Thread: List Iterators, erase() -- help

Right, so that fixes it:

Code:

for (iter = myList.begin() ; iter != myList.end() ; ++iter)
{
     std::cout << (*iter) << std::endl;
     if ((*iter) == 5)
          iter = myList.erase(iter);
}

Only problem is that since it returns the one AFTER, you get output like this:

Code:

$
0
1
2
3
4
5
7
8
9
0
1
2
3
4
6
7
8
9

So 6 is skipped. So to balance it out with that of the for loop, you have to do this:

Code:

for (iter = myList.begin() ; iter != myList.end() ; ++iter)
{
     std::cout << (*iter) << std::endl;
     if ((*iter) == 5)
     {
          iter = myList.erase(iter);
          iter--;
     }
}

To get desired results:

Code:

$
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
6
7
8
9

Arg. That's just annoying...

EDIT:
Or this:

Code:

for (iter = myList.begin() ; iter != myList.end() ; ++iter)
{
     std::cout << (*iter) << std::endl;
     if ((*iter) == 5)
          iter = myList.erase(iter)--;
}

Bit less retarded I guess...but still pretty bad...

Originally Posted by Syndacate

Code:

for (iter = myList.begin() ; iter != myList.end() ; ++iter)
{
     std::cout << (*iter) << std::endl;
     if ((*iter) == 5)
     {
          iter = myList.erase(iter);
          iter--;
     }
}

Arg. That's just annoying...

EDIT:
Or this:

Code:

for (iter = myList.begin() ; iter != myList.end() ; ++iter)
{
     std::cout << (*iter) << std::endl;
     if ((*iter) == 5)
          iter = myList.erase(iter)--;
}

Bit less retarded I guess...but still pretty bad...

Both of those suggestions are broken, do not try to do that. You must simply remove any iterator incrementing from the last part of the for-loop expression (the ++iter) and only increment if not erasing.
Otherwise:

Code:

iter = myList.erase(iter++);

Wouldn't that expression erase element 5, then increment the now possibly invalid iterator, then set the iterator to the element after (6) ? In essence, doing nothing? I'm pretty sure the postfix increment operator has higher precedence than the assignment operator.

That increment is pointless, since the value assigned to the left-hand side is the result of the function call anyway.

Perhaps this was meant.

Code:

list.erase(iter++);

This not incrementing an invalid iterator has to do with sequence points: side-effects have to be evaluated before entering the function body.

BTW, whether this is a right thing to do or not depends on the container. E.g don't try this with a vector.

If you want to erase elements from the entire list, then remove erases elements with a particular value and remove_if erases elements for which a condition is true. You'll only need a manual loop for a task more complicated, and I don't think there's much new to invent about how that looks.

Originally Posted by anon

Perhaps this was meant.

Code:

list.erase(iter++);

Yes indeed. Copy and paste error on my part.

since this is going on..i don't still understand why I can't just increment the iterator after erasing it..i really get the point of the else

if erase returns the element after the element to be deleted in question(in this case 5..so it should return the address that points to 6, right)..in the if statement i should be able to do..

Code:

     while(iter!=list.end())
     {
         if(*iter==5)
        {
             iter = list.erase(iter) ;
             cout << *iter ;
        }

        iter++ ;
        cout << *iter ;
     }

If I do that I get a seg fault...I don't get it..that should be perfectly safe..And I've been reading that a list is sequential in memory..i even printed it out myself..and the addresses using the STL list template are freaking sequential..so why do I get a access violation, when I try that?