Finding First Unique Number

• Download source code from Github

Introduction

Recently, I took a test to find the first unique number in an array. I did manage to produce the right answer, but it was computationally too expensive. This post is the result of looking into alternatives.

Original Solution

var y = numberArray.GroupBy(z => z).Where(z => z.Count() == 1)
                                   .Select(z => z.Key).ToList();
 if (y.Count > 0) actual = y[0];

Alternatives

I asked for help on Stack Overflow as I wanted to see how other devs approached the solution and following are alternatives that where suggested:

1. Change from y.Count to y.Any
2. numberArray.GroupBy(g => g).Where(w => w.Count() == 1).Select(s => s.Key).FirstOrDefault();
3. numberArray.ToLookup(x => x).First(x => x.Count() == 1).Key;
4. numberArray.GroupBy(x => x).First(x => x.Count() == 1).Key;

In order to make head or tail of the speed difference, I decided to run some tests, the template for each test is.

var sw = new System.Diagnostics.StopWatch2() { ShowStatsForEachLoop = false };
int actual = 0;
//////////////////////////////////////
sw.Restart();
sw.Start();
for (int i = 0; i < Iterations; i++)
{
   ///
     Find comparison here
   ///
    sw.RestartAndLog();
}
sw.Stop();

Test Results

It was surprising how big a difference switching to use .Any() made. The other surprise was the difference between .GroupBy() and .ToLookup()

If you want speed, then you should look at using traditional loops instead of LINQ.

All solutions were run 50 times with an array of 9 numbers, the last number being the unique number. The results below record the average number of CPU ticks for each test.

Ticks
   Min   Max     Avg  Result
    2  14502     584  -3     Original
    2    919      40  -3     if (y.Any()) actual = y[0];
    2   1423      60  -3     a.GroupBy(g => g).Where(w => w.Count() == 1)
                                            .Select(s => s.Key)
                                            .FirstOrDefault();
    2   1553      65  -3     a.ToLookup(i => i).First(i => i.Count() == 1).Key;
    2    317      15  -3     a.GroupBy(i => i).First(i => i.Count() == 1).Key;
    1     4        2  -4     Traditional loops

Source Code

The source code can be found on gitHub @ https://github.com/hungrydev/FirstUniqueNumberExporation.