VB.NET - Array of ArrayLists

Question

I've declared an array of 88 arraylists using the following code:

Dim Data_FRONT(88) As ArrayList

and then I try to add incoming data to it using the following code:

Dim Data_In(88) As Double
For i = 0 To 87
    Data_In(i) = 15 ' Hard-coding just to test it
Next

' ...

' Then later in the code after some processing 
For i = 0 To 87
    Data_FRONT(i).Add(Data_In(i))        
Next

and I get the following run-time error: "Object reference not set to an instance of an object."

I've even tried doing this:

For i = 0 to 87
    Data_FRONT(i).Add(15) ' Hard-coding to test it
Next

and I still get that error. Any thoughts?

Reed Copsey · Accepted Answer · 2012-10-31 15:29:00Z

2

You need to actually initialize each array list:

For i = 0 To 87
    Data_FRONT(i) = new ArrayList() ' Initialize each ArrayList
    Data_FRONT(i).Add(Data_In(i))        
Next

That being said, if you are always storing an array of doubles, why not just make a single multi-dimensional array?

Dim Data(88,88) As Double = new Double(88, 88)

answered Oct 31, 2012 at 15:29

Reed Copsey

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4 Comments

nsax91 Over a year ago

Thanks! I'm actually collecting data from 88 different sensors, so I want the size of the data structure to increase dynamically.

Reed Copsey Over a year ago

@nsax91 You should consider using List(Of T) then, instead of ArrayList.

nsax91 Over a year ago

What's the advantage of using Lists instead of ArrayLists?

Reed Copsey Over a year ago

@nsax91 Type safety + speed (no boxing with value types, like double)

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