사전을 반복하고 값을 변경하는 방법은 무엇입니까?

사전을 반복하고 값을 변경하는 방법은 무엇입니까?

---

Dictionary<string,double> myDict = new Dictionary();
//...
foreach (KeyValuePair<string,double> kvp in myDict)
 {
     kvp.Value = Math.Round(kvp.Value, 3);
}

"속성 또는 인덱서 'System.Collections.Generic.KeyValuePair.Value'를 할당 할 수 없습니다. 읽기 전용입니다."라는 오류가 발생합니다. 값을
반복 myDict하고 변경하려면 어떻게해야합니까?

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MSDN 에 따르면 :

foreach 문은 열거자를 둘러싼 래퍼로, 컬렉션에 쓰기가 아닌 읽기만 허용합니다.

이것을 사용하십시오 :

var dictionary = new Dictionary<string, double>();
// TODO Populate your dictionary here
var keys = new List<string>(dictionary.Keys);
foreach (string key in keys)
{
   dictionary[key] = Math.Round(dictionary[key], 3);
}

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게으른 프로그래머의 경우 :

Dictionary<string, double> dictionary = new Dictionary<string, double>();
foreach (var key in dictionary.Keys.ToList())
{
   dictionary[key] = Math.Round(dictionary[key], 3);
}

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반복하는 동안 사전을 변경해서는 안됩니다. 그렇지 않으면 예외가 발생합니다.

따라서 먼저 키-값 쌍을 임시 목록에 복사 한 다음이 임시 목록을 반복 한 다음 사전을 변경합니다.

Dictionary<string, double> myDict = new Dictionary<string, double>();

// a few values to play with
myDict["a"] = 2.200001;
myDict["b"] = 77777.3333;
myDict["c"] = 2.3459999999;

// prepare the temp list
List<KeyValuePair<string, double>> list = new List<KeyValuePair<string, double>>(myDict);

// iterate through the list and then change the dictionary object
foreach (KeyValuePair<string, double> kvp in list)
{
    myDict[kvp.Key] = Math.Round(kvp.Value, 3);
}


// print the output
foreach (var pair in myDict)
{
    Console.WriteLine(pair.Key + " = " + pair.Value);
}

// uncomment if needed
// Console.ReadLine();

출력 (내 컴퓨터에서) :

a = 2.2
b = 77777.333
c = 2.346

Note: in terms of performance, this solution is a bit better than currently posted solutions, since the value is already assigned with the key, and there's no need to fetch it again from the dictionary object.

---

passed some time, but maybe someone is interested in it:

yourDict = yourDict.ToDictionary(kv => kv.Key, kv => Math.Round(kv.Value, 3))

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I noticed that fastest way (at this moment) iterate over Dictionary with modify is:

//Just a dumb class
class Test<T>
{
    public T value;

    public Test() { }
    public Test(T v) { value = v; }
}

Dictionary<int, Test<object>> dic = new Dictionary<int, Test<object>>();
//Init dictionary
foreach (KeyValuePair<int, Test> pair in dic)
{
    pair.Value.value = TheObject;//Modify
}

VS

List<int> keys = new List<int>(dic.Keys); //This is fast operation   
foreach (int key in keys)
{
    dic[key] = TheObject;
}

First one takes about 2.2s and second one 4.5s (tested dictionary size of 1000 and repeated 10k time, changing dictionary size to 10 didn't change the ratios). Also there wasn't a big deal with getting the Key list, dictionary[key] value get is just slow VS built in iteration. Also if you want even more speed use hard coded type to dumb ("Test") class, with that I got it about 1.85s (with hard coded to "object").

EDIT:

Anna has posted the same solution before: https://stackoverflow.com/a/6515474/766304

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One solution would be to put the keys in a list (or another collection) beforehand and iterate through them while changing the dictionary:

Dictionary<string, double> dictionary = new Dictionary<string, double>();

// Populate it
List<string> keys = new List<string>(dictionary.Keys);

foreach (string key in keys)
{
   dictionary[key] = Math.Round(dictionary[key], 3);
}

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While iterating over the dictionary directly is not possible because you get an exception (like Ron already said), you don't need to use a temp list to solve the problem.

Instead use not the foreach, but a for loop to iterate through the dictionary and change the values with indexed access:

Dictionary<string, double> myDict = new Dictionary<string,double>();
//...    
for(int i = 0; i < myDict.Count; i++) {
    myDict[myDict.ElementAt(i).Key] = Math.Round(myDict.ElementAt(i).Value, 3);
}

---

Loop through the keys in the dictionary, not the KeyValuePairs.

Dictionary<string, double> myDict = new Dictionary<string, double>();
//...
foreach (string key in myDict.Keys)
{
    myDict[key] = Math.Round(myDict[key], 3);
}

참고URL : https://stackoverflow.com/questions/2260446/how-to-iterate-through-dictionary-and-change-values