: