Dictionary enumeration in c#

Question

Hi,

I want to enumerate dictionary. How i enumerate.

Thanks

Answer 1

To enumerate a dictionary you either enumerate the values within it:

Dictionary<int, string> dic;

foreach(string s in dic.Values)
{
   Console.WriteLine(s);
}

or the KeyValuePairs

foreach(KeyValuePair<int, string> kvp in dic)
{
   Console.WriteLine("Key : " + kvp.Key.ToString() + ", Value : " + kvp.Value);
}

or the keys

foreach(int key in dic.Keys)
{
    Console.WriteLine(key.ToString());
}

Answer 2

Foreach. There are three ways: You can enumerate over the Keys property, over the Values property or over the dictionary itself which is an enumerator of KeyValuePair<TKey, TValue>.

Matthias

Answer 3

Hi lan,

Suppose i use foreach() for dictionay enumeration . I can't update value/key inside foreach(). So i want some other methos

Thanks,

Answer 4

In answer to the problem "I can't update value/key inside foreach()", you cannot modify a collection while enumerating it. I would approach this by making a copy of the Keys collection:

Dictionary<int,int> dic=new Dictionary<int, int>();

//...fill the dictionary

int[] keys = dic.Keys.ToArray();
foreach (int i in keys)
{
    dic.Remove(i);
}

Answer 5

Indeed as Spender pointed out you can't modify the thing you are enumerating over. You could do one of two things:

1) Follow Spender's approach by creating a copy.

List<int> keys = new List<int>();

foreach(KeyValuePair<int,String> kvp in dic)
{
   if(needToRemoveKey)
      keys.Add(kvp.Key);
}

for(int i = 0; i < keys.Count; i++)
{
   //dic.Remove(keys[i]);
   //dic[keys[i]] = newValue;

   //etc 
}

2) Or a bit harder use a for loop and decrement the counter (but you can run into problems more easily so I won't post any code for this because I don't want to encourage you to do it).

Answer 6

I just answered the same (updated) question for lists, so here's the same thing for dictionaries.

public static void MutateEach(this IDictionary<TKey, TValue> dict, Func<TKey, TValue, KeyValuePair<TKey, TValue>> mutator)
{
    var removals = new List<TKey>();
    var additions = new List<KeyValuePair<TKey, TValue>>();

    foreach (var pair in dict)
    {
        var newPair = mutator(pair.Key, pair.Value);
        if ((newPair.Key != pair.Key) || (newPair.Value != pair.Value))
        {
            removals.Add(pair.Key);
            additions.Add(newPair);
        }
    }

    foreach (var removal in removals)
        dict.Remove(removal);

    foreach (var addition in additions)
        dict.Add(addition.Key, addition.Value);
}

Note that we have to do the updates outside the loop, so we aren't modifying the dictionary as we enumerate it. Also this detects clashes caused by making two keys the same - it will throw (due to the use of Add).

Example - make all keys lowercase and trim all values, with a Dictionary<string, string>:

myDict.MutateEach(key => key.ToLower(), value => value.Trim());

If the keys are not unique when made lowercase, this will throw.