c# Dictionary how to loop through items without knowing key

Question

I have a:

var selectedDates = new Dictionary<string, string>();
selectedDates.Add("2014-06-21", DateTime.Now.ToLongDateString());
selectedDates.Add("2014-07-21", DateTime.Now.AddDays(5).ToLongDateString());
selectedDates.Add("2014-08-21", DateTime.Now.AddDays(9).ToLongDateString());
selectedDates.Add("2014-09-21", DateTime.Now.AddDays(14).ToLongDateString());

How can I loop trough items without knowing the key?

For example I want to get the value of the item[0]

If I do:

var item = selectedDates[0].value; // I get an error

why use a dictionary then? Would an array or list not do the job? — Lee Willis, Jul 28 '14 at 14:15
Could you explain if you don't know how to loop over a dictionary or if you have problem to refer to a specific entry in the Dictionary without knowing the key value? — Steve, Jul 28 '14 at 14:23

score 9 · Answer 1 · edited Jun 20 '20 at 09:12

9

How can I loop trough items without knowing the key?

For example I want to get the value of the item[0]

You want to treat the dictionary as (ordered) collection similar to a list or array and get the first item in it?

You can because a Dictionary<string, string> is an IEnumerable<KeyValuePair<string, string>> implicitly. Just use First or FirstOrDefault:

string valueAtFirstPosition = selectedDates.First().Value;

However, note that a dictionary is not meant to be used as as an ordered collection. It is a collection which can be used to fast-lookup a value by a key. But you can enumerate it anyway.

foreach(KeyValuePair<string, string>keyVal in selectedDates)
{
    Console.WriteLine("Key: {0} Value: {1}", keyVal.Key, keyVal.Value);
}

You should simply not rely on that order. I think in the current implementation the order is stable as long as you don't delete items. Read

Read: Why is a Dictionary “not ordered”?

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 28 '14 at 14:14

Tim Schmelter

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who upvotes this? "how to loop through items" and you say "use first"? this answer is wrong – Dbl Jul 28 '14 at 14:18
@AndreasMüller: i understood "item[0]" as "give me the first item" and "loop trough items" as "use it as collection". – Tim Schmelter Jul 28 '14 at 14:19
@TimeSchmelter question asks for something else. using [0] is OP's way of expressing that he doesn't know how to use a dictionary, and is refusing to read msdn api – Dbl Jul 28 '14 at 14:21
@AndreasMüller: how do you know? OP is trying to find `0`(`int`) as key even if he adds `DateTime`s (as `string`)? Isn't that more unlikely as that he wants the first? – Tim Schmelter Jul 28 '14 at 14:22
he is adding string keys and expects the indexer to work with an integer. No. If you're using a dictionary that's not what you would want. If he would want the first item he HOPEFULLY would be using a list instead – Dbl Jul 28 '14 at 14:25
@AndreasMüller: He doesnt expect it to work, he just wants to use it like a list if he wants to access the first. That's why i've mentioned that a dictionary is not meant to be used as as an ordered collection. However, the order is stable as long as you don't modify the dictionary later (to be exact: delete items). – Tim Schmelter Jul 28 '14 at 14:28
fair enough. In this case you probably guessed his intentions correct and he just doesn't know what collection to use. – Dbl Jul 28 '14 at 14:30

score 3 · Answer 2 · answered Jul 28 '14 at 14:14

3

try this

  foreach (string key in selectedDates.Keys)
  {
        var item = selectedDates[key]; 
  }

answered Jul 28 '14 at 14:14

nsgocev

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6

though this is an amuzing way to do it, wouldn't a foreach(var v in sselectedDates){ var date = v.Value;} be better? – Vincent Jul 28 '14 at 14:20

score 2 · Answer 3 · answered Jul 28 '14 at 14:17

2

It's simple, loop trough it with a foreach or to get a specific index do:

var date = selectedDates.ElementAt(0).Value;

answered Jul 28 '14 at 14:17

Vincent

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Matthias Meid · Answer 4 · 2014-07-28T14:30:49.223

Let me put together two things for you. Firstly, you can loop or use LINQ to access elements, just as you could do it in a list as well:

var dict = new Dictionary<string, string>();

// loop
foreach (var item in dict)
{
    var key = item.Key;
    var value = item.Value;
}

// "first" (see below)
var firstItem = dict.First();

However, be aware that what you're referring to as the first item can be pretty much any item in the Dictionary. Dictionaries store elements in any order that is convenient for a lookup (so do sets).

This order is known for some implementations, but lists or arrays might fit better when the order of the elements is important. A Dictionary in .NET is an implementation of a hash table data structure (tree map is another map implementation).

score 1 · Answer 5 · answered Jul 28 '14 at 14:14

1

try this :

foreach(var key in selectedDates.Keys)
{
    var value = selectedDates[key];
}

answered Jul 28 '14 at 14:14

Parimal Raj

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As Vincent already commented at nsgocev's answer: *"wouldn't a foreach(var v in sselectedDates){ var date = v.Value;} be better?"* – Jul 28 '14 at 14:22

score 0 · Answer 6 · answered Jul 28 '14 at 14:13

0

Use this overload of Where:

var result = selectedDates.Where((d,i)=>i==0);

answered Jul 28 '14 at 14:13

Giannis Paraskevopoulos

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score 0 · Answer 7 · answered Jul 28 '14 at 14:14

0

Try:

foreach (var date in selectedDates)                
{                  
    var item = date.Value;              
}

answered Jul 28 '14 at 14:14

JKennedy

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c# Dictionary how to loop through items without knowing key

7 Answers7