我知道c#中实例化的值类型数组会自动填充该类型的默认值(例如bool为false, int为0，等等)。
是否有一种方法来自动填充一个不是默认的种子值的数组?无论是在创建或之后的内置方法(如Java的Arrays.fill())?假设我想要一个默认为true的布尔数组，而不是false。是否有一个内置的方法来做到这一点，或者你只需要通过一个for循环迭代数组?
// Example pseudo-code:
bool[] abValues = new[1000000];
Array.Populate(abValues, true);
// Currently how I'm handling this:
bool[] abValues = new[1000000];
for (int i = 0; i < 1000000; i++)
{
abValues[i] = true;
}
必须遍历数组并将每个值“重置”为true似乎效率不高。还有其他方法吗?也许通过翻转所有值?
在输入这个问题并思考之后，我猜默认值只是c#在幕后处理这些对象的内存分配的结果，所以我想这可能是不可能的。但我还是想确定一下!
如果你计划只设置数组中的几个值，但大多数时候想要获得(自定义)默认值，你可以尝试这样做:
public class SparseArray<T>
{
private Dictionary<int, T> values = new Dictionary<int, T>();
private T defaultValue;
public SparseArray(T defaultValue)
{
this.defaultValue = defaultValue;
}
public T this [int index]
{
set { values[index] = value; }
get { return values.ContainsKey(index) ? values[index] ? defaultValue; }
}
}
您可能需要实现其他接口以使其有用，例如在数组本身上的接口。
并行实现怎么样
public static void InitializeArray<T>(T[] array, T value)
{
var cores = Environment.ProcessorCount;
ArraySegment<T>[] segments = new ArraySegment<T>[cores];
var step = array.Length / cores;
for (int i = 0; i < cores; i++)
{
segments[i] = new ArraySegment<T>(array, i * step, step);
}
var remaining = array.Length % cores;
if (remaining != 0)
{
var lastIndex = segments.Length - 1;
segments[lastIndex] = new ArraySegment<T>(array, lastIndex * step, array.Length - (lastIndex * step));
}
var initializers = new Task[cores];
for (int i = 0; i < cores; i++)
{
var index = i;
var t = new Task(() =>
{
var s = segments[index];
for (int j = 0; j < s.Count; j++)
{
array[j + s.Offset] = value;
}
});
initializers[i] = t;
t.Start();
}
Task.WaitAll(initializers);
}
当只初始化一个数组时，这段代码的力量是看不出来的，但我认为你绝对应该忘记“纯”for。
这里给出的许多答案都可以归结为一个循环，每次初始化数组中的一个元素，它没有利用设计为一次操作内存块的CPU指令。
. net Standard 2.1(在撰写本文时的预览版中)提供了Array.Fill()，这有助于在运行时库中实现高性能(尽管到目前为止，. net Core似乎还没有利用这种可能性)。
For those on earlier platforms, the following extension method outperforms a trivial loop by a substantial margin when the array size is significant. I created it when my solution for an online code challenge was around 20% over the allocated time budget. It reduced the runtime by around 70%. In this case, the array fill was performed inside another loop. BLOCK_SIZE was set by gut feeling rather than experiment. Some optimizations are possible (e.g. copying all bytes already set to the desired value rather than a fixed-size block).
internal const int BLOCK_SIZE = 256;
public static void Fill<T>(this T[] array, T value)
{
if (array.Length < 2 * BLOCK_SIZE)
{
for (int i = 0; i < array.Length; i++) array[i] = value;
}
else
{
int fullBlocks = array.Length / BLOCK_SIZE;
// Initialize first block
for (int j = 0; j < BLOCK_SIZE; j++) array[j] = value;
// Copy successive full blocks
for (int blk = 1; blk < fullBlocks; blk++)
{
Array.Copy(array, 0, array, blk * BLOCK_SIZE, BLOCK_SIZE);
}
for (int rem = fullBlocks * BLOCK_SIZE; rem < array.Length; rem++)
{
array[rem] = value;
}
}
}
