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iterators: operators

Started by AngelForce Dec 17, 2002 at 1:17 PM 7 replies 750+ views

Original Post

Dec 17, 2002 at 1:17 PM

Ok, to me it seems, that you can only use the operator++ (and -- in bidirectional iterators) when dealing with iterators... But what should I do, when I want to access every second element of a list?? with vector I could write

        
std::vector<int> vec;
int i;
for(i=0; i<what_ever; i+=2)
    vec[i] = what_ever_2;

but a list doesn't have random access iterators and I want a way to solve the problem with iterators like

    
std::vector<int> vec;
std::vector<int>::iterator it;
for(it=vec.begin(); it!=vec.end();//vec.end() alone wouldn't work     but that's

                        //not the point

                       it+=2)
    *it = what_ever_2;

Is there a way, or would i have to use:

  
for(it=vec.begin(); it!=vec.end(); ++it)
{
     ++it;
     *it = what_ever_ever_ever;
}

????? Plz help me and thx for your answers! AngelForce EDIT: Fixed a source tag EDIT: these stupid tags... -- If you find any mistakes, you're allowed to keep 'em! 'When I look back I am lost.' [edited by - AngelForce on December 17, 2002 2:20:33 PM] [edited by - AngelForce on December 17, 2002 2:22:26 PM] [edited by - AngelForce on December 17, 2002 2:44:26 PM]

Sneftel

296

Sneftel

Dec 17, 2002 at 1:27 PM

For list iterators, there's no shortcut for skipping forwards; iteration is a O(n) operation. I would do this:


for(it=vec.begin(); it!=vec.end(); ++it, ++it)
{
     *it = what_ever_ever_ever;
}

just because it's clearer. Look up the comma operator in a C++ book if you're unsure of its syntax.

EDIT: make sure you've got the it=vec.begin() there.

Don't listen to me. I've had too much coffee.

[edited by - sneftel on December 17, 2002 2:27:34 PM]

AngelForce

Author

AngelForce

Dec 17, 2002 at 1:40 PM

quote:
Original post by Sneftel
For list iterators, there''s no shortcut for skipping forwards; iteration is a O(n) operation. I would do this:

for(it=vec.begin(); it!=vec.end(); ++it, ++it)
{
*it = what_ever_ever_ever;
}

just because it''s clearer. Look up the comma operator in a C++ book if you''re unsure of its syntax.

EDIT: make sure you''ve got the it=vec.begin() there.

Don''t listen to me. I''ve had too much coffee.

[edited by - sneftel on December 17, 2002 2:27:34 PM]

Sorry for the not existing .begin()....I wrote it in haste(as we all do

), but I''ll edit it!

Thanks for your answer, but I''m looking for another way...I know that I can do this, but I''d like to know if there''s a way to access every second(or third, or 4th...) element in O(n) (when writing

 ++it, ++it

, I would need twice as much time)

civguy

Dec 17, 2002 at 1:46 PM

++++it

Anyway, if you want to skip more, just make your own lil' function for that

    
template<class T>
inline T& increment(T& t, int amount) {
  for (int i=0; i<amount; ++i) {
    ++t;
  }
  return t;
}

for(it=vec.begin(); it!=vec.end(); increment(it, 10)) {}

(name it better than I did)

Or even (don't do this plz!)

  
template <class T, class A>
inline std::list<T,A>::iterator& operator+=(std::list<T,A>::iterator& it, int amount) {
  for (int i=0; i<amount; ++i) {
    ++it;
  }
  return it;
}

for(it=lst.begin(); it!=lst.end(); it+=10) {}

Also read what Fruny says below

[edited by - civguy on December 17, 2002 2:55:06 PM]

Fruny

203

Fruny

Dec 17, 2002 at 1:46 PM

quote:

Original post by Sneftel


for(it=vec.begin(); it!=vec.end(); ++it, ++it)
{
     *it = what_ever_ever_ever;
}

Odd number of elements + test for end() with != → boom.

Not that you have a choice in your test...

DrPizza

Dec 17, 2002 at 1:55 PM

quote:
Anyway, if you want to skip more, just make your own lil'' function for that

Is there something wrong with std::advance()?

But, no, such an approach is no good for this problem.

Because you have to handle odd-sized lists:


for(container::iterator it = container.begin(), end = container.end(); ; )
{
    // do something with it
    // advance once and test
    ++it;
    if(it == end) { break; }
    // advance again and test
    ++it;
    if(it == end) { break; }
}

I would prefer not to put any advancing or testing in the for''s conditions, simply because I don''t like splitting them up.

civguy

Dec 17, 2002 at 1:57 PM

quote:
Original post by DrPizza
Is there something wrong with std::advance()?

Yea: It''s existence is not known

AngelForce

Author

AngelForce

Dec 17, 2002 at 2:14 PM

quote:
Original post by civguy
[quote]Original post by DrPizza
Is there something wrong with std::advance()?

Yea: It''s existence is not known

  
//MS implementation:----------------------

		// TEMPLATE FUNCTION advance

template<class _InIt, class _Diff> inline
	void advance(_InIt& _Where, _Diff _Off)
	{	// increment iterator by offset, arbitrary iterators

	_Advance(_Where, _Off, _Iter_cat(_Where));
	}

template<class _InIt, class _Diff> inline
	void _Advance(_InIt& _Where, _Diff _Off, input_iterator_tag)
	{	// increment iterator by offset, input iterators

	for (; 0 < _Off; --_Off)
		++_Where;
	}

//SGI implementation:---------------------

template <class _InputIter, class _Distance>
inline void __advance(_InputIter& __i, _Distance __n, input_iterator_tag) {
  while (__n--) ++__i;
}

Ok, they aren''t doing it different...thanks for your answers guys, now I know a little bit more!

AngelForce

--
If you find any mistakes, you''re allowed to keep ''em!

''When I look back I am lost.''

civguy

Dec 17, 2002 at 2:49 PM

AngelForce, I didn''t mean it doesn''t exist or do the job correctly. I just meant I didn''t know about it

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