How to get index of all values of not sorted array based on lower value C#-CodePudding

I am currently working on a simulation system so i've an array like

int[] arr = {2,5,9,10,0, 4,1,5,3};

I want array of indexes of values based on lower and output result like

result = {4, 6, 0, 8, 1, 7, 2, 3};

I searched all over for almost 3 days i can't find.

CodePudding user response：

In the 2nd array you want the indexes of the elements in the first array as sorted in ascending order.

You can use LINQ to do this

int[] arr = { 2, 5, 9, 10, 0, 4, 1, 5, 3 };
int[] result = arr.Select((x, i) => (x, i))
    .OrderBy(t => t.x)
    .Select(t => t.i)
    .ToArray();

Here, we used an overload of Select that yields the index: Select<TSource,TResult>(IEnumerable, Func<TSource,Int32,TResult>).

The first Select creates a ValueTuple.

The test

Console.WriteLine(String.Join(", ", result));

Yields the result:

4, 6, 0, 8, 5, 1, 7, 2, 3

Note that the number 5 appears twice in the input array. Therefore, the result is ambiguous. (Your expected result has only 8 indexes but the input array has a length of 9)

My full .NET 6.0 test code (Console App):

namespace CoreConsoleApp;

internal static class SortedArrayIndexes
{
    public static void Test()
    {
        //  Indexes   0  1  2   3  4  5  6  7  8
        int[] arr = { 2, 5, 9, 10, 0, 4, 1, 5, 3 };
        int[] result = arr.Select((x, i) => (x, i))
            .OrderBy(t => t.x)
            .Select(t => t.i)
            .ToArray();
        Console.WriteLine(String.Join(", ", result));
    }
}

It is called in my Main method with:

SortedArrayIndexes.Test();
Console.ReadKey();

In case you are working with an older Framework or language version, you can also use the older System.Tuple Class

int[] result = arr.Select((x, i) => new Tuple<int, int>(x, i))
    .OrderBy(t => t.Item1)
    .Select(t => t.Item2)
    .ToArray();

CodePudding user response：

We can use the built-in Array.Sort() method. It has a second parameter that returns the sorted indices as well, but it requires us to make a new array with the indices (i.e. starts from 0 and ends at the length of the original array)

int[] arr = {2,5,9,10,0,4,1,5,3};
int[] indexArr = Enumerable.Range(0, arr.Length).ToArray();
Array.Sort(arr, indexArr);

Note that you will need to add using System.Linq; if you get an error that Enumerable doesn't exist.