I have an array of integers and I need to know the index of the largest number (not the actual value, just the index of whichever is highest).
However, if one or more indexes "tie" for the highest value, I need to have all of the indexes that share that high value.
I assume this function would need to return an array (since it could be one or more indexes), but I am not totally sure how to go about getting the more efficient solution.
If this is going to be a common thing you could write your own Extension. You should add some additional sanity/null checking but this will get you started:
Module Extensions
<System.Runtime.CompilerServices.Extension()> Function FindAllIndexes(Of T)(ByVal array() As T, ByVal match As Predicate(Of T)) As Integer()
''//Our return object
Dim Ret As New List(Of Integer)
''//Current array index
Dim I As Integer = -1
''//Infinite loop, break out when we no more matches are found
Do While True
''//Loop for a match based on the last index found, add 1 so we dont keep returning the same value
I = System.Array.FindIndex(array, I + 1, match)
''//If we found something
If I >= 0 Then
''//Append to return object
Ret.Add(I)
Else
''//Otherwise break out of loop
Exit Do
End If
Loop
''//Return our array
Return Ret.ToArray()
End Function
End Module
Then to call it:
Dim ints As Integer() = New Integer() {1, 2, 8, 6, 8, 1, 4}
Dim S = ints.FindAllIndexes(Function(c) c = ints.Max())
''//S now holds 2 and 4
If you are using .NET 3.5, you can use the Max() Extension function to easily find the highest value, and use Where to locate the matching records in your source array.
IList has an IndexOf member, which helps. This code is completely untested, and probably has at least one off-by-one error.
Public Function GetPostionsOfMaxValue(ByVal input() As Integer) As Integer()
Dim ints = New List(Of Integer)(input)
Dim maxval = ints.Max
Dim indexes As New List(Of Integer)
Dim searchStart As Integer = 0
Do Until searchStart >= ints.Count
Dim index = ints.IndexOf(maxval, searchStart)
If index = -1 Then Exit Do
indexes.Add(index)
searchStart = index + 1
Loop
Return indexes.ToArray
End Function
Related
I've got a question about using two-dimensional array.
Public twolist(,) As String
For i As Integer = 0 To twolist.length()-1
If Func(twolist(i, )) Then 'this part is hard for me
'doing something
End If
Public Function Func(ByVal CancelInput() As String) As Boolean
What i want to do is Passing two-dimensional array to an array.
I want to read one row in two-dimensional array and pass to function(Func), which is using an array.
Hope You can understand my question... and Thank you!
As an alternative to the For Next Loop, you could use Linq (if you are confortable with it) to perform the same task.
This transforms each element of the source array to a String, groups them in an IEnumerable(Of String) and the result is converted to an unidimensional Array of Strings:
Dim twolist(N, N) As String
Dim CancelInput() As String = twolist.Cast(Of String).Select(Function(str) str).ToArray()
Dim result As Boolean = Func(CancelInput)
I have just used an arbitrary size for your array. You need nested For loops to iterate through a 2 dimensional array. The outer loop goes through the rows and the inner loop adds the value in each field to another array that you are passing to your Function. Each row is passed individually as a single dimension array.
Private Sub TwoDimensionalArray()
Dim twolist(,) As String
ReDim twolist(10, 5)
'First you will need to add data to your array
For x As Integer = 0 To 10
Dim arrayRow(5) As String
For y As Integer = 0 To 5
arrayRow(y) = twolist(x, y)
Next
If Func(arrayRow) Then 'this part is hard for me
'doing something
End If
Next
End Sub
Public Function Func(ByVal CancelInput() As String) As Boolean
Return True
End Function
Mary's answer is good, but assumes you know the length of each dimension.
I have changed it slightly to use the Array.GetLength function:
Private Sub TwoDimensionalArray()
Dim twolist(,) As String
ReDim twolist(10, 5)
'First you will need to add data to your array
For x As Integer = 0 To 10
'Fetch the length of this dimension:
Dim i As Integer = twolist.GetLength(x)
Dim arrayRow(i) As String
For y As Integer = 0 To i - 1
arrayRow(y) = twolist(x, y)
Next
If Func(arrayRow) Then
'do something
End If
Next
End Sub
Public Function Func(ByVal CancelInput() As String) As Boolean
Return True
End Function
Note:
In VB.Net, ReDim twoList(10,5) actually gives you an array of (11,6).
Array.GetLength(0) will return 6 (0,1,2,3,4,5).
In short, Dim specifies the maximum index in each dimension, Length & GetLength return the count of elements.
I have this problem in vb.net. Lets say I got 2 Lists ListA and ListB both holds objects of same type.
Eg., one of the property of the object is ID. (ID is written in brackets)
ListA ListB
---------------------------
A(3818) A(3818)
B(3819) B(3819)
C(3820) C(3820)
D(3821) D(3821)
E(3823) F(0)
H(3824) G(0)
I(3825)
How do I merge these two Lists to have a new distinct list which holds objects only once whose ID matches and all other objects(whose ID dont match) are simply added to the new list.
Sample output be,
New List
--------
A(3818)
B(3819)
C(3820)
D(3821)
E(3823)
F(0)
G(0)
H(3824)
I(3825)
When I searched I found that AddRange() and Union are some of the methods to do the merge. But i am not able to find if this works for non standard objects(apart from Integer, String)
Use addRange() and then linq with distinct to filter out the duplicates.
Dim b = YourCollection.Distinct().ToList()
Could use a collection bucket
Dim oCol As New Collection
AddTitems(oCol, oListA)
AddTitems(oCol, olistB)
Public Function AddTitems(oSummaryList As Collection, oList As List(Of thing)) As Collection
For Each oThing As thing In oList
If Not oSummaryList.Contains(CStr(oThing.ID)) Then oSummaryList.Add(oList, CStr(oThing.ID))
Next
Return oSummaryList
End Function
Here are a couple simple functions that should do that for you. I'm not sure how efficient they are though. I don't think there is anything built in.
Private Function nameOfFunction(list1 as list(of type), list2 as list(of type)) as list(of type)
Dim result as new list(of type)
for a as integer = 0 to math.max(list1.count, list2.count) - 1 step 1
If a < list1.count AndAlso resultHasID(result, list1(a).ID) = False Then
result.add(list1(a))
end if
If a < list2.count AndAlso resultHasID(result, list2(a).ID) = False Then
result.add(list2(a))
end if
next
End Function
Private Function resultHasID(testList as list(of type), s as string) as boolean
Dim result as Boolean = False
for a as integer = 0 to testlist.count - 1 step 1
if(testlist(a).ID = s) then
result = true
exit for
End if
Next
Return result
End function
For each item as String in ListA
If Not ListB.Contains(item) Then
ListB.Add(item)
End If
Next
In the below code I am getting the following error on line If (Not hash.Add(Numbers(Num))) Then Value of Type 'Integer' cannot be converted to 'System.Array'. What am I doing wrong?
Module Module1
Sub Main()
Dim array() As Integer = {5, 10, 12, 8, 8, 14}
' Pass array as argument.
Console.WriteLine(findDup(array))
End Sub
Function findDup(ByVal Numbers() As Integer) As Integer
Dim hash As HashSet(Of Array)
For Each Num In Numbers
If (Not hash.Add(Numbers(Num))) Then
Return (Num)
End If
Next
End Function
End Module
Inside findDup, this
Dim hash As HashSet(Of Array)
Should be
Dim hash As HashSet(Of Integer)
EDIT: As suggested by #Damien_The_Unbeliever, the code
This line
If (Not hash.Add(Numbers(Num))) Then
Should be
If (Not hash.Add(Num)) Then
You have created a hashset of Array, rather than a hashset of Integer. You could change it to Integer, and alter how you try to add things in your loop to:
Function findDup(ByVal Numbers() As Integer) As Integer
Dim hash As New HashSet(Of Integer)
For Each Num In Numbers
If (Not hash.Add(Num)) Then
Return (Num)
End If
Next
End Function
I am hoping you realize that it will only ever find the first duplicate, and doesn't return a value of any type if it doesn't find a duplicate.
You've declared hash to be HashSet(Of Array), which means that it holds arrays. But you're trying to add integers to it.
You need to either change its declaration (HashSet(Of Integer)) or change the call to Add (Add(Numbers)).
Which solution you use will depend on your intent. My guess is that you want to change the type of hash.
Is this what you mean to do?
Sub Main()
Dim someArray() As Integer = {5, 10, 12, 8, 7, 8, 8, 10, 14, 10}
' Pass array as argument.
Dim foo As List(Of Integer) = findDups(someArray)
'foo contains a list of items that have more than 1 occurence in the array
End Sub
Function findDups(ByVal Numbers() As Integer) As List(Of Integer)
Dim rv As New List(Of Integer)
For idx As Integer = 0 To Numbers.Length - 1
If Array.LastIndexOf(Numbers, Numbers(idx)) <> idx Then
If Not rv.Contains(Numbers(idx)) Then rv.Add(Numbers(idx))
End If
Next
Return rv
End Function
I've written a short function for array intersection and wanted to know why one function is faster than the other.
1)
Dim list2() As String 'Assume it has values'
Dim list2length As Integer = list2.length
Function newintersect(ByRef list1() As String) As String()
Dim intersection As New ArrayList
If (list1.Length < list2length) Then
'use list2'
For Each thing As String In list2
If (Array.IndexOf(list1, thing) <> -1) Then
intersection.Add(thing)
End If
Next
Else
'use list1'
For Each thing As String In list1
If (Array.IndexOf(list2, thing) <> -1) Then
intersection.Add(thing)
End If
Next
End If
Return intersection
End Function
2)
Dim list2() As String 'Assume it has values'
Dim list2length As Integer = list2.length
Function newintersect(ByRef list1() As String) As String()
Dim intersection As New ArrayList
If (list1.Length > list2length) Then 'changed >'
'use list2'
For Each thing As String In list2
If (Array.IndexOf(list1, thing) <> -1) Then
intersection.Add(thing)
End If
Next
Else
'use list1'
For Each thing As String In list1
If (Array.IndexOf(list2, thing) <> -1) Then
intersection.Add(thing)
End If
Next
End If
Return intersection
End Function
3)
Dim list2() As String 'Assume it has values'
Dim list2length As Integer = list2.length
Function newintersect(ByRef list1() As String) As String()
For Each thing As String In list1
If (Array.IndexOf(list2, thing) <> -1) Then
intersection.Add(thing)
End If
Next
Return intersection
End Function
So for my testcase, 1 take 65 seconds, 3 takes 63 seconds, while 2 actually takes 75 seconds. Anyone know why 3 is the fastest? And why is 1 faster than 2?
(Sorry about the poor formatting...can't seem to paste it right)
That's not much of a difference. Also, it doesn't seem like the methods would produce the same result, so it would be pointless to compare the performance, right?
Anyhow, the Array.IndexOf is not very efficient way to find items, and doesn't scale well. You should get a dramatic improvement if you use a hash key based collection as lookup instead, something like this:
Function newintersect(ByRef list1 As String(), ByRef list2 As String()) As String()
Dim smaller As HashSet(Of String)
Dim larger As String()
If list1.Length < list2.Length Then
smaller = New HashSet(Of String)(list1)
larger = list2
Else
smaller = New HashSet(Of String)(list2)
larger = list1
End If
Dim intersection As New List(Of String)
For Each item As String In larger
If smaller.Contains(item) Then
intersection.Add(item)
End If
Next
Return intersection.ToArray()
End Function
I'd expect you'd find that with different test cases you can reverse the results you had above and reach a situation where 2 is fastest and 1 & 3 are slower.
It's difficult to comment without knowing the make-up of the test case, it'll depend on the location of 'intersecting' items within the two arrays - if these tend to be close the to front on one array and closer to the end of another then the nesting order of the array iteration / IndexOf will have markedly different performance.
BTW - there are better ways of performing an intersection - sorting one or other array and performing a BinarySearch is one means - using a Dictionary(Of String, ...) or similar is another - and either would result in far better performance.
This is from the MSDN documentation
Dim id1() As Integer = {44, 26, 92, 30, 71, 38}
Dim id2() As Integer = {39, 59, 83, 47, 26, 4, 30}
' Find the set intersection of the two arrays.
Dim intersection As IEnumerable(Of Integer) = id1.Intersect(id2)
For Each id As Integer In intersection
Debug.WriteLine(id.ToString)
Next
How can I delete an item from an array in VB.NET?
As Heinzi said, an array has a fixed size. In order to 'remove an item' or 'resize' it, you'll have to create a new array with the desired size and copy the items you need as appropriate.
Here's code to remove an item from an array:
<System.Runtime.CompilerServices.Extension()> _
Function RemoveAt(Of T)(ByVal arr As T(), ByVal index As Integer) As T()
Dim uBound = arr.GetUpperBound(0)
Dim lBound = arr.GetLowerBound(0)
Dim arrLen = uBound - lBound
If index < lBound OrElse index > uBound Then
Throw New ArgumentOutOfRangeException( _
String.Format("Index must be from {0} to {1}.", lBound, uBound))
Else
'create an array 1 element less than the input array
Dim outArr(arrLen - 1) As T
'copy the first part of the input array
Array.Copy(arr, 0, outArr, 0, index)
'then copy the second part of the input array
Array.Copy(arr, index + 1, outArr, index, uBound - index)
Return outArr
End If
End Function
You can use it as such:
Module Module1
Sub Main()
Dim arr = New String() {"abc", "mno", "xyz"}
arr.RemoveAt(1)
End Sub
End Module
The code above removes the second element ("mno") [which has an index of 1] from the array.
You need to be developing in .NET 3.5 or higher in order to use the extension method.
If you're using .NET 2.0 or 3.0, you can call the method as such
arr = RemoveAt(arr, 1)
I hope this is what you need.
Update
After running tests based on ToolMakerSteve's comment it appears the initial code does not modify the array you want to update because of the ByVal used in the function's declaration. However, writing code like arr = arr.RemoveAt(1) or arr = RemoveAt(arr, 1) does modify the array because it reassigns the modified array to the original.
Find below the updated method (subroutine) for removing an element from an array.
<System.Runtime.CompilerServices.Extension()> _
Public Sub RemoveAt(Of T)(ByRef arr As T(), ByVal index As Integer)
Dim uBound = arr.GetUpperBound(0)
Dim lBound = arr.GetLowerBound(0)
Dim arrLen = uBound - lBound
If index < lBound OrElse index > uBound Then
Throw New ArgumentOutOfRangeException( _
String.Format("Index must be from {0} to {1}.", lBound, uBound))
Else
'create an array 1 element less than the input array
Dim outArr(arrLen - 1) As T
'copy the first part of the input array
Array.Copy(arr, 0, outArr, 0, index)
'then copy the second part of the input array
Array.Copy(arr, index + 1, outArr, index, uBound - index)
arr = outArr
End If
End Sub
Usage of the method is similar to the original, except there is no return value this time so trying to assign an array from the return value will not work because nothing is returned.
Dim arr = New String() {"abc", "mno", "xyz"}
arr.RemoveAt(1) ' Output: {"abc", "mno"} (works on .NET 3.5 and higher)
RemoveAt(arr, 1) ' Output: {"abc", "mno"} (works on all versions of .NET fx)
arr = arr.RemoveAt(1) 'will not work; no return value
arr = RemoveAt(arr, 1) 'will not work; no return value
Note:
I use a temporary array for the process because it makes my intentions clear and that is exactly what VB.NET does behind the scenes when you do Redim Preserve. If you would like to modify the array in-place using Redim Preserve, see ToolmakerSteve's answer.
The RemoveAt methods written here are extension methods. In order for them to work, you will have to paste them in a Module. Extension methods will not work in VB.NET if they are placed in a Class.
Important If you will be modifying your array with lots of 'removes', it is highly recommended to use a different data structure such as List(Of T) as suggested by other answerers to this question.
You can't. I would suggest that you put the array elements into a List, at least then you can remove items. An array can be extended, for example using ReDim but you cannot remove array elements once they have been created. You would have to rebuild the array from scratch to do that.
If you can avoid it, don't use arrays here, use a List.
One line using LINQ:
Dim arr() As String = {"uno", "dos", "tres", "cuatro", "cinco"}
Dim indx As Integer = 2
arr = arr.Where(Function(item, index) index <> indx).ToArray 'arr = {"uno", "dos", "cuatro", "cinco"}
Remove first element:
arr = arr.Skip(1).ToArray
Remove last element:
arr = arr.Take(arr.length - 1).ToArray
That depends on what you mean by delete. An array has a fixed size, so deleting doesn't really make sense.
If you want to remove element i, one option would be to move all elements j > i one position to the left (a[j - 1] = a[j] for all j, or using Array.Copy) and then resize the array using ReDim Preserve.
So, unless you are forced to use an array by some external constraint, consider using a data structure more suitable for adding and removing items. List<T>, for example, also uses an array internally but takes care of all the resizing issues itself: For removing items, it uses the algorithm mentioned above (without the ReDim), which is why List<T>.RemoveAt is an O(n) operation.
There's a whole lot of different collection classes in the System.Collections.Generic namespace, optimized for different use cases. If removing items frequently is a requirement, there are lots of better options than an array (or even List<T>).
Yes, you can delete an element from an array. Here is an extension method that moves the elements as needed, then resizes the array one shorter:
' Remove element at index "index". Result is one element shorter.
' Similar to List.RemoveAt, but for arrays.
<System.Runtime.CompilerServices.Extension()> _
Public Sub RemoveAt(Of T)(ByRef a() As T, ByVal index As Integer)
' Move elements after "index" down 1 position.
Array.Copy(a, index + 1, a, index, UBound(a) - index)
' Shorten by 1 element.
ReDim Preserve a(UBound(a) - 1)
End Sub
Usage examples (assuming array starting with index 0):
Dim a() As String = {"Albert", "Betty", "Carlos", "David"}
a.RemoveAt(0) ' Remove first element => {"Betty", "Carlos", "David"}
a.RemoveAt(1) ' Remove second element => {"Betty", "David"}
a.RemoveAt(UBound(a)) ' Remove last element => {"Betty"}
Removing First or Last element is common, so here are convenience routines for doing so (I like code that expresses my intent more readably):
<System.Runtime.CompilerServices.Extension()> _
Public Sub DropFirstElement(Of T)(ByRef a() As T)
a.RemoveAt(0)
End Sub
<System.Runtime.CompilerServices.Extension()> _
Public Sub DropLastElement(Of T)(ByRef a() As T)
a.RemoveAt(UBound(a))
End Sub
Usage:
a.DropFirstElement()
a.DropLastElement()
And as Heinzi said, if you find yourself doing this, instead use List(Of T), if possible. List already has "RemoveAt" subroutine, and other routines useful for inserting/deleting elements.
My favorite way:
Imports System.Runtime.CompilerServices
<Extension()> _
Public Sub RemoveAll(Of T)(ByRef arr As T(), matching As Predicate(Of T))
If Not IsNothing(arr) Then
If arr.Count > 0 Then
Dim ls As List(Of T) = arr.ToList
ls.RemoveAll(matching)
arr = ls.ToArray
End If
End If
End Sub
Then in the code, whenever I need to remove something from an array I can do it by some property in some object in that array having a certain value, like:
arr.RemoveAll(Function(c) c.MasterContactID.Equals(customer.MasterContactID))
Or if I already know the exact object I want to remove, I can just do:
arr.RemoveAll(function(c) c.equals(customer))
The variable i represents the index of the element you want to delete:
System.Array.Clear(ArrayName, i, 1)
This may be a lazy man's solution, but can't you just delete the contents of the index you want removed by reassigning their values to 0 or "" and then ignore/skip these empty array elements instead of recreating and copying arrays on and off?
Public Sub ArrayDelAt(ByRef x As Array, ByVal stack As Integer)
For i = 0 To x.Length - 2
If i >= stack Then
x(i) = x(i + 1)
x(x.Length-1) = Nothing
End If
Next
End Sub
try this
Seems like this sounds more complicated than it is...
Dim myArray As String() = TextBox1.Lines
'First we count how many null elements there are...
Dim Counter As Integer = 0
For x = 0 To myArray.Count - 1
If Len(myArray(x)) < 1 Then
Counter += 1
End If
Next
'Then we dimension an array to be the size of the last array
'minus the amount of nulls found...
Dim tempArr(myArray.Count - Counter) As String
'Indexing starts at zero, so let's set the stage for that...
Counter = -1
For x = 0 To myArray.Count - 1
'Set the conditions for the new array as in
'It .contains("word"), has no value, length is less than 1, ect.
If Len(myArray(x)) > 1 Then
Counter += 1
'So if a value is present, we move that value over to
'the new array.
tempArr(Counter) = myArray(x)
End If
Next
Now you can assign tempArr back to the original or what ever you need done with it as in...
TextBox1.Lines = tempArr (You now have a textbox void of blank lines)
If the array is a string array you are able to then do the following:
AlphaSplit = "a\b\c".Split("\")
MaxIndex = AlphaSplit.GetUpperBound(0)
AlphaSplit = AlphaSplit.Where(Function(item, index) index <> MaxIndex).ToArray
AlphaJoin = String.Join("\", PublishRouteSplit)
How about this method:
Get a method which return an array, say tempArray
tempArray is supposed to have at least 1 less element to your array, say permArray
The method should take and integer param (this will be the index of the unwanted element) say ommitIndex and your permArray
In the method, copy all elements excluding the element as position ommitIndex from permArray to tempArray
The method returns tempArray so update permArray with the method.
Here's a snippet
Function updateArray(ommitIndex As Integer, array() As String) As Array
Dim tempArray(array.Length - 2) As String
Dim counter As Integer = 0
For i As Integer = 0 To (array.Length - 1)
If (i <> ommitIndex) Then
tempArray(counter) = array(i)
counter += 1
End If
Next
Return tempArray
End Function