R - check elements of vector against other vector

问题

I would like to match the elements of a column in a dataframe against another dataframe.

Consider these dataframes:

A=data.frame(par=c('long A story','long C story', 'blabla D'),val=1:3) 
B=data.frame(par=c('Z','D','A'),val=letters[1:3])

Each element of B column 'par' should be matched against A column par. If there is a match, it should be labeled in A. [This then gives a column of common values for merging A and B].

The desired result is therefore:

A=transform(A,label=c('A','NA','D'))

How can this be done?

Henk

回答1:

To do what you're asking for, try

A=data.frame(par=c('long A story','long C story', 'blabla D'),val=1:3) 
B=data.frame(par=c('Z','D','A'),val=letters[1:3])
A$label <- NA
for (x in B$par){
    is.match <- lapply(A$par,function(y) grep(x, y))
    A$label[which(is.match > 0)] <- x
}

(I assumed you meant a capital A in your example A=transform(a,label=c('A','NA','D')); in that case, these match exactly). EDIT: I see you made that edit. They do match then.

The above method will work only if there is exactly one B that fits every A (in other words, there can be multiple As to a B but not multiple Bs to an A). This is because of the structure you want in the output.

回答2:

Hi you can do something like this :

list <- lapply(1:length(B$par),function(x) grep(B$par[x],A$par))
list
[[1]]
integer(0)

[[2]]
[1] 3

[[3]]
[1] 1

label <- rep("NA",length(list))

B$par <-as.character(B$par)

label[unlist(list)] <- B$par[which(list != "integer(0)")]
label
[1] "A"  "NA" "D" 

A <- transform(A,label=label)
A
           par val label
1 long A story   1     A
2 long C story   2    NA
3     blabla D   3     D

Hope this helps.

回答3:

The approach I thought of:

M <- lapply(strsplit(as.character(A$par), " "), function(x) x[x %in% B$par])
M[sapply(M, function(x) {identical(x, character(0))})] <- NA
A$label <- unlist(M)
A

           par val label
1 long A story   1     A
2 long C story   2  <NA>
3     blabla D   3     D

Microbenchmarked the answers here and here are the results:

Unit: microseconds
       expr      min       lq   median       uq      max
1  EDWARD() 1638.815 1678.934 1698.061 1726.983 4973.823
2   SONAL()  705.348  725.874  734.738  747.334 2085.721
3     TLM()  268.705  281.300  287.831  294.362 1465.744
4 TRINKER()  156.278  168.407  173.538  177.737 1331.391

回答4:

Without loops in a handy function:

findkey <- function(key,terms) {
  result <- sapply(as.character(key),function(x) grepl(x,terms))
  result <- apply(result,1,function(x) names(x)[x==TRUE])
  result[(lapply(result,length)==0)] <- NA

  return(unlist(result))
}

Apply to current example:

A$label <- findkey(B$par,A$par)

Result:

> A
           par val label
1 long A story   1     A
2 long C story   2  <NA>
3     blabla D   3     D

来源：https://stackoverflow.com/questions/11667734/r-check-elements-of-vector-against-other-vector

标签

loops

indexing

grep

match