有没有在C＃中列出℃的方法; T>像C ++的调整矢量< T> [英] is there in C# a method for List<T> like resize in c++ for vector<T>

问题描述

当我使用调整（INT newsize）在C ++中的矢量< T> ，这意味着尺寸矢量> 设置为 newsize 和指标运行在范围 [0..newsize）。如何做到在C＃中同为列表< T> 结果
更改列表< T> 属性容量只改变了容量，但离开计数相同，此外指标仍然在范围内的 [0..Count）。帮助我，请。

When I use resize(int newsize) in C++ for vector<T>, it means that the size of this vector are set to newsize and the indexes run in range [0..newsize). How to do the same in C# for List<T>?
Changing the List<T> property Capacity only changes the Capacity but leaves the Count the same, and furthermore the indexes still are in range [0..Count). Help me out, please.

P.S。想象一下，我有一个矢量< T> TMP 与 tmp.size（）== 5 我不能引用 TMP [9] ，但是当我再使用 tmp.resize（10）我可参照 TMP [9] 。在C＃中，如果我有列表< T> TMP 与 tmp.Count == 5 我不能引用 TMP [9] （ IndexOutOfRangeException ），但即使当我设置 tmp.Capacity = 10 我将无法引用 TMP [9] 堂妹的 tmp.Count 还是5.我想找到大小调整的一些比喻在C＃。

P.S. Imagine I have a vector<T> tmp with a tmp.size() == 5 I cannot refer to tmp[9], but when I then use tmp.resize(10) I may refer to tmp[9]. In C# if I have List<T> tmp with tmp.Count == 5 I cannot refer to tmp[9] (IndexOutOfRangeException), but even when I set tmp.Capacity=10 I will not be able to refer to tmp[9] coz of tmp.Count is still 5. I want to find some analogy of resize in C#.

推荐答案

没有，但你可以使用扩展方法来添加自己的。以下具有相同的行为为的std ::矢量< T> ::调整（），其中包括同时复杂性。唯一的区别是，在C ++中，我们可以定义无效调整大小默认（SIZE_TYPE SZ，T C = T（））和方式的模板工作意味着那很好，如果我们叫它不默认为 T 有没有可访问的参数的构造函数。在C＃中，我们不能做到这一点，所以不是我们要创建一个方法，没有约束的非默认使用的情况下匹配，另一个与，其中新（）约束在调用它

No, but you can use extension methods to add your own. The following has the same behaviour as std::vector<T>::resize(), including the same time-complexity. The only difference is that in C++ we can define a default with void resize ( size_type sz, T c = T() ) and the way templates work means that that's fine if we call it without the default for a T that has no accessible parameterless constructor. In C# we can't do that, so instead we have to create one method with no constraint that matches the non-default-used case, and another with a where new() constraint that calls into it.

public static class ListExtra
{
    public static void Resize<T>(this List<T> list, int sz, T c)
    {
        int cur = list.Count;
        if(sz < cur)
            list.RemoveRange(sz, cur - sz);
        else if(sz > cur)
        {
            if(sz > list.Capacity)//this bit is purely an optimisation, to avoid multiple automatic capacity changes.
              list.Capacity = sz;
            list.AddRange(Enumerable.Repeat(c, sz - cur));
        }
    }
    public static void Resize<T>(this List<T> list, int sz) where T : new()
    {
        Resize(list, sz, new T());
    }
}

现在 myList中的喜欢.Resize（23）或 myList.Resize（23 myDefaultValue）将匹配什么人从C ++的矢量的期望。我注意到，虽然有时在那里与C ++，你就会有指针的载体，在C＃中你有一定的参考类型的列表。因此，在所在的C ++ T（）产生一个空指针（因为它是一个指针）的情况下，我们在这里期待它来调用参数的构造函数。出于这个原因，你可能会发现它更接近你习惯的行为，以取代与第二种方法：

Now the likes of myList.Resize(23) or myList.Resize(23, myDefaultValue) will match what one expects from C++'s vector. I'd note though that sometimes where with C++ you'd have a vector of pointers, in C# you'd have a list of some reference-type. Hence in cases where the C++ T() produces a null pointer (because it's a pointer), here we're expecting it to call a parameterless constructor. For that reason you might find it closer to the behaviour you're used to to replace the second method with:

  public static void Resize<T>(this List<T> list, int sz)
  {
      Resize(list, sz, default(T));
  }

这与值类型（调用参数的构造函数），但具有相同的效果引用类型，它就会用null填充。在这种情况下，我们就可以重写整个类：

This has the same effect with value types (call parameterless constructor), but with reference-types, it'll fill with nulls. In which case, we can just rewrite the entire class to:

public static class ListExtra
{
    public static void Resize<T>(this List<T> list, int sz, T c = default(T))
    {
        int cur = list.Count;
        if(sz < cur)
            list.RemoveRange(sz, cur - sz);
        else if(sz > cur)
            list.AddRange(Enumerable.Repeat(c, sz - cur));
    }
}

请注意，这是不是这么多的差异的std ::矢量< T> 和列表< T> 约在指针是如何在C中使用的差异++和C＃。

Note that this isn't so much about differences between std::vector<T> and List<T> as about the differences in how pointers are used in C++ and C#.

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