vector of lists: anyone know how to do this?

[craig]

I've declared the following as a "vector" of "lists" using STL

vector< list<Edge*> >

I've tried many ways to add objects to this but can't figure out how to
do it. Does anyone have any tips, or references to material that might
explain it?

Any help greatly appreciated.

[Default User]

craig wrote:

I've declared the following as a "vector" of "lists" using STL

vector< list<Edge*> >

I've tried many ways to add objects to this but can't figure out how
to do it. Does anyone have any tips, or references to material that
might explain it?

And what is reason you decided to hide your code from us? How can we
figure out what you're doing wrong?


Brian

[Kirit Sælensminde]

craig wrote:

I've declared the following as a "vector" of "lists" using STL

vector< list<Edge*> >

I've tried many ways to add objects to this but can't figure out how to
do it. Does anyone have any tips, or references to material that might
explain it?

Any help greatly appreciated.

Why not something like:

Edge *pEdge = NULL;
vector< list< Edge * > > edges( 10 );
edges[ 0 ].push_back( pEdge );
Putting pointers into STL containers is not always a good idea. Make
sure you know what you're doing.

Prefer something like this:

boost::shared_p tr< Edge > pEdge;
vector< list< boost::shared_p tr< Edge > > edges( 10 );
edges[ 0 ].push_back( pEdge );

Your choice of smart pointer depends on context though, although
boost::shared_p tr is for many uses a good one. std::auto_ptr is not
normally a good choice.
Of course including the code that doesn't work and telling us what
you're thinking about might help us to answer better.
K

[Jerry Coffin]

In article <1150530822.539 811.41420
@i40g2000cwc.go oglegroups.com> ,
ki************* ***@gmail.com says...

[ ... ]

Your choice of smart pointer depends on context though, although
boost::shared_p tr is for many uses a good one. std::auto_ptr is not
normally a good choice.

Let me put that more strongly. Anything you store in a
standard container needs to be copyable, and
std::auto_ptr isn't copyable, so it's simply not allowed.
In most cases, compilation will fail if you try, though
the error message(s) rarely make the source of the
problem very clear.

--
Later,
Jerry.

The universe is a figment of its own imagination.

[craig]

Brian,..Sorry, I didn't give more info (it was late and I was tired).
But here is a bit more:

vector<Vertex*> vertices;
// array of adjecacency lists
vector< list<Edge*> > adjLists; <<<<< the focus

This is an array(vector) of "adjacency lists" for a graph. The graph
is empty at first, and then added to dynamically. I don't know if more
initialization has to be done before I add to it, but I've tried to add
elements in the following ways. (Adding an element is essentially
adding an "Edge" to the i-th adjacency list). pEdge is a ptr to an
Edge:

I tried:
adjLists.push_b ack( pEdge );
then:

adjLists[0].push_back( pEdge);

then:
list <Edge*> L;
adjLists[0] = L

I've tried many other things also, but most don't compile, and the ones
that do crash.

-Craig
Default User wrote:

craig wrote:

I've declared the following as a "vector" of "lists" using STL

vector< list<Edge*> >

I've tried many ways to add objects to this but can't figure out how
to do it. Does anyone have any tips, or references to material that
might explain it?

And what is reason you decided to hide your code from us? How can we
figure out what you're doing wrong?


Brian

[craig]

Thanks, Kirit for your comments. I'm going to see if I can incorporate
your idea into what I'm doing. I've also provided a little more info
in my response to Brian. If my initial info was too vague, perhaps you
could look at this response.

Thanks,
-Craig

Kirit Sælensminde wrote:

craig wrote:

I've declared the following as a "vector" of "lists" using STL

vector< list<Edge*> >

I've tried many ways to add objects to this but can't figure out how to
do it. Does anyone have any tips, or references to material that might
explain it?

Any help greatly appreciated.

Why not something like:

Edge *pEdge = NULL;
vector< list< Edge * > > edges( 10 );
edges[ 0 ].push_back( pEdge );
Putting pointers into STL containers is not always a good idea. Make
sure you know what you're doing.

Prefer something like this:

boost::shared_p tr< Edge > pEdge;
vector< list< boost::shared_p tr< Edge > > edges( 10 );
edges[ 0 ].push_back( pEdge );

Your choice of smart pointer depends on context though, although
boost::shared_p tr is for many uses a good one. std::auto_ptr is not
normally a good choice.
Of course including the code that doesn't work and telling us what
you're thinking about might help us to answer better.


K

[Roland Pibinger]

On 17 Jun 2006 10:44:38 -0700, "craig" <cr**********@c omcast.net>
wrote:

vector<Vertex* > vertices;
// array of adjecacency lists
vector< list<Edge*> > adjLists; <<<<< the focus

This is an array(vector) of "adjacency lists" for a graph. The graph
is empty at first, and then added to dynamically. I don't know if more
initializati on has to be done before I add to it, but I've tried to add
elements in the following ways. (Adding an element is essentially
adding an "Edge" to the i-th adjacency list). pEdge is a ptr to an
Edge:

I tried:
adjLists.push_ back( pEdge );
This does not compile because you try to push a pointer to and Edge
object into a vector of lists(!) of pointers to Edges.
then:

adjLists[0].push_back( pEdge);
adjLists is empty, you access the fist element in an empty array ->
crash.
then:
list <Edge*> L;
adjLists[0] = L
again, you access to fist element in an empty array.
I've tried many other things also, but most don't compile, and the ones
that do crash.

You have an array of list <Edge*>. So, first you need an element in
your array. Since STL supports only value semantics you have to copy a
list <Edge*> into adjLists.

list <Edge*> lst;
adjLists.push_b ack (lst);

// alternatively: adjLists.resize (1);

Then you can access the inserted list<Edge*> (the first and only so
far) with operator[], which returns a refernce(!) to that list
<Edge*>.

adjLists[0].push_back( pEdge);

Good luck!
Roland Pibinger

[craig]

Thanks so much Roland!! I'm deeply greatful!

I'm in a pinch and am trying to get this to work properly after
considering other alternatives. I definitely need to study templates
much more when I get a chance.

If you (or any one else) has a moment, one more question:

Currently I am trying to use this "vector of lists" in the following
graph member:

void Graph::addEdge( Vertex &v1, Vertex &v2, int weight )
{
ind = "compute index of Vertex in vertex list"
Edge *e = new Edge (v1, v2, weight );
list <Edge*> lst; <<<<<<<<< important part to
make dynamic
adjLists.push_b ack (lst); <<<<<<<<<import ant part to make
dynamic
adjLists[ind].push_back( e );
}
I need to dynamically create a new "list <Edge*> " in place of "lst"
upon each call to "addEdge". How do I do this with templates?

Thanks,....
-Craig


Roland Pibinger wrote:

On 17 Jun 2006 10:44:38 -0700, "craig" <cr**********@c omcast.net>
wrote:

vector<Vertex* > vertices;
// array of adjecacency lists
vector< list<Edge*> > adjLists; <<<<< the focus

This is an array(vector) of "adjacency lists" for a graph. The graph
is empty at first, and then added to dynamically. I don't know if more
initializati on has to be done before I add to it, but I've tried to add
elements in the following ways. (Adding an element is essentially
adding an "Edge" to the i-th adjacency list). pEdge is a ptr to an
Edge:

I tried:
adjLists.push_ back( pEdge );

This does not compile because you try to push a pointer to and Edge
object into a vector of lists(!) of pointers to Edges.

then:

adjLists[0].push_back( pEdge);

adjLists is empty, you access the fist element in an empty array ->
crash.

then:
list <Edge*> L;
adjLists[0] = L

again, you access to fist element in an empty array.

I've tried many other things also, but most don't compile, and the ones
that do crash.

You have an array of list <Edge*>. So, first you need an element in
your array. Since STL supports only value semantics you have to copy a
list <Edge*> into adjLists.

list <Edge*> lst;
adjLists.push_b ack (lst);

// alternatively: adjLists.resize (1);

Then you can access the inserted list<Edge*> (the first and only so
far) with operator[], which returns a refernce(!) to that list
<Edge*>.

adjLists[0].push_back( pEdge);

Good luck!
Roland Pibinger

[craig]

Again , thanks so much Roland and others!!!

Roland you corrections worked perfectly. One more question for anyone:
I need to dynamically create new "list <Edge*>"'s in place of "lst"
upon subsequent calls to "addEdge" (code below). How do I do this with
templates?

Do I create a list <Edge*> lists[10]; somewhere and index into it?

void Graph::addEdge( Vertex &v1, Vertex &v2, int weight )
{
ind = "compute index of Vertex in vertex list"
Edge *e = new Edge (v1, v2, weight );
list <Edge*> lst; <- how to make dynamic ??
adjLists.push_b ack (lst); <-how to make dynamic??
adjLists[ind].push_back( e );
}


Roland Pibinger wrote:

On 17 Jun 2006 10:44:38 -0700, "craig" <cr**********@c omcast.net>
wrote:

vector<Vertex* > vertices;
// array of adjecacency lists
vector< list<Edge*> > adjLists; <<<<< the focus

This is an array(vector) of "adjacency lists" for a graph. The graph
is empty at first, and then added to dynamically. I don't know if more
initializati on has to be done before I add to it, but I've tried to add
elements in the following ways. (Adding an element is essentially
adding an "Edge" to the i-th adjacency list). pEdge is a ptr to an
Edge:

I tried:
adjLists.push_ back( pEdge );

This does not compile because you try to push a pointer to and Edge
object into a vector of lists(!) of pointers to Edges.

then:

adjLists[0].push_back( pEdge);

adjLists is empty, you access the fist element in an empty array ->
crash.

then:
list <Edge*> L;
adjLists[0] = L

again, you access to fist element in an empty array.

I've tried many other things also, but most don't compile, and the ones
that do crash.

You have an array of list <Edge*>. So, first you need an element in
your array. Since STL supports only value semantics you have to copy a
list <Edge*> into adjLists.

list <Edge*> lst;
adjLists.push_b ack (lst);

// alternatively: adjLists.resize (1);

Then you can access the inserted list<Edge*> (the first and only so
far) with operator[], which returns a refernce(!) to that list
<Edge*>.

adjLists[0].push_back( pEdge);

Good luck!
Roland Pibinger

[Kirit Sælensminde]

craig wrote:

void Graph::addEdge( Vertex &v1, Vertex &v2, int weight )
{
ind = "compute index of Vertex in vertex list"
Edge *e = new Edge (v1, v2, weight );
list <Edge*> lst; <- how to make dynamic ??
adjLists.push_b ack (lst); <-how to make dynamic??
adjLists[ind].push_back( e );
}

The list is already dynamic, the push_back will add to the end. The
vector is slightly different though. I think you missed the suggestion
of the resize() somewhere.

What you need is (I'm not g'teeing it will compile directly) is
something like:

vector< list< boost::shared_p tr< Edge > > >::size_type ind = /* Your
calc */;
if ( adjLists.size() < ind + 1 ) // Remember it is zero indexed
adjLists.resize ( ind + 1 );
adjLists[ ind ].push_back( boost::shared_p tr< Edge >( new Edge( v1, v2,
weight ) ) );

Things to note:
* The if statement checks to see that the vector is big enough _before_
you access the index.
* The use of the smart pointer. Don't get in the habit of using raw
pointers.
* The use of the type vector<>::size_ type for the index. Your compiler
vendor has gone to a lot of trouble making sure you won't have any
unexpected overflows etc. when they chose this type. Use it rather than
make your own guess about what is right.
* The vector is zero indexed so index 1 means it must be at least size
2
K