Find closest number in a list
1，Problem
There are two ordered list, the list a and the list b, such as:
a=>[1,14,20,36]
b=>[2,7,11,25,32,39]
the result that I want to get is:
result=>[(2,1),(7,null),(11,14),(25,null),(32,null),(39,36)]
Put it into word: find the closest number(which delta between counterpart is less than 5) in list a for each item in list b.
First of all, I decide to divide the problem to:
1,find closest number for each item in a, since a is less elements than b.
2,compose a result into a list. if no proper value be found, put it to null.
Because a&b are ordered list, so binary search is fast way to find out corresponding closest value. The complexity of binary search for a ordered list is O(log(N)).Each of item in list a have O(N), so the total complexity is O(N*log(N))。
2，Code
How to use the code:
static void Main(string[] args)
{
int[] arrTime = { 1, 3, 5, 7, 8, 9, 10, 12, 15, 18, 25 };
int[] arrAlign = { 3, 6, 11, 15, 22 };
FindClosestNumber finder = new FindClosestNumber(3);
var lst = finder.FindClosestAlignment(arrTime, arrAlign);
foreach (var item in lst)
{
Console.WriteLine("{0}\t\t{1}", item.Time, item.Alignment);
}
Console.ReadKey();
}
There is the code:
public class Pair
{
public Pair(int? time, int? alignment)
{
Time = time;
Alignment = alignment;
}
public int? Time { set; get; }
public int? Alignment { set; get; }
}
public class FindClosestNumber
{
public int Threshold { get; set; }
public FindClosestNumber()
{
this.Threshold = 50;
}
public FindClosestNumber(int threshold)
{
this.Threshold = threshold;
}
public List<Pair> FindClosestAlignment(int[] arrTime, int[] arrAlignment)
{
List<Pair> list = FindClosestTime(arrTime, arrAlignment);
List<Pair> reslut = new List<Pair>();
for (int i = 0; i < arrTime.Length; i++)
{
Pair pair = new Pair(arrTime[i], null);
foreach (var p in list)
{
if (pair.Time == p.Time)
pair.Alignment = p.Alignment;
}
reslut.Add(pair);
}
return reslut;
}
private List<Pair> FindClosestTime(int[] arrTime, int[] arrAlignment)
{
List<Pair> list = new List<Pair>();
for (int i = 0; i < arrAlignment.Length; i++)
{
int el = arrAlignment[i];
int index = FindClosestIndexInOrderList(el, arrTime, this.Threshold);
if (index == -1)
{
list.Add(new Pair(null, el));
}
else
{
list.Add(new Pair(arrTime[index], el));
}
}
return list;
}
private int FindClosestIndexInOrderList(int val, int[] arr, int threshold)
{
int index = Array.BinarySearch(arr, val);
if (index >= 0)
{//found
return index;
}
else
{
//bitwise complement for index
int compl = (~index);
if (compl == arr.Length)
{//val is biggest value in the arr
if (Math.Abs(arr[arr.Length - 1] - val) > threshold)
return -1;
else
return (arr.Length - 1);
}
else if (compl == 0)
{//val is smallest value in the arr
if (Math.Abs(arr[0] - val) > threshold)
return -1;
else
return 0;
}
else
{
int preIndex = compl - 1;
int deltaToPrevious = Math.Abs(arr[preIndex] - val);
int deltaToCurrent = Math.Abs(arr[compl] - val);
if (deltaToPrevious > threshold)
return -1;
if (deltaToCurrent > threshold)
return -1;
return deltaToPrevious <= deltaToCurrent ? preIndex : compl;
}
}
}
}