How to modify key in a dictionary in C#

Question

How can I change the value of a number of keys in a dictionary.

I have the following dictionary :

SortedDictionary<int,SortedDictionary<string,List<string>>>

I want to loop through this sorted dictionary and change the key to key+1 if the key value is greater than a certain amount.

This question sounds like you are trying to manually insert an item and keep it sorted (specifically, making room for one). Note the SortedDictionary does this for you already. — Russell, Dec 21 '09 at 02:30

Dan Tao · Accepted Answer · 2013-03-19T14:36:44.077

50

As Jason said, you can't change the key of an existing dictionary entry. You'll have to remove/add using a new key like so:

// we need to cache the keys to update since we can't
// modify the collection during enumeration
var keysToUpdate = new List<int>();

foreach (var entry in dict)
{
    if (entry.Key < MinKeyValue)
    {
        keysToUpdate.Add(entry.Key);
    }
}

foreach (int keyToUpdate in keysToUpdate)
{
    SortedDictionary<string, List<string>> value = dict[keyToUpdate];

    int newKey = keyToUpdate + 1;

    // increment the key until arriving at one that doesn't already exist
    while (dict.ContainsKey(newKey))
    {
        newKey++;
    }

    dict.Remove(keyToUpdate);
    dict.Add(newKey, value);
}

edited Mar 19 '13 at 14:36

answered Dec 21 '09 at 02:33

Dan Tao

125,917
54
300
447

I think you might encounter an OutOfBoundsException with keyToUpdate +1 – Dec 21 '09 at 02:39
@Roboto: You might get an `OverflowException` if `keyToUpdate == int.MaxValue`; other than that, though, I don't see how adding two integers is going to give an `OutOfBoundsException`... – Dan Tao Dec 21 '09 at 03:41
@Roboto: Maybe you meant on adding the new key to the dictionary? In that case, no, the `SortedDictionary.Add` method doesn't throw an `OutOfBoundsException`, only `ArgumentNullException` and `ArgumentException`; see http://msdn.microsoft.com/en-us/library/7620520y.aspx. – Dan Tao Dec 21 '09 at 03:44

score 26 · Answer 2 · edited Dec 21 '09 at 03:37

26

You need to remove the items and re-add them with their new key. Per MSDN:

Keys must be immutable as long as they are used as keys in the SortedDictionary(TKey, TValue).

edited Dec 21 '09 at 03:37

Matt Davis

45,297
16
93
124

answered Dec 21 '09 at 02:19

jason

236,483
35
423
525

marcel · Answer 3 · 2015-09-13T17:50:17.553

5

You can use LINQ statment for it

var maxValue = 10
sd= sd.ToDictionary(d => d.key > maxValue ? d.key : d.Key +1, d=> d.Value);

edited Sep 13 '15 at 17:50

answered Sep 13 '15 at 17:44

marcel

313
4
10

score 2 · Answer 4 · answered Jan 22 '14 at 10:44

If you don't mind recreating the dictionary, you could use a LINQ statment.

var dictionary = new SortedDictionary<int, SortedDictionary<string, List<string>>>();
var insertAt = 10;
var newValues = dictionary.ToDictionary(
    x => x.Key < insertAt ? x.Key : x.Key + 1,
    x => x.Value);
return new SortedDictionary<int, SortedDictionary<string, List<string>>>(newValues);

or

var dictionary = new SortedDictionary<int, SortedDictionary<string, List<string>>>();
var insertAt = 10;
var newValues = dictionary.ToDictionary(
    x => x.Key < insertAt ? x.Key : x.Key + 1,
    x => x.Value);
dictionary.Clear();
foreach(var item in newValues) dictionary.Add(item.Key, item.Value);

No need to create a new to create a variable for it. See my comment — marcel, Sep 13 '15 at 17:51

How to modify key in a dictionary in C#

4 Answers4

Linked

Related