I need a two column list like:
List<int,string> mylist= new List<int,string>();
it says
using the generic type System.collection.generic.List<T> requires 1 type arguments.
Depending on your needs, you have a few options here.
If you don't need to do key/value lookups and want to stick with a List<>, you can make use of Tuple<int, string>:
List<Tuple<int, string>> mylist = new List<Tuple<int, string>>();
// add an item
mylist.Add(new Tuple<int, string>(someInt, someString));
If you do want key/value lookups, you could move towards a Dictionary<int, string>:
Dictionary<int, string> mydict = new Dictionary<int, string>();
// add an item
mydict.Add(someInt, someString);
You could use an immutable struct
public struct Data
{
public Data(int intValue, string strValue)
{
IntegerData = intValue;
StringData = strValue;
}
public int IntegerData { get; private set; }
public string StringData { get; private set; }
}
var list = new List<Data>();
Or a KeyValuePair<int, string>
using Data = System.Collections.Generic.KeyValuePair<int, string>
...
var list = new List<Data>();
list.Add(new Data(12345, "56789"));
With the new ValueTuple from C# 7 (VS 2017 and above), there is a new solution:
List<(int,string)> mylist= new List<(int,string)>();
Which creates a list of ValueTuple type. If you're targeting .NET Framework 4.7+ or .NET/.NET Core, it's native, otherwise you have to get the ValueTuple package from nuget.
It's a struct opposing to Tuple, which is a class. It also has the advantage over the Tuple class that you could create a named tuple, like this:
var mylist = new List<(int myInt, string myString)>();
That way you can access like mylist[0].myInt and mylist[0].myString
Since your example uses a generic List, I assume you don't need an index or unique constraint on your data. A List may contain duplicate values. If you want to insure a unique key, consider using a Dictionary<TKey, TValue>().
var list = new List<Tuple<int,string>>();
list.Add(Tuple.Create(1, "Andy"));
list.Add(Tuple.Create(1, "John"));
list.Add(Tuple.Create(3, "Sally"));
foreach (var item in list)
{
Console.WriteLine(item.Item1.ToString());
Console.WriteLine(item.Item2);
}
Use C# Dictionary datastructure it good for you...
Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("one", 1);
dict.Add("two", 2);
You can retrieve data from Ditionary in a simple way..
foreach (KeyValuePair<string, int> pair in dict)
{
MessageBox.Show(pair.Key.ToString ()+ " - " + pair.Value.ToString () );
}
For more example using C# Dictionary... C# Dictionary
Navi.
Not sure about your specific scenario, but you have three options:
1.) use Dictionary<..,..>
2.) create a wrapper class around your values and then you can use List
3.) use Tuple
List<Tuple<string, DateTime, string>> mylist = new List<Tuple<string, DateTime,string>>();
mylist.Add(new Tuple<string, DateTime, string>(Datei_Info.Dateiname, Datei_Info.Datum, Datei_Info.Größe));
for (int i = 0; i < mylist.Count; i++)
{
Console.WriteLine(mylist[i]);
}
For that, you could use a Dictionary where the int is the key.
new Dictionary<int, string>();
If you really want to use a list, it could be a List<Tuple<int,string>>() but, Tuple class is readonly, so you have to recreate the instance to modifie it.
Related
Suppose I have a class Building with properties int Id, int Height, int NumberOfFloors.
If I have a Dictionary, where each key is the building id. Is there a way to convert this into a Dictionary where each key is the building Id, and each value is the number of floors. Obviously this is feasible with a loop. Just wondering if there is a simple way to do this using lambdas?
Thanks!
var intToBuilding = new Dictionary<int, Building>(); //pretend populated
var intToInt = new Dictionary<int, int>();
foreach(var intBuilding in intToBuilding)
{
var building = userPreference.Value;
intToInt.Add(intBuilding.Key, building.Height);
}
It should go something like:
var floorDictionary = buildingDictionary
.ToDictionary(kv => kv.Key, kv => kv.Value.NumberofFloors);
The source dictionary implements IEnumerable< KeyValuePair<TKey, TValue> > and for that there is an extension method ToDictionary(keySelector, valueSelector).
I have a dictionary:
Dictionary<String, List<Foo>> test = new Dictionary<String, List<Foo>>();
I then populate this dictionary hence why I need the list so I can call Add(). My problem is the function needs to return:
Dictionary<String, IEnumerable<Foo>>
Is there any easy way to do this without doing the obvious and looping through my original dictionary and doing it manually?
return dictionary.ToDictionary(x => x.Key,x => x.Value.AsEnumerable())
It's more efficient and easier to use the List<Foo> to add things but add it to a Dictionary<String, IEnumerable<Foo>>. That's no problem since List<Foo> implements IEnumerable<Foo>, it's not even necessary to cast.
So something like this(pseudo code):
var test = new Dictionary<String, IEnumerable<Foo>>();
foreach(var x in something)
{
var list = new List<Foo>();
foreach(var y in x.SomeCollection)
list.Add(y.SomeProperty);
test.Add(x.KeyProperty, list); // works since List<T> is also an IEnumerable<T>
}
I tried this route as well, converting Dictionary<string, List<Foo>> to a ReadOnlyDictionary<string, IEnumerable<Foo>>. While I was trying to convert to a read-only dictionary, the whole purpose of converting a List to IEnumerable is to make a read only collection. The problem with the OP's approach is:
Dictionary<string, List<string>> errors = new Dictionary<string, List<string>>();
errors["foo"] = new List<string>() { "You can't do this" };
Dictionary<string, IEnumerable<string>> readOnlyErrors = // convert errors...
readOnlyErrors["foo"] = new List<string>() { "I'm not actually read-only!" };
The appearance of IEnumerable<Foo> makes you think this is read only and safe, when in fact it is not. After reading the question LINQ Convert Dictionary to Lookup a Lookup object is more appropriate, because it allows you to:
Associate one key with multiple values
You cannot overwrite a key with a new value
// This results in a compiler error
lookUp["foo"] = new List<Foo>() { ... };
The "multiple values" are already defined as IEnumerable<T>
You can still use the same outer and inner loop algorithm to extract individual values:
ILookup<string, string> lookup = // Convert to lookup
foreach (IGrouping<string, string> grouping in lookup)
{
Console.WriteLine(grouping.Key + ":");
foreach (string item in grouping)
{
Console.WriteLine(" item: " + item);
}
}
Convert Dictionary<string, List<Foo>> to ILookup<string, Foo>
It's a quick two-liner:
Dictionary<string, List<Foo>> foos = // Create and populate 'foos'
ILookup<string, Foo> lookup = foos.SelectMany(item => item.Value, Tuple.Create)
.ToLookup(p => p.Item1.Key, p => p.Item2);
Now you can use the same two-step loop as you would have with a Dictionary<string, IEnumerable<Foo>>:
foreach (IGrouping<string, Foo> grouping in lookup)
{
string key = grouping.Key;
foreach (Foo foo in grouping)
{
// Do stuff with key and foo
}
}
Source: LINQ Convert Dictionary to Lookup
Converting to another Dictionary with an IEnumerable value is like trying to stuff a square peg into a round hole. The more appropriate, and safe way (from an object-oriented standpoint) is to convert your read/write Dictionary to a Lookup. This gives you the true intended safety of an object that is read-only (except for the Foo items, which might not be immutable).
I would go so far as to say that most times when a ReadOnlyDictionary is used, you could use ILookup and get the same functionality.
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))
I have some lines from text files that i want to add into the Dictionary.I am using Dictionary for the first time.While adding up starting lines it was Ok but suddenly i got error:
An item with the same key has already been added
Here in my code there are duplicate keys which i can not change.Here is my code in c#
Dictionary<string, string> previousLines = new Dictionary<string, string> { };
previousLines.Add(dialedno, line);
Here dialedno is the key and line is the textfile line.
Here is the code from which i am retrieving the given line based on key.
string tansferOrginExt = previousLines[dialedno];
So my concern is how to allow to add duplicate keys in Dictionary if possible and if not how can i get similar functionality.
how to allow to add duplicate keys in Dictionary
It is not possible. All keys should be unique. As Dictionary<TKey, TValue> implemented:
Every key in aDictionary<TKey, TValue> must be unique according to
the dictionary's equality comparer.
Possible solutions - you can keep collection of strings as value (i.e. use Dictionary<string, List<string>>), or (better) you can use Lookup<TKey, TValue> instead of dictionary.
how to check for duplicate keys and delete previous value from
Dictionary?
You can check if the key exists with previousLines.ContainsKey(dialedno) but if you always want to hold the last line, then just replace whatever dictionary had for the key, or add the new key if it is not in the dictionary:
previousLines[dialedno] = line;
We can Use a List of Key Value Pair
List<KeyValuePair<string, string>> myduplicateLovingDictionary= new List<KeyValuePair<string, string>>();
KeyValuePair<string,string> myItem = new KeyValuePair<string,string>(dialedno, line);
myduplicateLovingDictionary.Add(myItem);
Its not possible to add duplicate items to a Dictionary - an alternative is to use the Lookup class.
Enumerable.ToLookup Method
Creates a generic Lookup from an IEnumerable.
Example:
class Program
{
private static List<KeyValuePair<string, int>> d = new List<KeyValuePair<string, int>>();
static void Main(string[] args)
{
d.Add(new KeyValuePair<string, int>("joe", 100));
d.Add(new KeyValuePair<string, int>("joe", 200));
d.Add(new KeyValuePair<string, int>("jim", 100));
var result = d.Where(x => x.Key == "joe");
foreach(var q in result)
Console.WriteLine(q.Value );
Console.ReadLine();
}
}
List< KeyValuePair < string, string>> listKeyValPair= new List< KeyValuePair< string, string>>();
KeyValuePair< string, string> keyValue= new KeyValuePair< string, string>("KEY1", "VALUE1");
listKeyValPair.Add(keyValue);
If your question is if you can add the same key twice, the answer is No.
However if you want to just iterate through the item and then increase the count of the value for the particular Key, you can achieve that by using "TryAdd" method.
var dict = new Dictionary<int, int>();
foreach (var item in array)
{
dict.TryAdd(item, 0);
dict[item]++;
}
The same thing we are trying to achieve with if else, can be achieved with this method.``
https://learn.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.tryadd?view=netframework-4.7.2
I am storing a string and int value in Key value pair.
var list = new List<KeyValuePair<string, int>>();
While adding i need to check if string(Key) already exists in list, if exists i need to add it to Value instead of adding new key.
How to check and add?
Instead of List you can use Dictionary and check if it contains key then add the new value to the existing key
int newValue = 10;
Dictionary<string, int> dictionary = new Dictionary<string, int>();
if (dictionary.ContainsKey("key"))
dictionary["key"] = dictionary["key"] + newValue;
use dictonary. Dictionary in C# and I suggest you to read this post Dictonary in .net
Dictionary<string, int> dictionary =
new Dictionary<string, int>();
dictionary.Add("cat", 2);
dictionary.Add("dog", 1);
dictionary.Add("llama", 0);
dictionary.Add("iguana", -1);
to check. use ContainsKey ContainsKey
if (dictionary.ContainsKey("key"))
dictionary["key"] = dictionary["key"] + yourValue;
If you need use the list,you must foreach the list,and look for the keys.
Simplely,you can use hashtable.
For anyone who has to use a List (which was the case for me, since it does things Dictionary doesn't), you can just use a lambda expression to see if the List contains the Key:
list.Any(l => l.Key == checkForKey);
Your needs exactly describe the design of Dictionarys?
Dictionary<string, string> openWith =
new Dictionary<string, string>();
// Add some elements to the dictionary. There are no
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";
For sure, dictionary is preferable in your case. You can not modify the Value of KeyValue<string,int> class as it is Immutable.
But even if you still want to use List<KeyValuePair<string, int>>();. You can use IEqualityComparer<KeyValuePair<string, int>>. Code will be like.
public class KeyComparer : IEqualityComparer<KeyValuePair<string, int>>
{
public bool Equals(KeyValuePair<string, int> x, KeyValuePair<string, int> y)
{
return x.Key.Equals(y.Key);
}
public int GetHashCode(KeyValuePair<string, int> obj)
{
return obj.Key.GetHashCode();
}
}
And use it in Contains like
var list = new List<KeyValuePair<string, int>>();
string checkKey = "my string";
if (list.Contains(new KeyValuePair<string, int>(checkKey, int.MinValue), new KeyComparer()))
{
KeyValuePair<string, int> item = list.Find((lItem) => lItem.Key.Equals(checkKey));
list.Remove(item);
list.Add(new KeyValuePair<string, int>("checkKey", int.MinValue));// add new value
}
which does not sounds good way.
hope this info helps..