Get values from an array

Question

This code get a user to enter 5 numbers, stores the in an array, and works out the average

double average;
double variance;
int i;
int[] arr = new int[5]; // 5 size array

for (i = 0; i < 5; i++)
{
    Console.Write("\n Enter your number:\t");
    arr[i] = Convert.ToInt32(Console.ReadLine());
}

Console.WriteLine("\n");
average = arr.Sum() / 5.0;
Console.WriteLine("\n The average is {0}", average);
Console.ReadLine();

I am trying to work out the variance which is (number – average) * (number – average).

However I am unsure on how to pull out each number from the array and perform this calculation?

Any help much appreciated thanks

It's almost identical to what you did to put the values in. Just mimic that pattern. — Servy
– Servy, Commented May 27, 2015 at 18:00
Not sure why you compute average instead of using existing Average... would make sense if you compute sum with for, but you already use LINQ to get Sum... — Alexei Levenkov
– Alexei Levenkov, Commented May 27, 2015 at 18:09

MarvelousCactus · Accepted Answer · 2015-05-27 18:09:24Z

8

You can get the standard deviation as follows:

double average = arr.Average();
double sumOfSquaresOfDifferences = arr.Select(val => (val - average) * (val - average)).Sum();
double sd = Math.Sqrt(sumOfSquaresOfDifferences / arr.Length);

edited May 27, 2015 at 18:09

answered May 27, 2015 at 18:04

MarvelousCactus

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2 Comments

juharr Over a year ago

Or use var sd = arr.Select(v=>(v-average)*(v-average)).Average(); and skip the sumOfSquaresOfDifferences altogether.

Der Kommissar Over a year ago

@juharr That won't return the correct value. You forget the Math.Sqrt(double) portion. (Very important for Standard Deviation.)

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Your Question

Make an Extension Method.. not Tested but it should work if what is described here is correct

static class MathsExtensions
{
    public static double GetVariance(this double[] values)
    {
        var avg = values.Average();
        return values.Select(value => (value - avg) * (value - avg)).Sum() / values.Count();
    }
}

Just gave it a little test.

        double[] arr = { 8, 7, 9, 10, 6 };
        Console.WriteLine(arr.GetVariance());
        Console.ReadLine();

It returns 2 which - according to my link - is correct.

This might be helpful in the feauture

If you next Time have a similar Problem, you should propably just google like "how do i calculate variance" and then create your algorithm based on the theory. But alaways look up if there does already exist a solution

edited Jun 20, 2020 at 9:12

CommunityBot

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answered May 27, 2015 at 18:51

LuckyLikey

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1 Comment

LuckyLikey Over a year ago

I won't press you, but there's alaways the possibility to accept an answer :)

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