How to update value of a key in dictionary in c#?

Question

I have the following code in c# , basically it's a simple dictionary with some keys and their values.

Dictionary<string, int> dictionary =
    new Dictionary<string, int>();
dictionary.Add("cat", 2);
dictionary.Add("dog", 1);
dictionary.Add("llama", 0);
dictionary.Add("iguana", -1);

I want to update the key 'cat' with new value 5.
How could I do this?

Really? Did you try google/bing/prefered-search-engine first? — sovanesyan, Apr 12 '12 at 12:09
I did and this was the first result! way to be constructive! — Brandon Clapp, Aug 15 '14 at 15:26

J0HN · Answer 1 · 2012-04-12T12:07:29.997

41

Have you tried just

dictionary["cat"] = 5;

:)

Update

dictionary["cat"] = 5+2;
dictionary["cat"] = dictionary["cat"]+2;
dictionary["cat"] += 2;

Beware of non-existing keys :)

edited Apr 12 '12 at 12:07

answered Apr 12 '12 at 11:55

J0HN

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but what if i want to add new value into existing value ? – Ahsan Ashfaq Apr 12 '12 at 11:58
@AhsanAshfaq, what exactly do you mean by "add to existing one"? – walther Apr 12 '12 at 11:59
well @JHON it also works . Thanks budy you really help me thankyou very much – Ahsan Ashfaq Apr 12 '12 at 12:12
Just check if a key exist and update its value, if not then add a new dictionary entry. – cubski Apr 12 '12 at 12:13
thanks @cubski for your suggestion its a good one – Ahsan Ashfaq Apr 12 '12 at 12:14
@AhsanAshfaq: You can find a simple implementation below. – cubski Apr 12 '12 at 12:16

cubski · Answer 2 · 2013-11-05T15:27:36.047

19

Try this simple function to add an dictionary item if it does not exist or update when it exists:

    public void AddOrUpdateDictionaryEntry(string key, int value)
    {
        if (dict.ContainsKey(key))
        {
            dict[key] = value;
        }
        else
        {
            dict.Add(key, value);
        }
    }

This is the same as dict[key] = value.

edited Nov 05 '13 at 15:27

answered Apr 12 '12 at 12:07

cubski

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4

That is same as `dict[key] = value` in one line. Also I would call the funcation `AddOrUpdate` – nawfal Nov 05 '13 at 07:51
@nawfal They are not the same, the difference is it checks for the key first. If it exists, it adds the new value if not it creates a new entry. Good point on the function name though. – cubski Nov 05 '13 at 10:11
3

which is same as `dict[key] = value`.. What do you think `dict[key] = value` does? – nawfal Nov 05 '13 at 10:13
1

@nawfal You are right. I have always thought dict[key] would throw an exception when the key doesn't exist. You learn something new everyday. Thanks. – cubski Nov 05 '13 at 15:27
1

dict[key] will throw KeyNotfoundException when the key doesn't exist. You have to be setting it to create it. dict[key] = value – Dan But Mar 26 '15 at 04:35
@DanBut Actually, it doesn't. It will add the dictionary entry when the key doesn't exist and update the value of the entry if it does exist. – cubski Mar 26 '15 at 09:59
@DanBut I misunderstood your comment, you are right. When say doing dict[newKey] (without assigning a value), it would throw an exception. But when doing dict[newKey] = value, it wouldn't throw an exception instead will add new entry if the key value doesn't exist and update it if it does exist. Just want to clarify for others. – cubski Mar 26 '15 at 10:04
1

Yep I was not very clear. `var x = dict[key]` will throw when key does not exist. `dict[key] = x` will not throw when key does not exist – Dan But Mar 26 '15 at 20:47

score 0 · Answer 3 · answered Apr 12 '12 at 11:55

0

Just use the indexer and update directly:

dictionary["cat"] = 3

answered Apr 12 '12 at 11:55

yamen

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score 0 · Answer 4 · answered Apr 12 '12 at 12:13

0

Dictionary is a key value pair. Catch Key by

dic["cat"]

and assign its value like

dic["cat"] = 5

answered Apr 12 '12 at 12:13

Nikhil Agrawal

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How to update value of a key in dictionary in c#?

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