I have a dictionary, for example Dictionary<int, string>.
What would be the best way to get the string value if I know the key?
If you know the key is in the dictionary:
value = dictionary[key];
If you're not sure:
dictionary.TryGetValue(key, out value);
What do you mean by best?
This is the standard way to access Dictionary values by key:
var theValue = myDict[key];
If the key does not exist, this will throw an exception, so you may want to see if they key exists before getting it (not thread safe):
if(myDict.ContainsKey(key))
{
var theValue = myDict[key];
}
Or, you can use myDict.TryGetValue, though this required the use of an out parameter in order to get the value.
If you want to query against a Dictionary collection, you can do the following:
static class TestDictionary
{
static void Main() {
Dictionary<int, string> numbers;
numbers = new Dictionary<int, string>();
numbers.Add(0, "zero");
numbers.Add(1, "one");
numbers.Add(2, "two");
numbers.Add(3, "three");
numbers.Add(4, "four");
var query =
from n in numbers
where (n.Value.StartsWith("t"))
select n.Value;
}
}
You can also use the n.Key property like so
var evenNumbers =
from n in numbers
where (n.Key % 2) == 0
select n.Value;
var stringValue = dictionary[key];
Can't you do something like:
var value = myDictionary[i];?
string value = dictionary[key];
Dictionary.TryGetValue is the safest way
or use Dictionary indexer as other suggested but remember to catch KeyNotFoundException
Well I'm not quite sure what you are asking here but i guess it's about a Dictionary?
It is quite easy to get the string value if you know the key.
string myValue = myDictionary[yourKey];
If you want to make use like an indexer (if this dictionary is in a class) you can use the following code.
public class MyClass
{
private Dictionary<string, string> myDictionary;
public string this[string key]
{
get { return myDictionary[key]; }
}
}
Related
In my app I have a Dictionary<ContainerControl, int>.
I need to check if a key is present in the dictionary and alter its corresponding value if key is found or add the key if not already present.
The key for my dictionary is a ControlContainer object.
I could use this method:
var dict = new Dictionary<ContainerControl, int>();
/*...*/
var c = GetControl();
if (dict.ContainsKey(c))
{
dict[c] = dict[c] + 1;
}
else
{
dict.Add(c, 0);
}
but I think that this way if the key is already present, my dictionary is iterated three times: once in ContainsKey and twice in the if branch.
I wander if there is a more efficient way to do this, something like
var dict = new Dictionary<ContainerControl, int>();
/*...*/
var c = GetControl();
var kvp = dict.GetKeyValuePair(c); /* there is no such function in Dictionary */
if (kvp != null)
{
kvp.Value++;
}
else
{
dict.Add(c, 0);
}
This is possible using linq:
var kvp = dict.SingleOrDefault(x => x.Key == c);
but what about performance?
As noted in comments, finding a key in a dictionary doesn't mean iterating over the whole dictionary. But in some cases it's still worth trying to reduce the lookups.
KeyValuePair<,> is a struct anyway, so if GetKeyValuePair did exist, your kvp.Value++ wouldn't compile (as Value is read-only) and wouldn't work even if it did (as the pair wouldn't be the "original" in the dictionary).
You can use TryGetValue to reduce this to a single "read" operation and a single "write" operation:
// value will be 0 if TryGetValue returns false
if (dict.TryGetValue(c, out var value))
{
value++;
}
dict[c] = value;
Or change to ConcurrentDictionary and use AddOrUpdate to perform the change in a single call.
You could also store a reference type in the dict. This means an extra allocation when you insert an item, but you can mutate items without another dictionary access. You'll need a profiler to tell you whether this is a net improvement!
class IntBox
{
public int Value { get; set; }
}
if (dict.TryGetValue(c, out var box))
{
box.Value++;
}
else
{
dict[c] = new IntBox();
}
With .NET 6 you can use CollectionsMarshal.GetValueRefOrAddDefault for a single lookup:
ref int value = ref CollectionsMarshal.GetValueRefOrAddDefault(dict, c, out bool exists);
if(exists) value++; // changes the value in the dictionary even if it's a value type
Demo: https://dotnetfiddle.net/tnW9P5
this is my code
pritvate static Dictionary<int, string[]> _list = new Dictionary<int, string[]>();
how can i get the string[] out of this?
I have tried this and a lot more:
string[] s = _list.Values;
but it is all not working.
please help
If you want all string arrays for all keys merged into a single array, you can use LINQ's .SelectMany(...):
var strings = _list.Values.SelectMany(v => v).ToArray()
Reading your question again, I wonder if you're asking how to access a value for a single key. So, if you want the string array for a single key you can simply use the indexer:
var value = _list["keyname"];
But that will cause an exception if the key doesn't exist. If you're not sure that the key exists, you can use .TryGetValue(...):
string[] value;
if (_list.TryGetValue("keyname", out value))
{
// value was found
}
else
{
// value wasn't found
}
I have an application that use managed dlls. One of those dlls return a generic dictionary:
Dictionary<string, int> MyDictionary;
The dictionary contains keys with upper and lower case.
On another side I am getting a list of potential keys (string) however I cannot guarantee the case. I am trying to get the value in the dictionary using the keys. But of course the following will fail since I have a case mismatch:
bool Success = MyDictionary.TryGetValue( MyIndex, out TheValue );
I was hoping the TryGetValue would have an ignore case flag like mentioned in the MSDN doc, but it seems this is not valid for generic dictionaries.
Is there a way to get the value of that dictionary ignoring the key case?
Is there a better workaround than creating a new copy of the dictionary with the proper StringComparer.OrdinalIgnoreCase parameter?
There's no way to specify a StringComparer at the point where you try to get a value. If you think about it, "foo".GetHashCode() and "FOO".GetHashCode() are totally different so there's no reasonable way you could implement a case-insensitive get on a case-sensitive hash map.
You can, however, create a case-insensitive dictionary in the first place using:-
var comparer = StringComparer.OrdinalIgnoreCase;
var caseInsensitiveDictionary = new Dictionary<string, int>(comparer);
Or create a new case-insensitive dictionary with the contents of an existing case-sensitive dictionary (if you're sure there are no case collisions):-
var oldDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
var newDictionary = new Dictionary<string, int>(oldDictionary, comparer);
This new dictionary then uses the GetHashCode() implementation on StringComparer.OrdinalIgnoreCase so comparer.GetHashCode("foo") and comparer.GetHashcode("FOO") give you the same value.
Alternately, if there are only a few elements in the dictionary, and/or you only need to lookup once or twice, you can treat the original dictionary as an IEnumerable<KeyValuePair<TKey, TValue>> and just iterate over it:-
var myKey = ...;
var myDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
var value = myDictionary.FirstOrDefault(x => String.Equals(x.Key, myKey, comparer)).Value;
Or if you prefer, without the LINQ:-
var myKey = ...;
var myDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
int? value;
foreach (var element in myDictionary)
{
if (String.Equals(element.Key, myKey, comparer))
{
value = element.Value;
break;
}
}
This saves you the cost of creating a new data structure, but in return the cost of a lookup is O(n) instead of O(1).
For you LINQers out there that never use a regular dictionary constructor
myCollection.ToDictionary(x => x.PartNumber, x => x.PartDescription, StringComparer.OrdinalIgnoreCase)
There is much simpler way:
using System;
using System.Collections.Generic;
....
var caseInsensitiveDictionary = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
Its not very elegant but in case you cant change the creation of dictionary, and all you need is a dirty hack, how about this:
var item = MyDictionary.Where(x => x.Key.ToLower() == MyIndex.ToLower()).FirstOrDefault();
if (item != null)
{
TheValue = item.Value;
}
I have Dictionary from string key i want to get Value of corresponding key using Linq
Why do you want to get a value from a Dictionary using LINQ? You can just get the value using:
int value = dictionary[key];
You could use Single, but it's totally pointless and more code:
var keyValuePair = dictionary.Single(x => x.Key == key);
int value = keyValuePair.Value;
Why use Linq for something that is built in?
var val = myDict[key];
Use Linq where it makes sense (querying collections), not for something that is already well handled by the Dictionary classes.
returns the string value in this particular instance.
Disctionary<int, string> CustomValues = new Dictionary<int, string>();
CustomValues[int]; //CustomValues[key];
To Get the Value from the Dictionary Key
Dictionary<int,string>dict=new Dictionary<int,string>();
dict.Add(1,"Maha");
dict.Add(2,"Malathi");
dict.Add(3,"Mithra");
dict.Add(4,"Saravana");
var val=dict.Where(x=>x.Key==key).Select(x=>x.Value);//get the value from dictionary where key=3
(or)
var val=dict[Key];//get the value for Dictionary key 3.
Output: Mithra
Dictionary<int, List<Customer>> dictionary = new Dictionary<int, List<Customer>>();
I want to query based on the key and get a List back. Not sure how to structure the LINQ query for that.
Desired Output:
A List<Customer> for a particular key in the Dictionary.
That's what the Dictionary (as you've defined the generic arguments) will do. So, dictionary[key] will return the list. Note that it will throw an exception if you haven't initialized it already with dictionary[key] = new List<Customer>();.
You don't need to use LINQ for this, but if you really want to
int key = 1;
List<Customer> customers = dictionary.Single(item => item.Key == key).Value;
The simplest way is to just retrieve the value for the key using the regular [] operator
dictionary[key];