How to update value for a specific key in a dictionary Dictionary<string, int>
?
8 Answers
You can follow this approach:
void addOrUpdate(Dictionary<int, int> dic, int key, int newValue)
{
int val;
if (dic.TryGetValue(key, out val))
{
// yay, value exists!
dic[key] = val + newValue;
}
else
{
// darn, lets add the value
dic.Add(key, newValue);
}
}
The edge you get here is that you check and get the value of corresponding key in just 1 access to the dictionary.
If you use ContainsKey
to check the existance and update the value using dic[key] = val + newValue;
then you are accessing the dictionary twice.
This may work for you:
Scenario 1: primitive types
string keyToMatchInDict = "x";
int newValToAdd = 1;
Dictionary<string,int> dictToUpdate = new Dictionary<string,int>{"x",1};
if(!dictToUpdate.ContainsKey(keyToMatchInDict))
dictToUpdate.Add(keyToMatchInDict ,newValToAdd );
else
dictToUpdate[keyToMatchInDict] = newValToAdd; //or you can do operations such as ...dictToUpdate[keyToMatchInDict] += newValToAdd;
Scenario 2: The approach I used for a List as Value
int keyToMatch = 1;
AnyObject objInValueListToAdd = new AnyObject("something for the Ctor")
Dictionary<int,List<AnyObject> dictToUpdate = new Dictionary<int,List<AnyObject>(); //imagine this dict got initialized before with valid Keys and Values...
if(!dictToUpdate.ContainsKey(keyToMatch))
dictToUpdate.Add(keyToMatch,new List<AnyObject>{objInValueListToAdd});
else
dictToUpdate[keyToMatch] = objInValueListToAdd;
Hope it's useful for someone in need of help.
update - modify existent only. To avoid side effect of indexer use:
int val; if (dic.TryGetValue(key, out val)) { // key exist dic[key] = val; }
update or (add new if value doesn't exist in dic)
dic[key] = val;
for instance:
d["Two"] = 2; // adds to dictionary because "two" not already present d["Two"] = 22; // updates dictionary because "two" is now present
This extension method allows a match predicate delegate as the dictionary key selector, and a separate delegate to perform the dictionary value replacement, so it's completely open as to the type of key/value pair being used:
public static void UpdateAll<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, Func<TKey, TValue, bool> matchPredicate, Func<TValue, TValue> updatePredicate)
{
var keys = dictionary.Keys.Where(k => matchPredicate(k, dictionary[k])).ToList();
foreach (var key in keys)
{
dictionary[key] = updatePredicate(dictionary[key]);
}
}
Example usage:
Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "One");
dict.Add(2, "Two");
dict.Add(3, "Three");
//Before
foreach(var kvp in dict){
Console.WriteLine(kvp.Value);
}
dict.UpdateAll(
matchPredicate: (k, v) => k >= 2, //Update any dictionary value where the key is >= 2
updatePredicate: (v) => v = v + " is greater than One"
);
//After
foreach(var kvp in dict){
Console.WriteLine(kvp.Value);
}