Three ways to test a vector to see if it contains a given element. Do not tell me instead of using functions,  you want to traverse the vector from first element toward the last.

1. match () : return the first appearance, if not exist return NA

> vt <- c('a', 'b', 'c')
> match('b', vt)
[1] 2
> match('d', vt)
[1] NA

2. %in% : return a Boolean

> vt <- c('a', 'b', 'c')
> 'a' %in% vt
[1] TRUE
> 'd' %in% vt
[1] FALSE

3. any () : Given a set of logical vectors, to see if at least one of the values is true

> vt <- c('a', 'b', 'c')
> any(vt=='a')
[1] TRUE
> any(vt=='d')
[1] FALSE

When the vector is big, the time cost is what need to be considered.  I do some simulation and it shows the efficiency ranking for these three functions is (shorter time first) :

any () > match () > %in%