[R] how to combine apply and which or alternative ways to do so

[Guillaume Bal]

Dear useRs,

I am currently doing some data cleaning and data manipulation and I have the
following problem.
I have two vectors. Let say the size of the first one is 10 000 (vector 1)
and the size of the second one is 1 000 000 (vector 2).

I need to know for each cell of vector 1 which cells of vector 2 correspond.

I tried to combine the "which" and "apply"  but I dit not succeed and  I
have no idea about a possible vectorization.

Can someone help me ?

Thanks

Guillaume