Get the index of the values of one vector in another?
I would guess this is a duplicate, but I can't find that so here goes...
I'd like to return the index of second开发者_StackOverflow in first:
first = c( "a" , "c" , "b" )
second = c( "c" , "b" , "a" )
result = c( 2 , 3 , 1 )
I guarantee that first and second have unique values, and the same values between the two.
Getting indexes of values is what match()
is for.
first = c( "a" , "c" , "b" )
second = c( "c" , "b" , "a" )
match(second, first)
[1] 2 3 1
I was solving related problem, selecting the elements of a vector based on a pattern. Lets say we have vector 'a' and we would like to find the occurrences of vector 'b'. Can be used in filtering data tables by a multiply search patterns.
a=c(1, 1, 27, 9, 0, 9, 6, 5, 7)
b=c(1, 9)
match(a, b)
[1] 1 1 NA 2 NA 2 NA NA NA
So match() it is not really useful here. Applying binary operator %in% is more convenient:
a %in% b
[1] TRUE TRUE FALSE TRUE FALSE TRUE FALSE FALSE FALSE
a[a %in% b]
[1] 1 1 9 9
Actually from the match() help %in% is just a wrap around match() function:
"%in%" <- function(x, table) match(x, table, nomatch = 0) > 0
here is another not efficient solution with which
and sapply
:
sapply(second, function(x) which(first==x)[1])
for every element of the second
returns the first index found in first
. if no match found NA is returned for that element
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