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Beginner python, about keys and array [duplicate]

I only want the Keys and not the Values of a Dictionary.
I haven't been able to get any code to do this yet. Using another array proved to be too much work as I use remove also.
How do I get a List of the Keys in a Dictionary?

List<string> keyList = new List<string>(this.yourDictionary.Keys);

You should be able to just look at .Keys:
Dictionary<string, int> data = new Dictionary<string, int>();
data.Add("abc", 123);
data.Add("def", 456);
foreach (string key in data.Keys)
{
Console.WriteLine(key);
}

Update for .NET 3.5+
To get list of all keys:
using System.Linq;
List<String> myKeys = myDict.Keys.ToList();
If you face any issues using System.Linq, see the following:
Visual Studio Does not recognize System.Linq
System.Linq Namespace

Marc Gravell's answer should work for you. myDictionary.Keys returns an object that implements ICollection<TKey>, IEnumerable<TKey> and their non-generic counterparts.
I just wanted to add that if you plan on accessing the value as well, you could loop through the dictionary like this (modified example):
Dictionary<string, int> data = new Dictionary<string, int>();
data.Add("abc", 123);
data.Add("def", 456);
foreach (KeyValuePair<string, int> item in data)
{
Console.WriteLine(item.Key + ": " + item.Value);
}

I can't believe all these convoluted answers. Assuming the key is of type: string (or use 'var' if you're a lazy developer): -
List<string> listOfKeys = theCollection.Keys.ToList();

The question is a little tricky to understand but I'm guessing that the problem is that you're trying to remove elements from the Dictionary while you iterate over the keys. I think in that case you have no choice but to use a second array.
ArrayList lList = new ArrayList(lDict.Keys);
foreach (object lKey in lList)
{
if (<your condition here>)
{
lDict.Remove(lKey);
}
}
If you can use generic lists and dictionaries instead of an ArrayList then I would, however the above should just work.

Or like this:
List< KeyValuePair< string, int > > theList =
new List< KeyValuePair< string,int > >(this.yourDictionary);
for ( int i = 0; i < theList.Count; i++)
{
// the key
Console.WriteLine(theList[i].Key);
}

For a hybrid dictionary, I use this:
List<string> keys = new List<string>(dictionary.Count);
keys.AddRange(dictionary.Keys.Cast<string>());

I often used this to get the key and value inside a dictionary: (VB.Net)
For Each kv As KeyValuePair(Of String, Integer) In layerList
Next
(layerList is of type Dictionary(Of String, Integer))

how to iterate a dictionary<string,string> in reverse order(from last to first) in C#?

I have one Dictionary and added some elements on it.for example,
Dictionary<string, string> d = new Dictionary<string, string>();
d.Add("Content","Level0");
d.Add("gdlh","Level1");
d.Add("shows","Level2");
d.Add("ytye","Level0");
In C#, Dictionary keeps elements in natural order.But i now want to iterate those values from last to first(ie, Reverse order).i mean,
first i want to read ytye then shows,gdlh and finally Content.
Please guide me to get out of this issue...

Just use the Linq extension method Reverse
e.g.
foreach( var item in d.Reverse())
{
...
}

Use LINQ Reverse, but note that does not reverse in place:
var reversed = d.Reverse();
But note that this is not a SortedDictionary, so the order is not necessarily guaranteed in the first place. Perhaps you want OrderByDescending instead?

Maybe OrderByDescending on the key. Like this:
d.OrderByDescending (x =>x.Key)
Foreach like this:
foreach (var element in d.OrderByDescending (x =>x.Key))
{
}

It's available with Linq: d.Reverse()

Get index of a key/value pair in a C# dictionary based on the value

I would like to know if some property or method exists that gets the index of a specific value.
I found that dictionaries have the Contains() method which returns true if the value passed in exists, so this method almost implements what I need.
I know that I can loop through all the value pairs and check the condition, but I ask because maybe there's an optimized way of doing this.

Let's say you have a Dictionary called fooDictionary
fooDictionary.Values.ToList().IndexOf(someValue);
Values.ToList()
converts your dictionary values into a List of someValue objects.
IndexOf(someValue)
searches your new List looking for the someValue object in question
and returns the Index which would match the index of the Key/Value pair in the dictionary.
This method does not care about the dictionary keys, it simply returns the index of the value that you are looking for.
This does not however account for the issue that there may be several matching "someValue" objects.

There's no such concept of an "index" within a dictionary - it's fundamentally unordered. Of course when you iterate over it you'll get the items in some order, but that order isn't guaranteed and can change over time (particularly if you add or remove entries).
Obviously you can get the key from a KeyValuePair just by using the Key property, so that will let you use the indexer of the dictionary:
var pair = ...;
var value = dictionary[pair.Key];
Assert.AreEqual(value, pair.Value);
You haven't really said what you're trying to do. If you're trying to find some key which corresponds to a particular value, you could use:
var key = dictionary.Where(pair => pair.Value == desiredValue)
.Select(pair => pair.Key)
.FirstOrDefault();
key will be null if the entry doesn't exist.
This is assuming that the key type is a reference type... if it's a value type you'll need to do things slightly differently.
Of course, if you really want to look up values by key, you should consider using another dictionary which maps the other way round in addition to your existing dictionary.

Consider using System.Collections.Specialized.OrderedDictionary, though it is not generic, or implement your own (example).
OrderedDictionary does not support IndexOf, but it's easy to implement:
public static class OrderedDictionaryExtensions
{
public static int IndexOf(this OrderedDictionary dictionary, object value)
{
for(int i = 0; i < dictionary.Count; ++i)
{
if(dictionary[i] == value) return i;
}
return -1;
}
}

You can find index by key/values in dictionary
Dictionary<string, string> myDictionary = new Dictionary<string, string>();
myDictionary.Add("a", "x");
myDictionary.Add("b", "y");
int i = Array.IndexOf(myDictionary.Keys.ToArray(), "a");
int j = Array.IndexOf(myDictionary.Values.ToArray(), "y");

You can use LINQ to help you with this.
Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "hi");
dict.Add(2, "NotHi");
dict.Add(3, "Bah");
var item = (from d in dict
where d.Value == "hi"
select d.Key).FirstOrDefault();
Console.WriteLine(item); //Prints 1

If searching for a value, you will have to loop through all the data. But to minimize code involved, you can use LINQ.
Example:
Given Dictionary defined as following:
Dictionary<Int32, String> dict;
You can use following code :
// Search for all keys with given value
Int32[] keys = dict.Where(kvp => kvp.Value.Equals("SomeValue")).Select(kvp => kvp.Key).ToArray();
// Search for first key with given value
Int32 key = dict.First(kvp => kvp.Value.Equals("SomeValue")).Key;

In your comment to max's answer, you say that what you really wanted to get is the key in, and not the index of, the KeyValuePair that contains a certain value. You could edit your question to make it more clear.
It is worth pointing out (EricM has touched upon this in his answer) that a value might appear more than once in the dictionary, in which case one would have to think which key he would like to get: e.g. the first that comes up, the last, all of them?
If you are sure that each key has a unique value, you could have another dictionary, with the values from the first acting as keys and the previous keys acting as values. Otherwise, this second dictionary idea (suggested by Jon Skeet) will not work, as you would again have to think which of all the possible keys to use as value in the new dictionary.
If you were asking about the index, though, EricM's answer would be OK. Then you could get the KeyValuePair in question by using:
yourDictionary.ElementAt(theIndexYouFound);
provided that you do not add/remove things in yourDictionary.
PS: I know it's been almost 7 years now, but what the heck. I thought it best to formulate my answer as addressing the OP, but of course by now one can say it is an answer for just about anyone else but the OP. Fully aware of that, thank you.

no , there is nothing similar IndexOf for Dictionary although you can make use of ContainsKey method to get whether a key belongs to dictionary or not

A dictionary with multiple entries with the same key [duplicate]

This question already has answers here:
C# dictionary - one key, many values
(15 answers)
Closed 8 years ago.
I need a Dictionary like object that can store multiple entries with the same key. Is this avaliable as a standard collection, or do I need to roll my own?
To clarify, I want to be able to do something like this:
var dict = new Dictionary<int, String>();
dict.Add(1, "first");
dict.Add(1, "second");
foreach(string x in dict[1])
{
Console.WriteLine(x);
}
Output:
first
second

In .NET 3.5 you can use a Lookup instead of a Dictionary.
var items = new List<KeyValuePair<int, String>>();
items.Add(new KeyValuePair<int, String>(1, "first"));
items.Add(new KeyValuePair<int, String>(1, "second"));
var lookup = items.ToLookup(kvp => kvp.Key, kvp => kvp.Value);
foreach (string x in lookup[1])
{
Console.WriteLine(x);
}
The Lookup class is immutable. If you want a mutable version you can use EditableLookup from MiscUtil.

I would recommend doing something like this:
var dict = new Dictionary<int, HashSet<string>>();
dict.Add(1, new HashSet<string>() { "first", "second" });

Dictionary<T,K> does not support such behavior and there's no collection in the base class library providing such behavior. The easiest way is to construct a composite data structure like this:
var data = new Dictionary<int, List<string>>();
As the second parameter you should use a collection which provides the qualities you are looking for, i.e. stable order ⇒ List<T>, fast access HashSet<T>, etc.

You definitely want to use NameValueCollection:
using System.Collections.Specialized;
NameValueCollection nvc = new NameValueCollection();
nvc.Add("pets", "Dog");
nvc.Add("pets", "Rabbit");
Console.WriteLine(nvc["pets"]);
//returns Dog,Rabbit

What you're looking for isn't actually a Dictionary in the traditional sense (see Associative Array).
There's no class, as far as I'm aware, that offers this in the framework (System.Linq.Lookup doesn't expose a constructor), but you could create a class yourself that implements ILookup<TKey, TElement>

You could perhaps use a Dictionary on your primary key, in which each element is a List or other collection on your secondary key. To add an item to your data structure, see if the primary key exists. If not, create a new single-item list with your Value and store it in the dictionary. If the primary key does exist, add your Value to the list that's in the dictionary.

Using LINQ to find all keys from one collection that are not in another?

I'm trying to locate all the keys in one Dictionary that are not in another Dictionary. Obviously, I can do this using a nested loop, but I'm trying to learn LINQ at the moment and I was wondering if I might use it to accomplish this task?
Here's what I have so far:
Dictionary<string, List<string>> DBtables = this.CollectTableListings();
var generic = from Dictionary<string,List<string>> tab
in DBtables
where !_tables.ContainsKey(???)
select tab;
Any idea what should go in place of the question marks (or perhaps instead of the entire where clause)?

You can do:
var resultKeys = DBTables.Keys.Except( _tables.Keys );
The Except() method is essentially the same as the minus operations in SQL - it returns all items from the first collection excluding those in the second. Since dictionaries expose their keys, you can compute their difference that way.
The Except() operator uses the default equality for the type, but there is also an overload which allows you to specify your own IEqualityComparer to override the semantics of how to compare values. In your example, you probably don't need that - but it's nice to know it there.

Dictionary<string, List<string>> dictOne = ...
Dictionary<string, List<string>> dictTwo = ...
var missingKeys = dictOne.Keys.Where(x => !dictTwo.ContainsKey(x));

Dictionary<string, List<string>> dictionary = this.CollectTableListings();
Dictionary<string, List<string>> otherDictionary = getOtherTable();
var keys = from key in dictionary.Keys
where !otherDictionary.Keys.Contains(key)
select key;
(But LBuskin's answer is much better)

have a look at the Except extension method. HTH.

If you wanted to use query syntax I would do something akin to below:
var keys = from d1 in dictionary1
select d1.Key;
var items = from d2 in dictionary2
where d2.Key in keys
select d2;
foreach(var item in items)
{
}