Question

Lets say that I have,

Dictionary<string, int> dict = new Dictionary<string, int>();

and in there are already some items:

“A”, 1

“B”, 15

“C”, 9

….

Now, as I’m adding new ones I’m checking if the key already exist:

for(int i = 0; i<n; i++)
    { 
        if (dict.ContainsKey(newKey[i] == true)
        { 
            //I should add newValue to existing value(sum all of them) of existing key pair
        }
        else
        {
            dict.Add(newKey[i],newValue[i]);
        }
    }

How should I summarize all values for existing key, add new value to existing vale for existing key pair?

Answer 1

The simplest approach would be this:

for(int i = 0; i < n; i++)
{
    int currentValue;
    // Deliberately ignore the return value
    dict.TryGetValue(newKey[i], out currentValue);
    dict[newKey[i]] = currentValue + newValue[i];
}

This does a single “get” and then a single “put” for every key. It uses the fact that the default value of int is 0 – when TryGetValue returns false, currentValue will be set to 0, which is appropriate for a new entry.