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.
Related
I have two dictionaries:
Dictionary<string, Tuple<T, T>> dict1
Dictionary<Enum, Tuple<string, DateTime, DateTime>> dict2
The string value in the first item in the tuple is sometimes equal to the key in the first dictionary. I would like to sort the second dictionary by the enum value and then sort the first dictionary based on the order that dict2 has. How would I be able to do that while retaining the other keys that might be in dict1? I'm able to get as far as sorting the dictionary.
var positions = new Dictionary<Enum, string>();
//This foreach loop can be simplified by a linq expression.
foreach (var position in dict2)
{
var isFound = dict1.Any(x => x.Key == position.Value.Item1.Value);
if (isFound)
{
clubPositions.Add(position.Key, position.Value.Item1.Value);
}
}
var sortedPositions = positions.OrderByDescending(x => x.Key);
You are looking for the SortedDictionary documented on MSDN here. You will need to instantiate them as such:
var dict1 = new SortedDictionary<string, Tuple<DateTime, DateTime>>();
var dict2 = new SortedDictionary<Enum, Tuple<string, DateTime, DateTime>>();
Once you have the sorted dictionaries instances you can use LINQ to map or filter or do whatever else is desired on them. When items are added to the dictionaries they are automatically sorted based on either the ICompare<T> for the given TKey (or the default IComparer if not explicitly provided).
One downside is the fact that this sorts based on the key alone, there is no concept of sorting based on values.
The way I understand the question is not to sort the first dictionary, but to be able to iterate its elements in order defined by the second dictionary keys. If that's correct, the following should do the trick:
var orderedKeys = new HashSet<string>(dict2.OrderBy(e => e.Key).Select(e => e.Value.Item1));
var orderedEntries = orderedKeys.Where(dict1.ContainsKey)
.Select(key => new KeyValuePair<string, Tuple<T, T>>(key, dict1[key]))
.Concat(dict1.Where(e => !orderedKeys.Contains(e.Key)));
Note that it will put the entries that has no corresponding key last in the order.
You can not sort dictionary as it uses hash algorithm for constant search.
A regular dictionary cannot be sorted. You could use a SortedDictionary, it exists for this exact purpose.
This question already has answers here:
Multi Value Dictionary?
(10 answers)
Closed 8 years ago.
How can I store many different values in Dictionary under one key?
I have a code here:
Dictionary<string, DateTime> SearchDate = new Dictionary<string, DateTime>();
SearchDate.Add("RestDate", Convert.ToDateTime("02/01/2013"));
SearchDate.Add("RestDate", Convert.ToDateTime("02/28/2013"));
but in Dictionary i learned that only one unique key is allowed, so my code is producing error.
The simplest way is to make a Dictionary of some sort of container, for example
Dictionary<string,HashSet<DateTime>>
or
Dictionary<string,List<DateTime>>
Use Dictionary<string, List<DateTime>>. Access the list by the key, and then add the new item to the list.
Dictionary<string, List<DateTime>> SearchDate =
new Dictionary<string, List<DateTime>>();
...
public void AddItem(string key, DateTime dateItem)
{
var listForKey = SearchDate[key];
if(listForKey == null)
{
listForKey = new List<DateTime>();
}
listForKey.Add(dateItem);
}
You may try using a Lookup Class. To create it you may use Tuple Class:
var l = new List<Tuple<string,DateTime>>();
l.Add(new Tuple<string,DateTime>("RestDate", Convert.ToDateTime("02/01/2013")));
l.Add(new Tuple<string,DateTime>("RestDate", Convert.ToDateTime("02/28/2013")));
var lookup = l.ToLookup(i=>i.Item1);
However, if you need to modify the lookup, you'll have to modify the original list of tuples and update the lookup from it. So, it depends on how often this collection tends to change.
You can use Lookup class if you are using .NET 3.5
I have the following declaration:
Dictionary<string, Dictionary<string, string>> like = new Dictionary<string, Dictionary<string, string>>();
I need to get the first element out, but do not know the key or value. What's the best way to do this?
Note that to call First here is actually to call a Linq extension of IEnumerable, which is implemented by Dictionary<TKey,TValue>. But for a Dictionary, "first" doesn't have a defined meaning. According to this answer, the last item added ends up being the "First" (in other words, it behaves like a Stack), but that is implementation specific, it's not the guaranteed behavior. In other words, to assume you're going to get any defined item by calling First would be to beg for trouble -- using it should be treated as akin to getting a random item from the Dictionary, as noted by Bobson below. However, sometimes this is useful, as you just need any item from the Dictionary.
Just use the Linq First():
var first = like.First();
string key = first.Key;
Dictionary<string,string> val = first.Value;
Note that using First on a dictionary gives you a KeyValuePair, in this case KeyValuePair<string, Dictionary<string,string>>.
Note also that you could derive a specific meaning from the use of First by combining it with the Linq OrderBy:
var first = like.OrderBy(kvp => kvp.Key).First();
For anyone coming to this that wants a linq-less way to get an element from a dictionary
var d = new Dictionary<string, string>();
d.Add("a", "b");
var e = d.GetEnumerator();
e.MoveNext();
var anElement = e.Current;
// anElement/e.Current is a KeyValuePair<string,string>
// where Key = "a", Value = "b"
I'm not sure if this is implementation specific, but if your Dictionary doesn't have any elements, Current will contain a KeyValuePair<string, string> where both the key and value are null.
(I looked at the logic behind linq's First method to come up with this, and tested it via LinqPad 4)
Though you can use First(), Dictionaries do not have order per se. Please use OrderedDictionary instead. And then you can do FirstOrDefault. This way it will be meaningful.
EDIT:
Use an OrderedDictionary.
It's better to use FirstOrDefault() to retrieve the first value.
Ex:
var firstElement = like.FirstOrDefault();
string firstElementKey = firstElement.Key;
Dictinary<string,string> firstElementValue = firstElement.Value;
Dictionary does not define order of items. If you just need an item use Keys or Values properties of dictionary to pick one.
using System.Linq;
Dictionary<string, Dictionary<string, string>> like = new Dictionary<string, Dictionary<string, string>>();
Dictionary<string, string> first = like.Values.First();
ill find easy way to find first element in Dictionary :)
Dictionary<string, Dictionary<string, string>> like =
newDictionary<string,Dictionary<string, string>>();
foreach(KeyValuePair<string, Dictionary<string, string>> _element in like)
{
Console.WriteLine(_element.Key); // or do something
break;
}
convert to Array
var array = like.ToArray();
var first = array[0];
Easy way of to index a Collection in terms of performance, high compatibility (2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8) and easy implemention.
Save today!!
Its not only a items copy, this is items reference of a Collection!!
buy it!!
string [] arrayString = new string[like.Count];
like.Values.CopyTo( arrayString,0 );
arrayString[0] //First
References:
https://learn.microsoft.com/es-es/dotnet/api/system.collections.generic.icollection-1.copyto?view=net-5.0
https://social.msdn.microsoft.com/Forums/vstudio/en-US/dc5e4242-64d3-45ac-bdea-cf4f3d9abdbb/icollectioncopyto-vs-arraylisttoarray?forum=netfxbcl
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
IDictionary<string, string> or NameValueCollection
Any reason I should use Dictionary<string,string> instead of NameValueCollection?
(in C# / .NET Framework)
Option 1, using NameValueCollection:
//enter values:
NameValueCollection nvc = new NameValueCollection()
{
{"key1", "value1"},
{"key2", "value2"},
{"key3", "value3"}
};
// retrieve values:
foreach(string key in nvc.AllKeys)
{
string value = nvc[key];
// do something
}
Option 2, using Dictionary<string,string>...
//enter values:
Dictionary<string, string> dict = new Dictionary<string, string>()
{
{"key1", "value1"},
{"key2", "value2"},
{"key3", "value3"}
};
// retrieve values:
foreach (KeyValuePair<string, string> kvp in dict)
{
string key = kvp.Key;
string val = kvp.Value;
// do something
}
For these use cases, is there any advantage to use one versus the other? Any difference in performance, memory use, sort order, etc.?
They aren't semantically identical. The NameValueCollection can have duplicate keys while the Dictionary cannot.
Personally if you don't have duplicate keys, then I would stick with the Dictionary. It's more modern, uses IEnumerable<> which makes it easy to mingle with Linq queries. You can even create a Dictionary using the Linq ToDictionary() method.
NameValueCollection is string typed whereas Dictionary leverages generics to allow type variance. See Benefits of Generics.
Dictionary will be much faster. NameValueCollection allows duplicate keys. Which could be bad in certain situations, or desired in other. Dictionary does not allow duplicate keys.
From: http://msdn.microsoft.com/en-us/library/xfhwa508.aspx
The Dictionary<(Of <(TKey, TValue>)>)
generic class provides a mapping from
a set of keys to a set of values. Each
addition to the dictionary consists of
a value and its associated key.
Retrieving a value by using its key is
very fast, close to O(1), because the
Dictionary<(Of <(TKey, TValue>)>)
class is implemented as a hash table.
I just need to be able to insert a key/value pair into an object at a specific position. I'm currently working with a Hashtable which, of course, doesn't allow for this functionality. What would be the best approach?
UPDATE: Also, I do need the ability to lookup by the key.
For example (oversimplified and pseudocoded but should convey the point):
// existing Hashtable
myHashtable.Add("somekey1", "somevalue1");
myHashtable.Add("somekey2", "somevalue2");
myHashtable.Add("somekey3", "somevalue3");
// Some other object that will allow me to insert a new key/value pair.
// Assume that this object has been populated with the above key/value pairs.
oSomeObject.Insert("newfirstkey","newfirstvalue");
List<KeyValuePair<string, string>> kvpList = new List<KeyValuePair<string, string>>()
{
new KeyValuePair<string, string>("Key1", "Value1"),
new KeyValuePair<string, string>("Key2", "Value2"),
new KeyValuePair<string, string>("Key3", "Value3"),
};
kvpList.Insert(0, new KeyValuePair<string, string>("New Key 1", "New Value 1"));
Using this code:
foreach (KeyValuePair<string, string> kvp in kvpList)
{
Console.WriteLine(string.Format("Key: {0} Value: {1}", kvp.Key, kvp.Value);
}
the expected output should be:
Key: New Key 1 Value: New Value 1
Key: Key 1 Value: Value 1
Key: Key 2 Value: Value 2
Key: Key 3 Value: Value 3
The same will work with a KeyValuePair or whatever other type you want to use..
Edit -
To lookup by the key, you can do the following:
var result = stringList.Where(s => s == "Lookup");
You could do this with a KeyValuePair by doing the following:
var result = kvpList.Where (kvp => kvp.Value == "Lookup");
Maybe the OrderedDictonary will help you out.
Do you need to look up objects by the key? If not, consider using List<Tuple<string, string>> or List<KeyValuePair<string, string>> if you're not using .NET 4.
I would use the Dictionary<TKey, TValue> (so long as each key is unique).
EDIT: Sorry, realised you wanted to add it to a specific position. My bad. You could use a SortedDictionary but this still won't let you insert.
You could use an OrderedDictionary, but I would question why you would want to do that.
Use a linked list. It was designed for this exact situation.
If you still need the dictionary O(1) lookups, use both a dictionary and a linked list.
Hashtables are not inherently sorted, your best bet is to use another structure such as a SortedList or an ArrayList