How to iterate through Dictionary and change values?

Question

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

I get an error: "Property or indexer 'System.Collections.Generic.KeyValuePair.Value' cannot be assigned to -- it is read only."
How can I iterate through myDict and change values?

Answer 1

According to MSDN:

The foreach statement is a wrapper around the enumerator, which allows only reading from the collection, not writing to it.

Use this:

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);
}

Answer 2

For the lazy programmers:

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

Answer 3

You shouldn't change the dictionary while iterating it, otherwise you get an exception.

So first copy the key-value pairs to a temp list and then iterate through this temp list and then change your dictionary:

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();

output (on my machine):

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.

Answer 4

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

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

Answer 5

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

Answer 6

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);
}

Answer 7

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);
}

Answer 8

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);
}