Question

In C# i have a List which contains numbers in string format. Which is the best way to count all this numbers? For example to say i have three time the number ten..

I mean in unix awk you can say something like

tempArray["5"] +=1

it is similar to a KeyValuePair but it is readonly.

Any fast and smart way?

Answer 1

(As noted in digEmAll’s answer, I’m assuming you don’t really care that they’re numbers – everything here assumes that you wanted to treat them as strings.)

The simplest way to do this is to use LINQ:

var dictionary = values.GroupBy(x => x)
                       .ToDictionary(group => group.Key, group => group.Count());

You could build the dictionary yourself, like this:

var map = new Dictionary<string, int>();
foreach (string number in list)
{
    int count;
    // You'd normally want to check the return value, but in this case you
    // don't care.
    map.TryGetValue(number, out count);
    map[number] = count + 1;
}

… but I prefer the conciseness of the LINQ approach 🙂 It will be a bit less efficient, mind you – if that’s a problem, I’d personally probably create a generic “counting” extension method:

public static Dictionary<T, int> GroupCount<T>(this IEnumerable<T> source)
{
    if (source == null)
    {
        throw new ArgumentNullException("source");
    }
    var map = new Dictionary<T, int>();
    foreach (T value in source)
    {
        int count;
        map.TryGetValue(number, out count);
        map[number] = count + 1;
    }
    return map;
}

(You might want another overload accepting an IEqualityComparer<T>.) Having written this once, you can reuse it any time you need to get the counts for items:

var counts = list.GroupCount();