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Possible Duplicate:
finding all numbers less than x in a BST
How would I modify a binary search to find the number of numbers in a sorted array that are less than a certain number?
If you have an already sorted array of numbers simply find the insertion point for your item in the sorted array usng a binary search algorithm. The index of the insertion point gives you the number of elements that are less than your target number.
In your comments you raised two good questions:
To handle this you keep searching until you find the point where the number should be if it were present, that is the index where the current element is greater than x and the previous element is less than x.
To handle this, instead of stopping when you first find an element, continue searching until you lower bound and upper bound meet. If you hit a value that is equal to x treat it in the same way as if you found a number that was too high and continue bisecting.