Dictionary Examples
This page was last reviewed on Nov 30, 2023.
Dot Net Perls
Dictionary. In VB.NET the Dictionary allows fast key lookups. A generic type, it can use any types for its keys and values. Its syntax is at first confusing.
Shows a dictionaryShows a dictionary
Compared to alternatives, a Dictionary is easy to use and effective. It has many functions (like ContainsKey and TryGetValue) that do lookups.
Add example. Most Dictionaries we use will be added to with the Add() Subroutine. Usually we create an empty Dictionary and then populate it with keys and values.
Step 1 We create Dictionary with String keys, and Integer values. The Dictionary is empty here.
Step 2 We invoke Add to populate the Dictionary. For the arguments, we pass the key we want to add, and the value.
Dictionary TryAdd
Step 3 The Count of this dictionary, after 4 Add() calls have run, is 4—a key and value are counted together as 1 entry.
Shows a dictionary
Module Module1 Sub Main() ' Step 1: create a Dictionary. Dim dictionary As New Dictionary(Of String, Integer) ' Step 2: add 4 entries. dictionary.Add("bird", 20) dictionary.Add("frog", 1) dictionary.Add("snake", 10) dictionary.Add("fish", 2) ' Step 3: display count. Console.WriteLine("DICTIONARY COUNT: {0}", dictionary.Count) End Sub End Module
TryGetValue. Often we test a value through a key in a Dictionary collection. Then we act upon that value. The TryGetValue function combines the 2 steps—this enables optimizations.
Dictionary TryGetValue
Here We create a Dictionary with string keys and values, and then add 2 pairs to it. We then invoke TryGetValue and print the result.
Also Consider GetValueOrDefault for a more convenient way to access values in a VB.NET dictionary.
Dictionary GetValueOrDefault
Shows a dictionary
Module Module1 Sub Main() Dim values As Dictionary(Of String, String) = New Dictionary(Of String, String) values.Add("A", "uppercase letter A") values.Add("c", "lowercase letter C") ' Get value with TryGetValue. Dim result As String = Nothing If values.TryGetValue("c", result) Then Console.WriteLine("RESULT: {0}", result) End If End Sub End Module
RESULT: lowercase letter C
ContainsKey. This function returns a Boolean value, which means you can use it in an If conditional statement. One common use of ContainsKey is to prevent exceptions before calling Add.
Dictionary ContainsKey
Module Module1 Sub Main() ' Declare new Dictionary with String keys. Dim dictionary As New Dictionary(Of String, Integer) ' Add two keys. dictionary.Add("carrot", 7) dictionary.Add("perl", 15) ' See if this key exists. If dictionary.ContainsKey("carrot") Then ' Write value of the key. Dim num As Integer = dictionary.Item("carrot") Console.WriteLine(num) End If ' See if this key also exists (it doesn't). If dictionary.ContainsKey("python") Then Console.WriteLine(False) End If End Sub End Module
Add, error. If you add keys to the Dictionary and one is already present, you will get an exception. We often must check with ContainsKey that the key is not present.
Also You can catch possible exceptions with Try and Catch. This often causes a performance loss.
Module Module1 Sub Main() Dim lookup As Dictionary(Of String, Integer) = New Dictionary(Of String, Integer) lookup.Add("cat", 10) ' This causes an error. lookup.Add("cat", 100) End Sub End Module
Unhandled Exception: System.ArgumentException: An item with the same key has already been added. at System.ThrowHelper.ThrowArgumentException...
KeyNotFoundException. To see if a key exists in a Dictionary, we should use ContainsKey or TryGetValue. If we just access the key directly, we might get a KeyNotFoundException.
Note We can use exception handling to detect the KeyNotFoundException, but this will be slower than not causing an exception.
Module Module1 Sub Main() Dim dict = New Dictionary(Of String, String)() ' We must use ContainsKey or TryGetValue. If dict("car") = "vehicle" Then Return End If End Sub End Module
Unhandled Exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary. at System.Collections.Generic.Dictionary`2.get_Item(TKey key)...
Item. The Item member is a Property accessor. It sets or gets an element in the Dictionary. It is equivalent to Add when you assign it.
And You can assign it to a nonexistent key, but you cannot access a nonexistent key without an exception.
Note Add() may be a better choice than assigning to a key in a Dictionary—it will warn you with an exception if you have a duplicate.
Module Module1 Sub Main() Dim dictionary = New Dictionary(Of Integer, Integer)() ' Add data by assigning to a key. dictionary(10) = 20 ' Look up value. Console.WriteLine(dictionary(10)) End Sub End Module
For Each loop. We can loop over the entries in a Dictionary. It is usually easiest to use the For Each loop. We can access each individual KeyValuePair structure in the loop body.
Step 1 Here we create a Dictionary and add 4 color names to it—blue, yellow, green and red.
Step 2 We access the Key and Value properties on each pair. They have the types of the Dictionary keys and values—no casts are required.
Module Module1 Sub Main() ' Step 1: create Dictionary with 4 keys. Dim colors As New Dictionary(Of String, Integer) colors.Add("blue", 32) colors.Add("yellow", 16) colors.Add("green", 256) colors.Add("red", 100) ' Step 2: use For Each loop over pairs. For Each pair As KeyValuePair(Of String, Integer) In colors Console.WriteLine("COLOR: {0}, VALUE: {1}", pair.Key, pair.Value) Next End Sub End Module
COLOR: blue, VALUE: 32 COLOR: yellow, VALUE: 16 COLOR: green, VALUE: 256 COLOR: red, VALUE: 100
Keys. We can get a List of the Dictionary keys. Dictionary has a Keys property, and we can use this with types like the List that act on IEnumerable.
Step 1 We create a Dictionary and place 4 String keys (which are all animal names) in it, with Integer values.
Step 2 We use the List constructor on the Keys property. The keys have the same type as that from the source Dictionary.
Step 3 We can loop over the resulting collection. With Item() we can access the value from the Dictionary based on the String key.
Module Module1 Sub Main() ' Step 1: add 4 string keys. Dim animals As New Dictionary(Of String, Integer) animals.Add("bird", 12) animals.Add("frog", 11) animals.Add("cat", 10) animals.Add("elephant", -11) ' Step 2: get List of String Keys. Dim list As New List(Of String)(animals.Keys) ' Step 3: loop over Keys and print the Dictionary values. For Each value As String In list Console.WriteLine("ANIMAL: {0}, VALUE: {1}", value, animals.Item(value)) Next End Sub End Module
please, 12 help, 11 poor, 10 people, -11
Types. Dictionary uses typed keys and values. We specify these types when the Dictionary is declared. Here we use more a more complex value type, a String array.
Step 1 We create a Dictionary. It has Integer keys and String array values—so each int can point to an entire array.
Step 2 We use ContainsKey. When the key type is Integer, we must pass an Integer to ContainsKey.
Step 3 We get the String array value with Item, and then call String.Join to concatenate its values.
Module Module1 Sub Main() ' Step 1: use Integer keys, String array values. Dim dictionary As New Dictionary(Of Integer, String()) dictionary.Add(100, New String() {"cat", "bird"}) dictionary.Add(200, New String() {"dog", "fish"}) ' Step 2: see if key exists. If dictionary.ContainsKey(200) Then ' Step 3: get array value, join elements, and print it. Dim value() As String = dictionary.Item(200) Console.WriteLine("RESULT: {0}", String.Join(",", value)) End If End Sub End Module
RESULT: dog,fish
ContainsValue. This returns a Boolean that tells whether any value in the Dictionary is equal to the argument. It is implemented as a For-loop over the entries in the Dictionary.
Tip ContainsValue has no performance advantage over a List that uses linear searching. Accessing keys in the Dictionary is faster.
Module Module1 Sub Main() ' Create new Dictionary with Integer values. Dim dictionary As New Dictionary(Of String, Integer) dictionary.Add("pelican", 11) dictionary.Add("robin", 21) ' See if Dictionary contains the value 21 (it does). If dictionary.ContainsValue(21) Then ' Prints true. Console.WriteLine(True) End If End Sub End Module
Remove. Here we use Remove. You must pass one parameter to this method, indicating which key you want to have removed from the Dictionary instance.
Module Module1 Sub Main() ' Create Dictionary and add two keys. Dim dictionary As New Dictionary(Of String, Integer) dictionary.Add("fish", 32) dictionary.Add("microsoft", 23) ' Remove two keys. dictionary.Remove("fish") ' Will remove this key. dictionary.Remove("apple") ' Doesn't change anything. End Sub End Module
Copy. It is possible to copy the entire contents of a Dictionary. You can do this by declaring a new Dictionary reference and using the copy constructor.
Tip In the Dictionary constructor, pass the Dictionary you want to copy as the parameter.
Module Module1 Sub Main() Dim source = New Dictionary(Of String, Integer)() source.Add("bird", 20) ' Copy the Dictionary. Dim copy = New Dictionary(Of String, Integer)(source) ' Write some details. Console.WriteLine("COPY: {0}, COUNT = {1}", copy("bird"), copy.Count) End Sub End Module
COPY: 20, COUNT = 1
Field. We use a Dictionary in a class. We store it as Private member variable, and then access it through Public methods on the enclosing class.
Part 1 We begin in the Main Sub. We create a new instance of the Example class.
Part 2 Control enters the New() Sub in the Example class. The Dictionary field is created. We add 3 entries to the Dictionary.
Part 3 We call GetValue() on the Example instance. This returns a value from the Dictionary that was stored with the String key "make."
Module Module1 Class Example Private _dictionary Public Sub New() ' Part 2: allocate and populate the field Dictionary. Me._dictionary = New Dictionary(Of String, Integer) Me._dictionary.Add("make", 55) Me._dictionary.Add("model", 44) Me._dictionary.Add("color", 12) End Sub Public Function GetValue() As Integer ' Return value from private Dictionary. Return Me._dictionary.Item("make") End Function End Class Sub Main() ' Part 1: allocate an instance of the class. Dim example As New Example ' Part 3: write a value from the class. Console.WriteLine(example.GetValue()) End Sub End Module
Count. You can count the number of entries with the Count property. Internally, the Count property subtracts 2 integers—it does no looping.
Module Module1 Sub Main() Dim dictionary As New Dictionary(Of String, Integer) dictionary.Add("a", 5) dictionary.Add("b", 8) dictionary.Add("c", 13) dictionary.Add("d", 14) ' Get count. Console.WriteLine(dictionary.Count) End Sub End Module
ToDictionary. We can quickly construct a new Dictionary from a collection (array, List) with the ToDictionary extension method. ToDictionary returns a Dictionary.
Detail We provide 2 functions. These indicate how the key and value are created from the collection's elements.
Here ToDictionary has 2 lambda arguments. They both receive one argument: the String from the source array.
And They return a String (for the first, key selector function) and an Integer (for the value selector function).
Module Module1 Sub Main() ' Create an array of four string literal elements. Dim array() As String = {"dog", "cat", "rat", "mouse"} ' Use ToDictionary. ' ... Use each string as the key. ' ... Use each string length as the value. Dim dict As Dictionary(Of String, Integer) = array.ToDictionary(Function(value As String) Return value End Function, Function(value As String) Return value.Length End Function) ' Display dictionary. For Each pair In dict Console.WriteLine(pair) Next End Sub End Module
[dog, 3] [cat, 3] [rat, 3] [mouse, 5]
Benchmark, ContainsKey. Dictionary is an optimization for key lookups. It can make a slow program many times faster. Consider this benchmark, which tests Dictionary and List.
Version 1 This version of the code calls ContainsKey on a Dictionary with 1000 String keys.
Version 2 Here we call Contains to find a String key in a List that exists. The element is the 900th one in the List.
Result The Dictionary is faster. In the List 900 items are looped over, but with hashing in the Dictionary, fewer items are scanned.
Module Module1 Sub Main() ' Create Dictionary and List. Dim lookup As Dictionary(Of String, Integer) = New Dictionary(Of String, Integer) For i As Integer = 0 To 1000 lookup.Add(i.ToString(), 1) Next Dim list As List(Of String) = New List(Of String) For i As Integer = 0 To 1000 list.Add(i.ToString()) Next Dim m As Integer = 1000 ' Version 1: search Dictionary. Dim s1 As Stopwatch = Stopwatch.StartNew For i As Integer = 0 To m - 1 If Not lookup.ContainsKey("900") Then Return End If Next s1.Stop() Dim s2 As Stopwatch = Stopwatch.StartNew ' Version 2: search List. For i As Integer = 0 To m - 1 If Not list.Contains("900") Then Return End If Next s2.Stop() Dim u As Integer = 1000000 Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns")) Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns")) End Sub End Module
35.10 ns Dictionary ContainsKey (1000 keys) 6801.10 ns List Contains (1000 elements)
Summary. This generic type is powerful. Designed for super-fast lookups, Dictionary often improves the performance of programs. And it can simplify our logic by checking for duplicates.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Nov 30, 2023 (edit link).
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