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Index › Programming Questions & Help › Syntax Questions › ArrayList problem

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ArrayList problem (Read 649 times)

10production

ArrayList problem
Dec 28^th, 2009, 3:16pm

Hello i have a problem with an ArrayList variables.
Here is function, aList is an arraylist variable with size=10:

Code:


void generate(ArrayList aList) {
  ArrayList tmp = aList;
  
  for(int i=0; i<2; i++) { // #1 loop
    for(int j=0; j<10; j++) { // #2 loop
	int index = int(random(tmp.size()));
	Obj o = (Obj) tmp.get(index); // get element with random index

	//some instructions

	tmp.remove(index); // remove element from arraylist
    }
    // this loop clears tmp arraylist
    // so i want to start next iteration with previous content of  tmp arraylist before removing elements.
    
    
    tmp = aList; // restoring previous content.
  }
  
   
}

When the #2 loop ends aList size is 0
Why aList variable do not contain any elements ?
Thatat's why tmp = aList make no sens
Help...

blindfish God Member Offline Posts: 793	Re: ArrayList problem Reply #1 - Dec 28^th, 2009, 4:14pm IIRC when you do tmp=alist you just create a reference to the array, not a true copy. So when you empty tmp you also empty alist. This is something of a FAQ. i.e. search if you want more detail
	Where is the what if the what is in why?

PhiLho

Re: ArrayList problem
Reply #2 - Dec 29^th, 2009, 2:13am

You can do instead:
Code:

ArrayList tmp = new ArrayList(aList);

It should copy the content of the list, so you can alter tmp without changing aList.

...
Checking...
Code:

ArrayList al = new ArrayList();
al.add("Foo");
al.add("Bar");
ArrayList nal = new ArrayList(al);
nal.add("Buz");
nal.remove(1);
for (int i = 0; i < al.size(); i++) print(al.get(i) + " "); println();
for (int i = 0; i < nal.size(); i++) print(nal.get(i) + " "); println();

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