How to delete stl<map> element from iterator?

[Steve Edwards]

Hi,

After making an iterator to a map and stepping through a loop, I want to
delete any entries that satisfy a test from an external function.

MyMapType::cons t_iterator iter;

for(iter = m->begin(); iter != m->end(); ++iter)
{
a = iter->first;
b = iter->second;
if ( Test(a,b) == true)
iter->delete?();
}

How do I actually delete the element that iter is pointing to?

Thanks

Steve

[Mike Wahler]

Re: How to delete stl&lt;map&gt; element from iterator?


"Steve Edwards" <gfx@lineone.ne t> wrote in message
news:gfx-D6FC33.17325521 022006@news.bti nternet.com...[color=blue]
> Hi,
>
> After making an iterator to a map and stepping through a loop, I want to
> delete any entries that satisfy a test from an external function.
>
> MyMapType::cons t_iterator iter;
>
> for(iter = m->begin(); iter != m->end(); ++iter)[/color]

This assumes that 'm' is the name of a pointer to your map,
not of the map itself. I'll keep that assumption below.
[color=blue]
> {
> a = iter->first;
> b = iter->second;
> if ( Test(a,b) == true)
> iter->delete?();
> }
>
> How do I actually delete the element that iter is pointing to?[/color]

for(iter = m->begin(); iter != m->end(); ++iter)
{
if(condition)
iter = m->erase(iter);
}


An excellent reference for the C++ standard library:

https://www.josuttis.com/libbook

-Mike

[Victor Bazarov]

Re: How to delete stl&lt;map&gt; element from iterator?

Steve Edwards wrote:[color=blue]
> After making an iterator to a map and stepping through a loop, I want to
> delete any entries that satisfy a test from an external function.
>
> MyMapType::cons t_iterator iter;
>
> for(iter = m->begin(); iter != m->end(); ++iter)
> {
> a = iter->first;
> b = iter->second;
> if ( Test(a,b) == true)
> iter->delete?();
> }
>
> How do I actually delete the element that iter is pointing to?[/color]

You ask your map to erase it.

Something like

m->erase(iter);

Beware, though, that after erasing the 'iter' becomes invalid. You cannot
increment it after that. You might want to rewrite your loop like this:

for (.... ; ) // no increment at the end
{
a =
b =
if (..
m->erase(iter++ );
else
++iter;
}

V
--
Please remove capital As from my address when replying by mail

[Mike Wahler]

Re: How to delete stl&lt;map&gt; element from iterator?

"Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message
news:mrIKf.1333 $a97.1001@newsr ead1.mlpsca01.u s.to.verio.net. ..[color=blue]
> Steve Edwards wrote:[/color]
[color=blue][color=green]
>> How do I actually delete the element that iter is pointing to?[/color]
>
> You ask your map to erase it.
>
> Something like
>
> m->erase(iter);
>
> Beware, though, that after erasing the 'iter' becomes invalid. You cannot
> increment it after that. You might want to rewrite your loop like this:[/color]

Well, I think we both botched our replies. I forgot the
'else' part, and yours disregards the invalid iterator. :-)
[color=blue]
>
> for (.... ; ) // no increment at the end
> {
> a =
> b =
> if (..
> m->erase(iter++ );[/color]

iter = m->erase(iter);
[color=blue]
> else
> ++iter;
> }[/color]

-Mike

[Victor Bazarov]

Re: How to delete stl&lt;map&gt; element from iterator?

Mike Wahler wrote:[color=blue]
> if(condition)
> iter = m->erase(iter);[/color]

IIRC, 'map::erase' has 'void' return type. Some Standard Library
implementations actually violate that to be consistent with other
(sequential) containers, and probably justifiedly, but generally
speaking your code is ill-formed.
[color=blue]
> }
>
>
> An excellent reference for the C++ standard library:
> www.josuttis.com/libbook[/color]

Yep. Check out table 6.31 in it.

V
--
Please remove capital As from my address when replying by mail

[Victor Bazarov]

Re: How to delete stl&lt;map&gt; element from iterator?

Mike Wahler wrote:[color=blue]
> "Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message
> news:mrIKf.1333 $a97.1001@newsr ead1.mlpsca01.u s.to.verio.net. ..
>[color=green]
>>Steve Edwards wrote:[/color]
>
>[color=green][color=darkred]
>>>How do I actually delete the element that iter is pointing to?[/color]
>>
>>You ask your map to erase it.
>>
>>Something like
>>
>> m->erase(iter);
>>
>>Beware, though, that after erasing the 'iter' becomes invalid. You cannot
>>increment it after that. You might want to rewrite your loop like this:[/color]
>
>
> Well, I think we both botched our replies. I forgot the
> 'else' part, and yours disregards the invalid iterator. :-)[/color]

Think again.
[color=blue]
>
>[color=green]
>> for (.... ; ) // no increment at the end
>> {
>> a =
>> b =
>> if (..
>> m->erase(iter++ );[/color]
>
>
> iter = m->erase(iter);
>
>[color=green]
>> else
>> ++iter;
>> }[/color]
>
>
> -Mike
>
>[/color]


--
Please remove capital As from my address when replying by mail

[Steve Edwards]

Re: How to delete stl&lt;map&gt; element from iterator?

[color=blue]
>
> Beware, though, that after erasing the 'iter' becomes invalid. You cannot
> increment it after that. You might want to rewrite your loop like this:
>
> for (.... ; ) // no increment at the end
> {
> a =
> b =
> if (..
> m->erase(iter++ );
> else
> ++iter;
> }
>
> V[/color]

Thanks, but sorry, I don't quite understand. If you say the iterator
becomes invalid, should I break at that point. (in which case I haven't
processed all my elements yet.)
On the other hand, I can't continue if the iterator's invalid, can I?

Steve

[Steve Edwards]

Re: How to delete stl&lt;map&gt; element from iterator?

In article <0mIKf.2502$VI6 .433@newsread1. news.pas.earthl ink.net>,
"Mike Wahler" <mkwahler@mkwah ler.net> wrote:
[color=blue]
> "Steve Edwards" <gfx@lineone.ne t> wrote in message
> news:gfx-D6FC33.17325521 022006@news.bti nternet.com...[color=green]
> > Hi,
> >
> > After making an iterator to a map and stepping through a loop, I want to
> > delete any entries that satisfy a test from an external function.
> >
> > MyMapType::cons t_iterator iter;
> >
> > for(iter = m->begin(); iter != m->end(); ++iter)[/color]
>
> This assumes that 'm' is the name of a pointer to your map,
> not of the map itself. I'll keep that assumption below.
>[color=green]
> > {
> > a = iter->first;
> > b = iter->second;
> > if ( Test(a,b) == true)
> > iter->delete?();
> > }
> >
> > How do I actually delete the element that iter is pointing to?[/color]
>
> for(iter = m->begin(); iter != m->end(); ++iter)
> {
> if(condition)
> iter = m->erase(iter);
> }
>
>
> An excellent reference for the C++ standard library:
> www.josuttis.com/libbook
>
> -Mike[/color]



Thanks for the link, it's just what I need.

My compiler is complaining about "iter = m->erase(iter); "

(error: no match for 'operator = ')

[TB]

Re: How to delete stl&lt;map&gt; element from iterator?

Steve Edwards skrev:[color=blue][color=green]
>> Beware, though, that after erasing the 'iter' becomes invalid. You cannot
>> increment it after that. You might want to rewrite your loop like this:
>>
>> for (.... ; ) // no increment at the end
>> {
>> a =
>> b =
>> if (..
>> m->erase(iter++ );
>> else
>> ++iter;
>> }
>>
>> V[/color]
>
> Thanks, but sorry, I don't quite understand. If you say the iterator
> becomes invalid, should I break at that point. (in which case I haven't
> processed all my elements yet.)
> On the other hand, I can't continue if the iterator's invalid, can I?
>[/color]

The statement:

iter++;

increments the iterator and returns the previous position. Compare with

int x = 8, y = 0;
y = x++;
// y = 8, x == 9

so the following

m->erase(iter++ );

safely erases the desired element and retains a valid iterator pointing
to the next element or m->end().

--
TB @ SWEDEN

[Victor Bazarov]

Re: How to delete stl&lt;map&gt; element from iterator?

Steve Edwards wrote:[color=blue][color=green]
>>Beware, though, that after erasing the 'iter' becomes invalid. You cannot
>>increment it after that. You might want to rewrite your loop like this:
>>
>> for (.... ; ) // no increment at the end
>> {
>> a =
>> b =
>> if (..
>> m->erase(iter++ );
>> else
>> ++iter;
>> }
>>
>>V[/color]
>
>
> Thanks, but sorry, I don't quite understand. If you say the iterator
> becomes invalid, should I break at that point. (in which case I haven't
> processed all my elements yet.)[/color]

Why should you? By passing the value of the post-incremented iterator you
keep it valid. If the 'iter' isn't post-incremented, after 'erase' does
its job, 'iter' cannot be used -- it's invalid. In the proposed code, the
value of 'iter' is passed to 'erase', but _outside_ the object 'iter' is
already pointing to the next element of the map, and therefore is OK.
[color=blue]
> On the other hand, I can't continue if the iterator's invalid, can I?[/color]

No, if it's invalid, you can't. The whole point of using post-increment
is to keep it valid.

V
--
Please remove capital As from my address when replying by mail

[Victor Bazarov]

Re: How to delete stl&lt;map&gt; element from iterator?

Steve Edwards wrote:[color=blue]
> In article <0mIKf.2502$VI6 .433@newsread1. news.pas.earthl ink.net>,
> "Mike Wahler" <mkwahler@mkwah ler.net> wrote:[color=green]
>>"Steve Edwards" <gfx@lineone.ne t> wrote in message
>>news:gfx-D6FC33.17325521 022006@news.bti nternet.com...
>> [..][color=darkred]
>>>How do I actually delete the element that iter is pointing to?[/color]
>>
>>for(iter = m->begin(); iter != m->end(); ++iter)
>>{
>> if(condition)
>> iter = m->erase(iter);
>>}
>> [..][/color]
>
> My compiler is complaining about "iter = m->erase(iter); "[/color]

Of course it does. 'm->erase(iter)' returns 'void'. Mike was too hasty
to suggest that solution. It would work for a 'list', for example. It
just doesn't work for 'map'.

V
--
Please remove capital As from my address when replying by mail

[Mike Wahler]

Re: How to delete stl&lt;map&gt; element from iterator?


"Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message
news:VJIKf.1336 $a97.597@newsre ad1.mlpsca01.us .to.verio.net.. .[color=blue]
> Mike Wahler wrote:[color=green]
>> "Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message
>> news:mrIKf.1333 $a97.1001@newsr ead1.mlpsca01.u s.to.verio.net. ..
>>[color=darkred]
>>>Steve Edwards wrote:[/color]
>>
>>[color=darkred]
>>>>How do I actually delete the element that iter is pointing to?
>>>
>>>You ask your map to erase it.
>>>
>>>Something like
>>>
>>> m->erase(iter);
>>>
>>>Beware, though, that after erasing the 'iter' becomes invalid. You
>>>cannot
>>>increment it after that. You might want to rewrite your loop like this:[/color]
>>
>>
>> Well, I think we both botched our replies. I forgot the
>> 'else' part, and yours disregards the invalid iterator. :-)[/color]
>
> Think again.[/color]

OK I did. I see I wasn't thinking before. :-)
[color=blue]
>[color=green]
>>
>>[color=darkred]
>>> for (.... ; ) // no increment at the end
>>> {
>>> a =
>>> b =
>>> if (..
>>> m->erase(iter++ );[/color]
>>
>>
>> iter = m->erase(iter);[/color][/color]

You've already pointed out my error with this line elsethread.

My excuse is ... we've got a new dog, she ate my attention. :-)

-Mike