I have a two-dimensional array, sorted along one dimension:

e.g.

array(n,1) array(n,2)
10 10020
10 20127
10 30568
11 09654
11 21678
11 54689
12 65894
..
..
99 11111

I want to quickly find only those rows (n) within the array
that match a particular value so that I can limit some looping to only
that area of the array. For instance, let's say I was able to find out
that for the value "11", the lower bound is 4 (if n starts at 1) and
the upper bound is 6.

Any suggestion?