Python 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 -1

Watch for: lo = mid instead of mid + 1 can loop for ever when two items are left.