Find most common numbers in array

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The question which algorithm I need to solve this problem:

I have an array of floats:
10.21313
10.456
10.234324
10.45758
11.4564747
10.45647
10.32425
9.34536
9.4578689
100.345345
129.3453
1.456456
10.345
10.235363
10.23425

I need to extract from this array only elements, which are most common, like 10.*, 11.* and 9.* - they are the nearest in value to each other. At a conceptual level, is there already an algorithm to do that or I have to invent my own?

What I have tried:

I have not tried anything yet.

Posted 28-Sep-20 22:31pm

csrss

Updated 28-Sep-20 23:16pm

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OriginalGriff · Answer 1 · 2020-09-28T22:49:00

To an extent, it's going to depend on what exactly you mean by "near": 10.4 is "near" 10, but is 10.5 near 10 or 11? What about 10.4999? 10.50001?
Decide on what constitutes "nearness" and you can work from there.

Then the way I'd do it would depend on the language and / or framework I was working in: a C# solution could be one line of code, an SQL one would be longer but use the same ideas. A javascript solution would be very different!

For example, in C# it's just this:

C#

double [] data = { 10.21313, 10.456, 10.234324, 10.45758, 11.4564747, 10.45647, 10.32425, 9.34536, 9.4578689, 100.345345, 129.3453, 1.456456, 10.345, 10.235363, 10.23425 };
var common = data.GroupBy(d => GetNearest(d)).Select(g => g.Key);

You'd have to write or replace the GetNearest method yourself, based on your decision as above.

Maciej Los · Answer 2 · 2020-09-28T23:17:00

In addition to OriginalGriff's solution... Please, read carefully, what OriginalGriff wrote.

As a GetNearest method you can use one of the following:
Math.Floor Method (System) | Microsoft Docs[^]
Math.Ceiling Method (System) | Microsoft Docs[^]
See the difference:

Value          Ceiling          Floor
7.03                8              7
7.64                8              7
0.12                1              0
-0.12                0             -1
-7.1               -7             -8
-7.6               -7             -8

You can also use Math.Round Method (System) | Microsoft Docs[^] or explicit conversion[^] to integer:

C#

double [] data = { 10.21313, 10.456, 10.234324, 10.45758, 11.4564747, 10.45647, 10.32425, 9.34536, 9.4578689, 100.345345, 129.3453, 1.456456, 10.345, 10.235363, 10.23425 };

var NearestDown = data
	.GroupBy(x=> (int)x)
	.Select(grp=> new
	{
		Key = grp.Key,
		Count = grp.Count(),
		Values = string.Join(";", grp.Select(x=>x))
	})
	.OrderByDescending(x=>x.Count)
	.ToList();

Result:

Key Count Values
10   9    10.21313;10.456;10.234324;10.45758;10.45647;10.32425;10.345;10.235363;10.23425 
9    2    9.34536;9.4578689 
11   1    11.4564747 
100  1    100.345345 
129  1    129.3453 
1    1    1.456456

Find most common numbers in array

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