removeNamedItem problem

[Julius Mong]

Dear all,

I'm trying to search for "width" and "height" in the root node in an SVG doc
and remove them accordingly, what I don't understand is if I did the
following code, it'll only remove "width" (the earlier in the attr list) in
<svg ... width="..." height="..." > and height is left untouched.

// SVG DOM Root
MSXML2::IXMLDOMElementPtr pDoc = pDom->documentElement;
MSXML2::IXMLDOMNamedNodeMapPtr pNamedNodeMap = NULL;
MSXML2::IXMLDOMNodePtr pRemovedNode = NULL;

pNamedNodeMap = pDoc->Getattributes();
CComBSTR attrWidth("width");
CComBSTR attrHeight("height");
for (int numAttr = 0; numAttr < pNamedNodeMap->Getlength(); numAttr++) {
BSTR attrName = pNamedNodeMap->Getitem(numAttr)->GetnodeName();
CComBSTR bstrAttrName;
bstrAttrName.AppendBSTR(attrName);
if ( bstrAttrName == attrWidth || bstrAttrName == attrHeight )
pRemovedNode = pNamedNodeMap->removeNamedItem(attrName);
bstrAttrName.Empty();
pRemovedNode = NULL;
}

But if I did the following, they're both removed... I don't understand why I
have to do 2 passes?

// Create SVG DOM Root
MSXML2::IXMLDOMElementPtr pDoc = pDom->documentElement;

// Remove any width/height attrs in svg (mattercast)
MSXML2::IXMLDOMNamedNodeMapPtr pNamedNodeMap = NULL;
MSXML2::IXMLDOMNodePtr pRemovedNode = NULL;

pNamedNodeMap = pDoc->Getattributes();
CComBSTR attrWidth("width");
CComBSTR attrHeight("height");
for (int numAttr = 0; numAttr < pNamedNodeMap->Getlength(); numAttr++) {
BSTR attrName = pNamedNodeMap->Getitem(numAttr)->GetnodeName();
CComBSTR bstrAttrName;
bstrAttrName.AppendBSTR(attrName);
if (bstrAttrName == attrWidth) pRemovedNode =
pNamedNodeMap->removeNamedItem(attrName);
bstrAttrName.Empty();
pRemovedNode = NULL;
}
for (numAttr = 0; numAttr < pNamedNodeMap->Getlength(); numAttr++) {
BSTR attrName = pNamedNodeMap->Getitem(numAttr)->GetnodeName();
CComBSTR bstrAttrName;
bstrAttrName.AppendBSTR(attrName);
if (bstrAttrName == attrHeight) pRemovedNode =
pNamedNodeMap->removeNamedItem(attrName);
bstrAttrName.Empty();
pRemovedNode = NULL;
}

Very confused... please enlighten...

Thanks, Juliues

[Derek Harmon]

"Julius Mong" <jx****@hotmail.com> wrote in message news:uv**************@tk2msftngp13.phx.gbl...

following code, it'll only remove "width" (the earlier in the attr list) in
<svg ... width="..." height="..." > and height is left untouched. : : for (int numAttr = 0; numAttr < pNamedNodeMap->Getlength(); numAttr++) {
BSTR attrName = pNamedNodeMap->Getitem(numAttr)->GetnodeName(); : : pRemovedNode = pNamedNodeMap->removeNamedItem(attrName);
If the width attribute is always immediately followed by the height attribute, then the
reason height is left untouched is because this for loop skips it. It's a common mistake
when removing items while you're enumerating their collection.

Let's walkthru the loop, and assume there are only two attributes: width and height.

First numAttr <- 0, which is less than the NodeMap length (2). The attribute in slot 0
is "width," so it is removed.

Next, numAttr <- 1, which is NOT less than the NodeMap length (1; remember, we
removed one of the nodes), so the for-loop ends.

Notice how the "height" attribute is left untouched?

Even had the NodeMap contained additional attributes, when "width" is removed, the
"height" slides up into slot 0, so it is never seen and is again untouched.

When removing nodes from a collection that you're enumerating, it's usually best to work
from the end of the NodeMap towards the beginning (if for no other reason than it sidesteps
this common pitfall entirely).

- - - RemoveNamedItem.cpp (excerpt)
// . . .
for ( int numAttr = pNamedNodeMap->Getlength( ) - 1; numAttr >= 0; --numAttr) {
BSTR attrName = pNamedNodeMap->Getitem( numAttr)->GetnodeName( );
CComBSTR bstrAttrName;
bstrAttrName.AppendBSTR( attrName);
if ( bstrAttrName == attrWidth || bstrAttrName == attrHeight ) {
pRemovedNode = pNamedNodeMap->removeNamedItem( attrName);
}
bstrAttrName.Empty( );
pRemovedNode = NULL;
}
// . . .
- - -
But if I did the following, they're both removed... I don't understand why I
have to do 2 passes? : : Very confused... please enlighten...

It's not necessary to do two passes. The second pass here is just catching the "height" node
that would be skipped in the original one-pass for loop.

Enlightenment comes when it's realized: the act of removing nodes changes the order and length
of the collection. When the loop goes from the length of the NamedNodeMap to the beginning,
then it no longer risks missing any nodes from the collection, since it always removes from the
end.
Derek Harmon

[Julius Mong]

Of course! :-) Thanks so much Derek!

Regards, Julius