Get the index of the values of one vector in another?

Question

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 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.

Answer 1

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 

Answer 2

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