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how to modify the value of an int Key in C# [duplicate]

This question already has answers here:
How to modify key in a dictionary in C#
(4 answers)
Closed 4 years ago.
I have a:
Dictionary<string, Dictionary<int, List<string>>> res = new Dictionary<string,
Dictionary<int, List<string>>>();
and I need to modify/change the int value of the nested Dictionary Key and keep all Dictionary values( List ) for the int Key.

If I understood everything correctly:
res[stringKey].Add(newKey, res[oldKey]);
res[stringKey].Remove(oldKey);

There is no native way to achieve this that I know of but you can try the following:
private void ModifyKey(int oldKey, int newKey, Dictionay<int, List<string>> dict)
{
var data = dict[oldKey];
// Now remove the previous data
dict.Remove(key);
try
{
dict.Add(newKey, data);
}
catch
{
// one already exists..., perhaps roll back or throw
}
}
You would then call the method as follows when you want to change the key:
// Assuming the dictionary is called myData
ModifyKey(5, 7, myData);

c# Dictionary<string,string> how to loop through items without knowing key [duplicate]

This question already has answers here:
How to iterate over a dictionary?
(29 answers)
Closed 8 years ago.
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

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”?

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

It's simple, loop trough it with a foreach or to get a specific index do:
var date = selectedDates.ElementAt(0).Value;

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).

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

Use this overload of Where:
var result = selectedDates.Where((d,i)=>i==0);

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

Dictionary with multiple values for each key? [duplicate]

This question already has answers here:
C# dictionary - one key, many values
(15 answers)
Closed 8 years ago.
I want the functionality of a Dictionary, but I want to store multiple values for each key. How can this be done in .NET/C#? Is there any built in collection that supports this scenario?
If I do the following:
collection.Add("key1", new Order(1));
collection.Add("key1", new Order(2));
collection.Add("key1", new Order(3));
collection.Add("key2", new Order(4));
Then running the following should return 3 orders.
collection["key1"];

Simple: Use Dictionary<string,List<Order>>

You may use lists for this purpose, if the number of orders per key needs to be variable.
collection.Add("key1"), new List<order>(){new Order(1), new Order(2)});

I would recommend using a List, Set, or Dictionary as appropriate for your Value in your Dictionary.
For example, you might do:
Dictionary<string, List<string>>

You should think about this in a different way. The Dictionary can hold a list of Orders rather than just a single one, like so:
var collection = Dictionary<string,List<Order>>();
collection.Add("key1", new List<Order>(new { new Order(1), new Order(2), new Order(3) });
collection.Add("key2", new Order(4));

Try this
Dictionary<string,order[] order> dict = new Dictionary<string,order[] order>();
how to access :
order[] orderArray = dict["key1"];
order order1 = orderArray[1] ;
order order2 = orderArray[2] ;
and goes on...

Concatenate two Dictionaries [duplicate]

This question already has answers here:
Merging dictionaries in C#
(29 answers)
Closed 9 years ago.
Given some Dictionaries
Dictionary<string, string> GroupNames = new Dictionary<string, string>();
Dictionary<string, string> AddedGroupNames = new Dictionary<string, string>();
I am unable to merge them into one:
GroupNames = GroupNames.Concat(AddedGroupNames);
because "the type can't be implicitly converted". I believe (and my code proves me true) their type is the same - what am I overlooking?

I think you defined your GroupNames as Dictionary<string,string>, so you need to add ToDictionary like this:
GroupNames = GroupNames.Concat(AddedGroupNames)
.ToDictionary(x=>x.Key,x=>x.Value);
Note that 2 original dictionaries would have different keys, otherwise we need some rule to merge them correctly.

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.