C# List .Add function but for arrays instead of Lists [duplicate]

Question

Let's say I have a completely empty array

int num = 1;
int[] arr = new int[0];

I just want to add the variable num to the arrat in the next possible position, like the .Add function for lists in C#. How would I do this without having to specify where I put it, only what to put. I do not want to use any Lists, basically just how the .Add function works for lists, but for arrays

I tried just using the .Add function but that didn't work. This was my full code:

public class Program
{
    public static int[] MultiplyByLength(int[] arr)
    {
        int[] finallist = new int[0];
        int curnum = 0;
        foreach(int item in arr)
        {
            curnum = item * arr.Length;
            finallist.Add(curnum);
        }
        return(finallist);
    }
}

But, clearly, it didn't work. how would I make the finallist.Add(curnum) line to work?

You wouldn't, because you can't add to an array... its length is fixed on creation. You'd have to create an array of the right size to start with. (Or potentially create a new array on each iteration, but that would be horribly inefficient.) — Jon Skeet
– Jon Skeet, Commented Aug 21, 2023 at 16:28
finallist = finallist.Concat(arr).ToArray(); This will add arr items to finallist array. See this link for more info: code-maze.com/add-values-to-csharp-array — SoftwareDveloper
– SoftwareDveloper, Commented Aug 21, 2023 at 16:31
I do not want to use any Lists why? Is there an actual reason for this or just an assumption? List<T> stores items in an internal array anyway. When a new item is added and the array is full, a new array with double the size is allocated and the existing data copied over. — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Aug 21, 2023 at 16:38
@SoftwareDveloper that's the slow way to do what List.Add also does. Concat returns an IEnumerable<T>. ToArray() internally adds items to a buffer with reallocation once it's empty. At least with List<T>(capacity) you can specify the expected size and avoid reallocations. — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Aug 21, 2023 at 16:45
This (my answer) is likely pertinent: stackoverflow.com/questions/51511209/…. Note that lists keep track of their capacity (how many elements have currently been allocated - though that is changeable) and how many elements have actually been added. That way, when you Add to a list, it knows where to do the insert (if the capacity runs out, it resizes itself internally). Arrays have no concept of "next possible position" — Flydog57
– Flydog57, Commented Aug 21, 2023 at 16:46

Guru Stron · Accepted Answer · 2023-08-21 16:29:07Z

how would I make the finallist.Add(curnum) line to work?

You can't, array's in C# have fixed size and it can't be changed after creation:

The number of dimensions and the length of each dimension are established when the array instance is created. These values can't be changed during the lifetime of the instance.

So the Add function does not make sense in case of arrays.

But you actually do not need it, just create an array of needed size and use for loop:

static int[] MultiplyByLength(int[] arr)
{
    int[] finallist = new int[arr.Length];
    for (var index = 0; index < arr.Length; index++)
    {
        var item = arr[index];
        var curnum = item * arr.Length;
        finallist[index] = curnum;
    }

    return finallist;
}

P.S.

There is Array.Resize but it actually does the following:

This method allocates a new array with the specified size, copies elements from the old array to the new one, and then replaces the old array with the new one. array must be a one-dimensional array.

(For the OP) the next evolution would be to look at LINQ and IEnumerables. The whole function can be a single line return arr.Select(x => x * arr.Length).ToArray();.
@gunr2171: although the for loop is more efficient. You know the final size of the array, so you can create it with the correct size. Your solution will always resize the array during enumeration. You could maybe use: var list = new List<int>(arr.Length); list.AddRange(arr.Select(x => x * arr.Length)); return list.ToArray();

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