This is the classic binary search algorithm for finding the location of a value in a sorted array.

``````public class BinarySearchGood {

//@ requires sortedArray != null && 0 < sortedArray.length < Integer.MAX_VALUE;
//@ requires \forall int i; 0 <= i < sortedArray.length; \forall int j; i < j < sortedArray.length; sortedArray[i] <= sortedArray[j];
//@ old boolean containsValue = (\exists int i; 0 <= i < sortedArray.length; sortedArray[i] == value);
//@ ensures containsValue <==> 0 <= \result < sortedArray.length;
//@ ensures !containsValue <==> \result == -1;
//@ pure
public static int search(int[] sortedArray, int value) {
//@ ghost boolean containsValue = (\exists int i; 0 <= i < sortedArray.length; sortedArray[i] == value);
if (value < sortedArray[0]) return -1;
if (value > sortedArray[sortedArray.length-1]) return -1;
int lo = 0;
int hi = sortedArray.length-1;
//@ loop_invariant 0 <= lo < sortedArray.length && 0 <= hi < sortedArray.length;
//@ loop_invariant containsValue ==> sortedArray[lo] <= value <= sortedArray[hi];
//@ loop_invariant \forall int i; 0 <= i < lo; sortedArray[i] < value;
//@ loop_invariant \forall int i; hi < i < sortedArray.length; value < sortedArray[i];
//@ loop_decreases hi - lo;
while (lo <= hi) {
int mid = lo + (hi-lo)/2;
if (sortedArray[mid] == value) {
return mid;
} else if (sortedArray[mid] < value) {
lo = mid+1;
} else {
hi = mid-1;
}
}
return -1;
}
}
``````