Removing from an ArrayList while looping thru

[pahiker]

I started working on a section of code where I will be looping through an ArrayList looking for entries matching a certain criteria. As there may be multiple occurrences I need to continue the loop after removing an entry. That got me to wondering, what happens if I remove an entry, is the loop process smart enough to pick up with where it left off, or will it skip an entry?

Here is a sample of what the code might look like:

int idx = -1;

for ( VIN VINentry : VINrecords )
{
	idx++;
	if (VINentry.getType().equals("SVC"))
		VINrecords.remove(idx);
}

If the data looks like this, what will happen?

INS
SPC
SVC
SVC
TTL

Or, is there a better way to handle this process? The intent is to delete all ArrayList entries with a type of SVC.

Thanks,

Mike

[masijade]

Use an iterator and its remove method.

[Norm]

what happens if

Often the fastest way is to write a test program and see.

[JosAH]

Use an iterator and its remove method.

Or do it the old fashioned way and loop through the ArrayList from back to front and remove the elements you want to remove. Pencil and paper show why you have to walk from back to front.

kind regards,

Jos

[pahiker]

Or do it the old fashioned way and loop through the ArrayList from back to front and remove the elements you want to remove. Pencil and paper show why you have to walk from back to front.

kind regards,

Jos

I understand the problem, and looping backwards through the list. I wasn't sure of how ArrayList worked when you remove an entry, if it actually compressed the list, or just marked the entry as being removed. I've seen it done both ways in different languages. Doing it the latter way would preserve the index number of the entries, the former way essentially renumbers everything after the deleted entry.

[masijade]

Backwards reindexes the entries, too, of course, but you'll have already covered those indexes, so it doesn't matter.

[pahiker]

Use an iterator and its remove method.

Interesting, I've not heard of iterators before, looks easier and safer, thanks.

I may change some of the other sections that loop through the AL to use the iterator as well. Any drawbacks to it that you may have run across?

[masijade]

The "new" or "enhanced" for loop uses an Iterator as well (effectively when, maybe not, actually), but you, as the programmer, don't have access to it. Generally, I would use the enhanced for loop, unless I will need to remove something from the list, its simply less code. Then I usually use an Iterator, also, because its simply less code, although I sometimes get sentimental and loop backwards. ;-)

Essentially when you write

for (String s : someStringList) {

what is actually done is the same as

Iterator<String> it = someStringList.iterator();
while (it.hasNext()) {
  String s = it.next();