我正试图从字典中建立一个饼图。在显示饼图之前，我想整理一下数据。我去掉了所有小于5%的派片，把它们放到“其他”派片里。然而，我得到一个集合被修改;枚举操作在运行时不能执行异常。
我理解为什么在遍历字典时不能从字典中添加或删除项。但是，我不明白为什么不能简单地在foreach循环中更改现有键的值。
任何建议:修复我的代码，将不胜感激。
Dictionary<string, int> colStates = new Dictionary<string,int>();
// ...
// Some code to populate colStates dictionary
// ...
int OtherCount = 0;
foreach(string key in colStates.Keys)
{
double Percent = colStates[key] / TotalCount;
if (Percent < 0.05)
{
OtherCount += colStates[key];
colStates[key] = 0;
}
}
colStates.Add("Other", OtherCount);
这个答案用于比较两个解决方案，而不是建议的解决方案。
您可以使用for循环，使用字典Count作为循环停止条件，并使用Keys.ElementAt(i)获取键，而不是像其他答案建议的那样创建另一个列表。
for (int i = 0; i < dictionary.Count; i++)
{
dictionary[dictionary.Keys.ElementAt(i)] = 0;
}
起初，我认为这将是更有效的，因为我们不需要创建一个键列表。在运行测试后，我发现for循环解决方案的效率要低得多。原因是ElementAt在字典上是O(n)。属性时，它从集合的开始搜索，直到找到第n个项。
测试:
int iterations = 10;
int dictionarySize = 10000;
Stopwatch sw = new Stopwatch();
Console.WriteLine("Creating dictionary...");
Dictionary<string, int> dictionary = new Dictionary<string, int>(dictionarySize);
for (int i = 0; i < dictionarySize; i++)
{
dictionary.Add(i.ToString(), i);
}
Console.WriteLine("Done");
Console.WriteLine("Starting tests...");
// for loop test
sw.Restart();
for (int i = 0; i < iterations; i++)
{
for (int j = 0; j < dictionary.Count; j++)
{
dictionary[dictionary.Keys.ElementAt(j)] = 3;
}
}
sw.Stop();
Console.WriteLine($"for loop Test: {sw.ElapsedMilliseconds} ms");
// foreach loop test
sw.Restart();
for (int i = 0; i < iterations; i++)
{
foreach (string key in dictionary.Keys.ToList())
{
dictionary[key] = 3;
}
}
sw.Stop();
Console.WriteLine($"foreach loop Test: {sw.ElapsedMilliseconds} ms");
Console.WriteLine("Done");
结果:
Creating dictionary...
Done
Starting tests...
for loop Test: 2367 ms
foreach loop Test: 3 ms
Done
从。net 4.5开始，你可以使用ConcurrentDictionary来实现:
using System.Collections.Concurrent;
var colStates = new ConcurrentDictionary<string,int>();
colStates["foo"] = 1;
colStates["bar"] = 2;
colStates["baz"] = 3;
int OtherCount = 0;
int TotalCount = 100;
foreach(string key in colStates.Keys)
{
double Percent = (double)colStates[key] / TotalCount;
if (Percent < 0.05)
{
OtherCount += colStates[key];
colStates[key] = 0;
}
}
colStates.TryAdd("Other", OtherCount);
但是请注意，它的性能实际上比简单的foreach dictionary差得多。
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
public class ConcurrentVsRegularDictionary
{
private readonly Random _rand;
private const int Count = 1_000;
public ConcurrentVsRegularDictionary()
{
_rand = new Random();
}
[Benchmark]
public void ConcurrentDictionary()
{
var dict = new ConcurrentDictionary<int, int>();
Populate(dict);
foreach (var key in dict.Keys)
{
dict[key] = _rand.Next();
}
}
[Benchmark]
public void Dictionary()
{
var dict = new Dictionary<int, int>();
Populate(dict);
foreach (var key in dict.Keys.ToArray())
{
dict[key] = _rand.Next();
}
}
private void Populate(IDictionary<int, int> dictionary)
{
for (int i = 0; i < Count; i++)
{
dictionary[i] = 0;
}
}
}
public class Program
{
public static void Main(string[] args)
{
BenchmarkRunner.Run<ConcurrentVsRegularDictionary>();
}
}
结果:
Method | Mean | Error | StdDev |
--------------------- |----------:|----------:|----------:|
ConcurrentDictionary | 182.24 us | 3.1507 us | 2.7930 us |
Dictionary | 47.01 us | 0.4824 us | 0.4512 us |
