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std::list<> & iterator

Started by V3rt3x Feb 26, 2003 at 4:48 PM 12 replies 650+ views

Original Post

Feb 26, 2003 at 4:48 PM

Hi all! I have a list of std::string(s) which I want to display only the X last of them. But I have''nt found any better solution than :

  
itor = m_szBuffer.end();
for( int i = 0; i < szCount; i++, itor-- )
   ;

to set it at the good position in the linked list (I hope you understand me :/) Furthermore I find this code very poor that the next piece of code is :

  
std::string text = "";
for( i = 0; i < szCount; i++, itor++ )
   text += (*itor);

So is there a better solution to adjust my iterator on the wanted string on my list ? I don''t know much about iterators, I hope there is a better way to do that...

Ironside

Feb 26, 2003 at 4:58 PM

just dereference your iterator...

itor = m_szBuffer.back();
text += (*itor);

I think that's what you want...

front( ) and back( ) return an iterator at the first and last elemetns in the list

begin() and end() return an itiorator before the first and after the last elements respectively.

oh. wait i see what your doing.. if your displaying the last X strings in your list why don't you do something like thiis

    
itor = m_szBuffer.end();
for( int i = 0; i < szCount; i++, itor--)
     text = (*itor) + text;

[edited by - Ironside on February 26, 2003 6:02:40 PM]

[edited by - Ironside on February 26, 2003 6:03:22 PM]

civguy

Feb 26, 2003 at 5:36 PM

Ironside, that's inefficient. A lot of temporary strings need to be created.

Standard algorithms can be used, but they aren't any faster. They're a nice way to confuse people:

  std::list<std::string>::reverse_iterator ritor = m_szBuffer.rbegin();
std::advance(ritor, szCount);
std::string text = std::accumulate(ritor.base(), m_szBuffer.end(), std::string());

(damn forums, needed to press 'post' five times to get through..)

[edited by - civguy on February 26, 2003 6:44:26 PM]

V3rt3x

Author

V3rt3x

Feb 26, 2003 at 6:02 PM

Oh thanx civguy and anonymous poster! it works perfectly!!!

Ironside

Feb 27, 2003 at 2:29 PM

Ooh.. advance, that outta help my code all over the place.

Ironside

Feb 27, 2003 at 2:29 PM

Ooh.. advance, that outta help my code all over the place.

Lorenzo

Feb 27, 2003 at 3:02 PM

quote:
Original post by Ironside

front( ) and back( ) return an iterator at the first and last elemetns in the list

begin() and end() return an itiorator before the first and after the last elements respectively.

That''s wrong! front() and begin() both return an iterator to the first element!

civguy

Feb 27, 2003 at 3:08 PM

quote:
Original post by Lorenzo
That''s wrong! front() and begin() both return an iterator to the first element!

Since when? Why would there be two functions to do the same?

http://www.sgi.com/tech/stl/Vector.html

See what''s said about front()

fizban75

Feb 27, 2003 at 3:56 PM

front() and back() return *references* to the first and last elements in the list.

begin() returns an *iterator* to the first element in the list. end() returns an *iterator* to just beyond the end of the list.

Ironside

Feb 27, 2003 at 4:59 PM

The pointer, or first element in the list doesn''t contain a value though, it''s also not counted as part of the lists size.
So you you use begin() you are getting an iterator at the first element in your list. however the content you instert into the list doesn''t start until the following element.

that''s why i said begin() returns an iterator before the first element in the list. Symantics

And yes fizban75 is correct, front and back return *references* my bad.

Doc

Feb 27, 2003 at 7:15 PM

quote:
Original post by Ironside
The pointer, or first element in the list doesn''t contain a value though, it''s also not counted as part of the lists size.
So you you use begin() you are getting an iterator at the first element in your list. however the content you instert into the list doesn''t start until the following element.

that''s why i said begin() returns an iterator before the first element in the list. Symantics

What the hell are you talking about? Of course the first element contains a value (unless the list is empty). begin() returns an iterator to the first element in the list. Where a new element will be inserted into the list given an iterator is a quirk of the insert method, not of what begin gives you.

Peace,
Doc

Ironside

Feb 27, 2003 at 7:23 PM

fire it up in the debugger and take a look...

list points;

int size = points.size();

set a breakpoint on the above line and look at the points variable in the watch window. There''s a node structure called Head, with members _Next, _Prev, and _Value.

points.push_back(2);

look at points in the watch window again...
the Head element''s _Value member is still garbage..
however if you expand Heads''s _Next member and look at that _Value member you''ll see it''s been initialized to 2.

Shannon Barber

Mod

368

Shannon Barber

Feb 28, 2003 at 2:18 AM

You may want to use .rbegin(), .rend(), & reverse iterators if X is small compared to the length of the list.

.rend() returns an iterator prior to the first element - as .end() returns an iterator after the last.

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