Removing elements from a List which respect a condition on the index

I'm asked to create a function which removes all the elements from a list of integers respecting a specified condition i.e. the element is multiple of his index.

It is very easy to do it with iterations and things like that but I wanted to do it in a "cuter" way just to get into more complex writing codes, Linq and Lambdas.

That's what I've tried

public static List<int> MultipleOfIndex(List<int> xs) =>
    xs.RemoveAll(s => (s % xs.IndexOf(s) == 0));

but I get the following error: error CS0029: Cannot implicitly convert type 'int' to 'System.Collections.Generic.List<int>'

I can't really understand where I'm doing an implicit conversion since my logic is this: I want to remove all the elements s where s modulo his index is 0.
I'd like to have an explanation and also I'd like to see how can I also implement it with Where/Select clauses.

Thanks in advance!

If you want to modify the original list, you can use the following (note that you need to handle a divide-by-zero)

public static List<int> MultipleOfIndex(List<int> xs)
{
    xs.RemoveAll(s => xs.IndexOf(s) == 0 && s != 0 || s % xs.IndexOf(s) == 0);
    return xs;
}

A more efficient version might be:

public static List<int> MultipleOfIndex(List<int> xs)
{
    for (var i = xs.Count - 1; i >= 0; i--)
    {
        if(i == 0 && xs[i] != 0 || xs[i] % i == 0)
            xs.RemoveAt(i);
    }
    return xs;
}

If you want to create a new list, you can use LINQ

public static List<int> MultipleOfIndex(List<int> xs) =>
    xs.Where((s, index) => index == 0 && s == 0 || s % index != 0).ToList();

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# 答案2
**得分**: 0

我认为将列表映射到包含索引和值的元组列表会更好，其中第一个元素是索引，第二个元素是值，原因是 `removeAll()` 可能是并行的，从而使算法不一致。这样我认为你也不会遇到问题（尽管我没有尝试过这种方法）。

<details>
<summary>英文:</summary>

I think it would be better to map the list to a list of tuples, with the first element being the index and the second the value, the reason being that `removeAll()` might be parallel, thusly making the algorithm inconsistent. This way I believe that you also won&#39;t get issues (although I didn&#39;t try this).


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# 答案3
**得分**: 0

你只需使用 Where 谓词（存在索引为 0 的问题）

```csharp
var result = listofInt.Where((x, i) => i != 0 && x % i != 0).ToList();

i 表示 listOfInt 中项目 x 的索引。

var result = listofInt.Where((x, i) => i != 0 && x % i != 0).ToList();