The Binary Search Algorithm

In this lesson you will learn how to implement binary search in C#, and understand the thinking behind searching efficiently. This is for anyone searching for "C# binary search implementation".

Binary search finds a value efficiently in already-sorted data. Picture opening a dictionary in the middle and halving the range you still have to look through each time.

The sample code computes the midpoint mid from the two boundaries low and high, compares it with the target, and updates low or high to narrow the range. Compared with checking each item from the start, it finds the value dramatically faster as the data grows.

A common stumbling block for beginners is that binary search requires the array to be sorted first. Sorted input is a precondition, so do not skip that step.

In real-world development, there is also a built-in List<T>.BinarySearch(), so you can get the same behaviour without writing it yourself.

📖 Reference code

int BinarySearch(int[] arr, int target) {
    int low = 0, high = arr.Length - 1;
    while (low <= high) {
        int mid = (low + high) / 2;
        if (arr[mid] == target) return mid;
        if (arr[mid] < target) low = mid + 1;
        else high = mid - 1;
    }
    return -1;
}

int[] nums = { 1, 3, 5, 7, 9, 11 };
Console.WriteLine(BinarySearch(nums, 7));
Console.WriteLine(BinarySearch(nums, 4));

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