Dictionary ++ not working as expected

Question

Let's say we have a dictionary like this:

var dic = new Dictionary<Int32, Int32>();

Whereas the key is the ID and the value is the count. Now we want to add a new key. This works perfectly fine with the following line:

dic[1] = 1; //adding ID 1, Count 1 to the current Dictionary

Assuming we have a list of integer with the following values:

var ids = new List<int> {1, 2, 3 , 1, 2};

Where we would like to get a dictionary with the following content:

[1, 2] ==> (ID 1, Count 2)
[2, 2] ==> (ID 2, Count 2)
[3, 1] ==> (ID 3, Count 1)

The obvious solution would be:

foreach (var id in ids)
{
    dic[id]++;
}

But this is throwing a KeyNotFoundException. So obviously the operator += is not supported for dictionaries.

I already attached an answer to this problem. Is there any better way of how to achieve this?

Answer 1

My solution:

var dic = new Dictionary<Int32, Int32>();
var ids = new List<int> { 1, 2, 3, 1, 2 };
foreach (var id in ids)
{
    if (dic.ContainsKey(id))
    {
        dic[id]++;
    } else {
        dic[id] = 1;
    }
}

Answer 2

You can use LINQ for simpler:

var dic = ids.GroupBy(id => id)
             .ToDictionary(g => g.Key, g => g.Count());