Sorting arrays C#: key array changes unintendedly

Question

I am trying to sort different item arrays based on one key array. The following simple code represents my more complex code:

int[] first = new int[] { 1, 9, 2, 3, 8, 4, 5 };
string[] second = new string[] { "one", "nine", "two", "three", "eight", "four", "five" };

int[] temp = first;
Array.Sort(temp, second);

foreach (int v in temp)
{
    Debug.WriteLine(v.ToString());
}

foreach (string v in second)
{
    Debug.WriteLine(v);
}

int[] third = new int[] { 11, 99, 22, 33, 88, 44, 55 };

foreach (int v in first)
{
    Debug.WriteLine(v.ToString());
}

Array.Sort(first, third);

foreach (int v in first)
{
    Debug.WriteLine(v.ToString());
}

foreach (int v in third) 
{
    Debug.WriteLine(v);
}

The arrays, called 'second' and 'third', should be sorted based on the ordering of array 'first'. I found that I can do this with:

Array.Sort(first, second)

This perfectly works, until I add another Array.Sort to sort 'third'. Since I want to keep 'first' as key array for other sorting actions, I use a temporary array called 'temp' to save the initial sequence for 'first' so that I can reuse each time. When I reuse first to also sort 'third' using Array.Sort(first, third), the sorting does not work (see output). It seems that 'first' gets sorted together with 'temp' during the first Array.Sort, even though it is not in the command.

Output:

1
2
3
4
5
8
9

one
two
three
four
five
eight
nine

1   //--> 'first' before it is used in Array.Sort, so it seems already sorted
2
3
4
5
8
9

1
2
3
4
5
8
9

11  //--> 'third' does not get sorted because 'first' seemed already sorted
99
22
33
88
44
55

How do I make sure that my key array does not get sorted so that I can use it multiple times?

public static void Sort(Array keys, Array items); It will sort based on keys, which present in first array, which was already sorted. — Nilay Mehta
– Nilay Mehta, Commented Jul 14, 2017 at 10:08
yes @TimSchmelter, you can see in debug array first is already sorted and used as key for that method so it does not sort it's item array. Try with out sorting firstArray. — Nilay Mehta
– Nilay Mehta, Commented Jul 14, 2017 at 10:13
@Nilay: Array "first" was initially not sorted and my question was why "first" got sorted even though I didn't use "first" in the first Array.Sort. Instead I used "temp" and still "first" was sorted, even without a specific command from my side. InBetween's answer explains what I did wrong. — 10a
– 10a, Commented Jul 14, 2017 at 11:36

InBetween · Accepted Answer · 2017-07-14 10:25:52Z

3

An array is a reference type. A variable is a placeholder for a value. The value stored in a reference type variable is the "address" (so to speak) in memory of the object it is referencing. When you assign the value of one reference type variable to another you are simply copying the value stored in the variable.

So what does this have to do with anything? Everything!

int[] temp = first;

This line here is not doing what you think it should. It is simply copying the value stored in first to the variable named temp. And what was that value? Yup, the address of the array, so now, both temp and first point to the exact same array; so, whatever changes you make to the array vía temp will also change the array referenced by first because both are the same array.

You need to create a fresh copy of the array each time. The easiest way to do that is shown in Pablo notPicasso's answer.

edited Jul 14, 2017 at 10:25

answered Jul 14, 2017 at 10:20

InBetween

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1 Comment

10a Over a year ago

Thanks for this clarification!

Pablo notPicasso · Accepted Answer · 2017-07-14 10:05:32Z

3

Create new array:

Array.Sort(first.ToArray(), second);
Array.Sort(first.ToArray(), third);

answered Jul 14, 2017 at 10:05

Pablo notPicasso

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3 Comments

Chris Over a year ago

I would imagine first.Clone() would be better since I would expect it to be optimised for arrays which ToArray() probably won't be (though its possible it is).

InBetween Over a year ago

@Chris true, but IClonable is simply such a mess (you are never sure if it does what it should) that I simply run away from it whenever I can. ToArray is clear what it does, and unless there is a performance reason to do it otherwise, I like this solution better.

Enigmativity Over a year ago

@Chris - .ToArray() is optimized for arrays. It's effectively an Array.Copy call.

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