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Create an array dynamically

Posted on 2014-04-20
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Last Modified: 2014-04-21
My Java code will prompt users to input some data, each piece of this data will need to have an array attached to it. Problem is I will not be privy to how many pieces of data will be entered, it could be just one or 10,000. So I won't know how many arrays or array lists I will need. I want to create the arrays as I need them. In other words I do not want to declare a bunch of arrays prior to knowing how many I need.
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Question by:CrazyOne
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dpearson earned 2000 total points
ID: 40011827
You can create arrays or array lists dynamically.

Here's an example of having a list of arrays, where the arrays have a random size from 0-99 elements.  In your example you'd probably want to decide on this size based on what the user inputs instead.

Then it collects up the arrays in a list, so you can find them again later.

		List<Integer[]> listOfArrays = new ArrayList<Integer[]>() ;

		boolean done = false ;

		while (!done) {
			int arraySize = (int)Math.round(Math.random() * 100) ;
			Integer[] myNewArray = new Integer[arraySize] ;
			listOfArrays.add(myNewArray) ;

			if (listOfArrays.size() > 10)
				done = true ;
		}

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Does that make sense?

Doug
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by:CrazyOne
ID: 40011837
Um, thanks. I am not looking for sizing the array, I am looking to create arrays on the fly. For instances lets say per the users input I will need twenty arrays and another may need 40 arrays. I would like to be able to create the arrays dynamically in code. Something like:

   for(int i = 0; i < HowManyArrays; i++) {
      int Di = new int[size];
      //where Di means the letter D 
      //and the value of i
   }

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Expert Comment

by:dpearson
ID: 40011859
I am looking to create arrays on the fly

Yes I understand that.  The code I posted shows you how to dynamically create 10 arrays in a loop.

If they are all the same size, then just set the "arraySize" variable to a constant, instead of the random value I posted.

I used a while loop in case you don't know at the start how many arrays to create.  If you know that at the start, then a for loop (like you posted) is simpler.

Doug
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LVL 44

Author Comment

by:CrazyOne
ID: 40011914
OK . Excuse my ignorance it has been 10 years or more since I sat down, and did some real hard-coding for an app, but how do I access the various newly created arrays. In your code there are 10 arrays, lets say I need to add or extract info from the 5th array how do I do it. Thanks

P.S. If you like could I ask this in a separate question, so as to get you more point, since this is an add to my original question.
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Expert Comment

by:dpearson
ID: 40011981
OK here's a somewhat simplified version of my original code - removing the while loop and the varying array size and then looking up the 5th element in the list and then adding an extra element as the 5th element.

               List<Integer[]> listOfArrays = new ArrayList<Integer[]>() ;

		for (int i = 0 ; i < howManyArrays ; i++) {
			int arraySize = 10 ;
			Integer[] myNewArray = new Integer[arraySize] ;
			listOfArrays.add(myNewArray) ;
		}

              // index 4 is the 5th element since we start at 0
               Integer[] fifthArray = listOfArrays.get(4) ; 

               // Add an extra array in at the 5th position (i.e. index 4)               
               Integer[] addArray = new Integer[10] ;
               listOfArrays.add(4, extraArray) ;

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Author Closing Comment

by:CrazyOne
ID: 40012776
Thanks
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