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Binary search
Find a number in a sorted list with binary search: look at the middle and keep whichever half can still contain it. Print its position (from 0), or -1.
Input: sorted whole numbers on one line (all different), then the number to find. Output: its position or -1
Examples
Input
1 3 5 7 9 11 7
Output
3
Input
1 3 5 4
Output
-1
Input
2 2
Output
0
2 more tests check other inputs when you press Check.
Stuck? The idea behind searching
Linear search looks at each item in turn. Binary search works on a sorted list: look at the middle, then throw away the half that can't hold the answer.
Binary search finds an item among a million in about 20 steps.
def find(a, target):
lo, hi = 0, len(a) - 1
while lo <= hi:
mid = (lo + hi) // 2
if a[mid] == target:
return mid
if a[mid] < target:
lo = mid + 1
else:
hi = mid - 1
return -1Watch for: lo = mid instead of mid + 1 can loop for ever when two items are left.