C# - Working with values in a Dictionary

Question

This may be a simple question, but I can't figure out a simple way of doing this. I currently have a simple dictionary with KeyValuePairs like the following:

<1, 100> <2, 200> <3, -999> <4, 134> <5, -999> <6, 223> <7, 123>

The value's (-999) are an indicator for an error that occurs. What I am trying to do is iterate through the dictionary and for whenever there occurs a -999 use the value for the following key. The result, given the above example, would be:

<1, 100> <2, 200> <3, 134> <4, 134> <5, 223> <6, 223> <7, 123>

Any thoughts would be much appreciated!

Dictionaries are unordered, so I'm not sure it would go well. — Etienne de Martel, Jan 13 '11 at 21:17
Dictionaries are unordered?? That could explain a few things... I work a lot with KeyValuePairs (with unique keys), but the ordering is quite important. Is there a similar object that I could use?? — keynesiancross, Jan 13 '11 at 21:21
@Jason - the actual keys I'm using though are strings, and are not alphabetical. Would that make a difference? And also, is a List ordered? — keynesiancross, Jan 13 '11 at 21:25
List preserves the order in which items were added to the collection. — n8wrl, Jan 13 '11 at 21:29
I'm curious about what you're doing, using a number to indicate an error and then replacing that value with whatever the next value happens to be, while still keeping both the modified and unmodified pairs. — Anthony Pegram, Jan 13 '11 at 21:33
SortedDictionary does take a Comparer so you could enforce your own non-alphabetical ordering. — Damian, Jan 13 '11 at 21:44
is there a way that I could use an enumeration to have the SortedDictionary sort on? — keynesiancross, Jan 13 '11 at 21:53

score 4 · Accepted Answer · answered Jan 13 '11 at 21:19

You need to assume that your keys have an ordering (this is necessary because dictionaries otherwise don't have an ordering). You might want to consider using a SortedDictionary.

Either way, you can say:

var keys = dictionary.Keys.OrderByDescending(k => k);
int current = -999;
foreach (var key in keys) {
    if (dictionary[key] == -999) {
        dictionary[key] = current;
    }
    current = dictionary[key];
}

Note that you didn't specify what to do if the "last" item is -999. For now it remains at -999.

score 1 · Answer 2 · answered Jan 13 '11 at 21:28

Do you have access to the n+1-th element when you're adding the nth element? If so, you could substitute n+1-th value when you add nth key.

If not then you need an ordered collection. I'm not sure SortedDictionary would be what you need becuase the keys might not be your sort order? It is also puzzling that you list several K,V pairs with identical keys. Dictionaries won't like that.

Is the key within the V? Like an ID property? If so, You could try to derive a class from KeyedCollection which preserves the order in which you added items and lets you decide what property of the value is the key if it works that way for you. But again, no duplicate keys.

Whoops, that's my fault. When I did that above I did a bit of copy-pasting — keynesiancross, Jan 13 '11 at 21:28

score 0 · Answer 3 · answered Jan 13 '11 at 21:31

I'm going to assume that you want to process the values in the order they are added to the dictionary, and not the order they might happen to sort in. I think you might be better off using a Stack<>. This way you can push all the KeyValuePairs onto the stack, then pop them off backwards. When you find a -999 you can apply the value of the previous KeyValuePair that was popped off the stack and move on.

score 0 · Answer 4 · answered Jan 13 '11 at 21:41

If you don't need them in a dictionary you could use a linked list, then you could start at the end and loop through the list to update the -999 values.

LinkedList<KeyValuePair<int, int>> myList = GetMyKeyValues();

LinkedListNode<KeyValuePair<int, int>> curr = myList.Last;
LinkedListNode<KeyValuePair<int, int>> last = null;

while (curr != null)
{
    if (curr.Value == -999 && last != null)
    {
        curr.Value = last.Value;
    }

    last = curr;
    curr = curr.Previous;
}

C# - Working with values in a Dictionary

4 Answers4