R help - Apr 2011 - search functions for use on vector of ordered data

Hello,

I am looking for some search functions that deal with _ordered data_.
I suppose they already exist but I cannot find any reference of them.


Explanation:

I have a matrix, who's first column is ordered incrementally, with each 
value unique and regularly spaced.
(timestamps of observations: number of miliseconds since begining).

I want to be able to find the the column index of the value X listed in 
the matrix's first col.

The matrix is quite large, so the use of which(), or other similar 
functions are not time efficient enough.

Thanks,

Regards,

Jonathan Gabris