See if Dictionary Item is the last one in the dictionary

Question

Given the code ..

var dictionary = new Dictionary<string, string>{
  { "something", "something-else" },
  { "another", "another-something-else" }
};

dictionary.ForEach( item => {
  bool isLast = // ... ? 

  // do something if this is the last item
});

I basically want to see if the item I am working with inside of the ForEach iteration is the last item in the dictionary. I tried

bool isLast = dictionary[ item.Key ].Equals( dictionary.Last() ) ? true : false;

but that did not work...

Have you tried using a counter and comparing the iterations through the foreach to the size of the dictionary? Dictionaries are theoretically unordered, so I wouldn't trust any comparison to Last value of the dictionary. — ben f., Jun 02 '11 at 14:55
This question doesn't make much sense because the `Dictionary` collection is not ordered. Take a look at this: [Why is a Dictionary “not ordered”?](https://stackoverflow.com/questions/6384710/why-is-a-dictionary-not-ordered) — Theodor Zoulias, Dec 02 '20 at 05:26

Chris Walsh · Accepted Answer · 2011-06-02T15:02:10.020

15

Dictionary.Last returns a KeyValuePair, and you are comparing that to just the value of a key. You'd instead need to check:

dictionary[item.Key].Equals( dictionary.Last().Value )

Also IAbstract was correct that you'd probably need to use an OrderedDictionary.

edited Jun 02 '11 at 15:02

answered Jun 02 '11 at 14:55

Chris Walsh

1,863
17
16

Thanks, I hadn't thought about this. – Ciel Jun 02 '11 at 15:09

score 2 · Answer 2 · answered Jun 02 '11 at 14:52

2

You will want to use an OrderedDictionary<TKey, TValue>. Check MSDN ref.

With the standard Dictionary, items are not guaranteed to be persisted in any specific order.

answered Jun 02 '11 at 14:52

IAbstract

19,551
15
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146

score 1 · Answer 3 · answered May 27 '16 at 15:16

Some people have mentioned to compare the value of the current item in the iteration to the last item's value, example:

    dictionary[item.Key].Equals(dictionary.Last().Value)

Warning: This could result in true for any item in the dictionary if the value of the item is equal to the value of the last item in the dictionary. This is not an indicator that the item is the last item in dictionary.

Instead, if you are really trying to find if the current item in the iteration is the last item, I would suggest comparing the Key because you know it is unique, So it might look something like:

    item.Key.Equals(dictionary.Last().Key)

score 1 · Answer 4 · answered Jun 02 '11 at 14:52

1

You could test if value == dictionary.Values.Last();

answered Jun 02 '11 at 14:52

Kelly Robins

7,168
6
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66

score 1 · Answer 5 · answered Jun 02 '11 at 16:37

1

Would it not be simpler to just perform the operation on the last item outside of the loop?

string requiredForSomething = dictionary.Last().Value;

answered Jun 02 '11 at 16:37

Jon Eastwood

813
10
22

score 1 · Answer 6 · answered Dec 02 '20 at 01:03

1

using the System.Linq namespace, we can do MyDictionary[item.Key].Equals( MyDictionary.Last().Key ); the Last() method should show us, every array, dictionary, list, stack, queue the last element of it.

answered Dec 02 '20 at 01:03

Cueia Dev

11
3

score 1 · Answer 7 · answered Feb 28 '12 at 14:07

You can always do this with a counter.

int itemsCount = yourDictionary.Count;
bool isLast = false;

foreach(var item in yourDictionary)
{
   itemsCount--;       
   isLast = itemsCount == 0; 

   if(isLast)
   {
     // this is the last item no matter the order of the dictionary        
   }
   else
  {
    //not the last item
  }

}

score 0 · Answer 8 · edited May 23 '17 at 12:31

First, there is no ForEach extension method for a Dictionary or even an IEnumerable. So you will have to fix that problem first.

Second, the Last extension method will be painfully slow since it has to enumerate the entire collection.

Third, I am not sure it makes a whole lot of sense to do something special on the last item from a collection with an unpredictable order, but that is mostly tangential to your specific question.

Here is how I would approach the problem. Create two new extensions methods that operate on IEnumerable<T> instances. ForEach will be the equivalent of the List<T>.ForEach method and WithIndex will return another enumerator that contains the sequential index and an IsLast flag. This is a variation of another one of my answers to a similiar problem.

dictionary.WithIndex().ForEach(
  (item) =>
  {
    var kvp = item.Value; // This extracts the KeyValuePair
    if (item.IsLast)
    {
      Console.WriteLine("Key=" + kvp.Key.ToString() + "; Value=" + kvp.Value.ToString());
    }
  });

Here are the new extension methods.

public static class ForEachHelperExtensions
{
    public sealed class Item<T>
    {
        public int Index { get; set; }
        public T Value { get; set; }
        public bool IsLast { get; set; }
    }

    public static void ForEach<T>(this IEnumerable<T> enumerable, Action<T> action)
    {
        foreach (T item in enumerable)
        {
            action(item);
        }
    }

    public static IEnumerable<Item<T>> WithIndex<T>(this IEnumerable<T> enumerable)
    {
        Item<T> item = null;
        foreach (T value in enumerable)
        {
            Item<T> next = new Item<T>();
            next.Index = 0;
            next.Value = value;
            next.IsLast = false;
            if (item != null)
            {
                next.Index = item.Index + 1;
                yield return item;
            }
            item = next;
        }
        if (item != null)
        {
            item.IsLast = true;
            yield return item;
        }
    }
}

See if Dictionary Item is the last one in the dictionary

8 Answers8