Dictionary linq select previous

Question

How to get a previous element in select(foreach) some like this dic[index -1].Key

new Dictionary<int, double>();

dic.Select(x => x.Value*(x.Equals(dic.FirstOrDefault()) ? x.Key : //x.Key - dic[index -1].Key//)).Sum()

Dictionaries are unordered. `FirstOrDefault()` will not work correctly here. — SLaks, Dec 03 '14 at 15:25
Can you expand your answer and tell us why you need to do this? It'll help us help you — Binary Worrier, Dec 03 '14 at 15:25
Dictionaries don't have an order. If you used an `OrderedDictionary` then a `foreach` may be cleaner since you'll need to keep track of the "last" key. — D Stanley, Dec 03 '14 at 15:26
possible duplicate of [how do access previous item in list using linQ?](http://stackoverflow.com/questions/4460106/how-do-access-previous-item-in-list-using-linq) — Daniel Edholm Ignat, Dec 03 '14 at 15:26

Vsevolod Goloviznin · Accepted Answer · 2014-12-03T15:35:40.100

3

Select method provides an index as a second parameter.

dic.Select((x, index) => dict[index])

If you want to select an element at previous index you can use ElementAt

dic.Select((x, index) => dic.ElementAt(index).Key)

edited Dec 03 '14 at 15:35

answered Dec 03 '14 at 15:24

Vsevolod Goloviznin

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And how would you apply that to the problem? `dict[index]` will NOT select the "nth" item; it will return the value where `index` is the key. – D Stanley Dec 03 '14 at 15:28
but how access to key because dict[index].(value only) - my dictionary contain for ex: 23,10.00; 150,56.7; and i need to getting exactly key – AleksP Dec 03 '14 at 15:34
@AleksP I've updated the answer. In fact I think it would be easier done with just an ordinary `for` loop – Vsevolod Goloviznin Dec 03 '14 at 15:36
@AleksP You still need to deal with the fact that a `Dictionary` isn't necessarily ordered by its key. – D Stanley Dec 03 '14 at 15:40

score 0 · Answer 2 · answered Dec 03 '14 at 15:32

If you use an OrderedDictionary then it may be faster to use a straight foreach:

double sum = 0.0;
int prev = 0;
foreach(var kvp in dic)
{
    sum += kvp.Value * (kvp.Key - prev);
    prev = kvp.Key;
}

In any case, I would first come up with something that works and then refactor it if you can make it better. And "better" may not be using Linq; a convoluted Linq query that's hard to understand and debug is worse than a clean, efficient loop in most cases.

Dictionary linq select previous

2 Answers2