Binary search is one of the fundamental algorithms in computer science. In order to explore it, we'll first build up a theoretical backbone, then use that to implement the algorithm properly and avoid those nasty off-by-one errors everyone's been talking about.
Finding a value in a sorted sequence
In its simplest form, binary search is used to quickly find a value in a sorted sequence (consider a sequence an ordinary array for now). We'll call the sought value the target value for clarity. Binary search maintains a contiguous subsequence of the starting sequence where the target value is surely located. This is called the search space. The search space is initially the entire sequence. At each step, the algorithm compares the median value in the search space to the target value. Based on the comparison and because the sequence is sorted, it can then eliminate half of the search space. By doing this repeatedly, it will eventually be left with a search space consisting of a single element, the target value.
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