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Return last match from vector

Is there an easy way to get the index of the last match of a vector?

lastInflectionRow(c(TRUE,FALSE,TRUE,FALSE,FALSE))

lastInflectionRow<-function(temp){
    m<-match(TRUE,temp,nomatch=NA)
    m
}

GOAL: 3

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4 answers

Another simple way would be to use maxelements for the index TRUE.

x <- c(TRUE,FALSE,TRUE,FALSE,FALSE)

max(which(x))

#[1] 3
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?Positioncreated for this kind of thing when using an argument right=TRUE. Everything below should be essentially equivalent.

Position(I, x, right=TRUE)
#[1] 3
Position(identity, x, right=TRUE)
#[1] 3
Position(isTRUE, x, right=TRUE)
#[1] 3
Position(function(x) x, x, right=TRUE)
#[1] 3
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==,

tail(which(v1 == TRUE),1)
#[1] 3

== ,

tail(which(v1),1)
#[1] 3

. , OP TRUE/FALSE, .

match,

v1 <- c(TRUE,FALSE,TRUE,FALSE,FALSE)
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If performance is a consideration, the best way I have found for this is

length(x) + 1L - match(TRUE, rev(x))

This is much faster, especially in the general case when one person has the right to the rightmost match for more than one entry.

MatchLast <- function (needles, haystack) # This approach
  length(haystack) + 1L - match(needles, rev(haystack))  

MaxWhich <- function (needles, haystack)  # Ronak Shah approach
  vapply(needles, function (needle) max(which(haystack==needle)), integer(1))

Pos <- function (needles, haystack)       # thelatemail suggestion
  vapply(needles, function (needle) 
    Position(function (x) x == needle, haystack, right=TRUE),
    integer(1))

Tail <- function (needles, haystack)      # akrun solution
  vapply(needles, function (needle) tail(which(haystack==needle), 1), integer(1))

With Rilkon42 data:

x <- c(TRUE, FALSE, TRUE, FALSE, FALSE)
microbenchmark(MatchLast(TRUE, x), MaxWhich(TRUE, x), Pos(TRUE, x), Tail(TRUE, x))

##  function    min      lq     mean  median      uq       max
## MatchLast 10.730 19.1270 175.3851 23.7920  28.458 14757.131   
##  MaxWhich 11.663 22.1600 275.4657 25.1920  28.224 24355.120       
##       Pos 25.192 47.5845 194.1296 52.7160  64.612 12890.622  
##      Tail 39.187 69.7435 223.1278 83.0395 101.233  9223.848

In a more general case:

needles <- 24:45
haystack <- c(45, 45, 44, 44, 43, 43, 42, 42, 41, 41, 40, 40, 39, 39, 38, 38, 37, 37,
  36, 36, 35, 35, 34, 34, 33, 33, 32, 32, 31, 31, 30, 30, 29, 29, 28, 28,
  27, 27, 26, 26, 25, 25, 24, 24)

microbenchmark(MatchLast(needles, haystack), MaxWhich(needles, haystack),
               Pos(needles, haystack), Tail(needles, haystack))

##  function     min       lq      mean    median       uq      max
## MatchLast  15.395  30.3240  137.3086   36.8550   48.051 9842.441
##  MaxWhich  90.971 102.1665  161.1100  172.3765  214.829  238.854
##       Pos 709.563 733.8220 1111.7000 1162.7780 1507.530 1645.383
##      Tail 654.981 690.2035 1017.7400  882.6385 1404.197 1595.933
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