deleting entries in a list

[Christoff Pale]

Hi,
suppose I have a list of strings;
I have to iterate of the strings and delete certain entries.
I and not sure how to do this?

for example:
#include<list>
#include<string >
#include<iostre am>
int main(){
list<string> l;
l.push_back("fi rst");
l.push_back("se cond");
l.push_back("de leteme");
l.push_back("fo urth");
l.push_back("de leteme");
l.push_back("si xth");
// I want to erase entries that say "deleteme"

I can get and iterator, but that screws up things
list<string>::i terator it=l.begin();
for( ; it!=l.end(); it++){
if( (*it).compare(" deleteme")==0){
l.erase(it);
}
}
}

This gives me segmentation fault

I think that when I delete the first "deleteme" the
it now points to "fourth";

Basically, this is what I want.
I will have a huge list of some object.
I want to traverse it all the way and delete
objects that meet a certain criteria.
any help will be much appreciated.
thanks

[Alf P. Steinbach]

* Christoff Pale:

suppose I have a list of strings;
I have to iterate of the strings and delete certain entries.
I and not sure how to do this?

for example:
#include<list>
#include<string >
#include<iostre am>
int main(){
list<string> l;
l.push_back("fi rst");
l.push_back("se cond");
l.push_back("de leteme");
l.push_back("fo urth");
l.push_back("de leteme");
l.push_back("si xth");
// I want to erase entries that say "deleteme"

I can get and iterator, but that screws up things
list<string>::i terator it=l.begin();
for( ; it!=l.end(); it++){
if( (*it).compare(" deleteme")==0){
l.erase(it);
}
}
}

This gives me segmentation fault

#include <algorithm>
#include <iostream>
#include <list>
#include <string>

int main()
{
typedef std::list<std:: string> StringList;
typedef StringList::ite rator Iterator;

StringList l;
l.push_back( "first" );
l.push_back( "second" );
l.push_back( "deleteme" );
l.push_back( "fourth" );
l.push_back( "deleteme" );
l.push_back( "sixth" );

for( Iterator it = l.begin(); it != l.end(); )
{
it->compare( "deleteme" ) == 0? it = l.erase( it ) : ++it;
}

std::copy(
l.begin(), l.end(),
std::ostream_it erator<std::str ing>( std::cout, "\n" )
);
}

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Mike Wahler]

"Christoff Pale" <ch************ @yahoo.com> wrote in message
news:73******** *************** ***@posting.goo gle.com...

Hi,
suppose I have a list of strings;
I have to iterate of the strings and delete certain entries.
I and not sure how to do this?

for example:
#include<list>
#include<string >
#include<iostre am>
int main(){
list<string> l;
std::list<std:: string> l; /* I *hate* that identifier! :-) */
l.push_back("fi rst");
l.push_back("se cond");
l.push_back("de leteme");
l.push_back("fo urth");
l.push_back("de leteme");
l.push_back("si xth");
// I want to erase entries that say "deleteme"

I can get and iterator, but that screws up things
list<string>::i terator it=l.begin();
std::list<std:: string>::iterat or it = l.begin();
for( ; it!=l.end(); it++){
if( (*it).compare(" deleteme")==0){
l.erase(it);
}
}
}

This gives me segmentation fault
When you delete an element to which the iterator points,
that iterator becomes invalidated. Then you try to increment
that invalid iterator. Undefined behavior.

I think that when I delete the first "deleteme" the
it now points to "fourth";
No it points to the outer moon of sixth planet circling
Alpha Seti. But that's just today. Tomorrow it would
point somewhere else.

std::list::eras e() returns an iterator that designates
the first element remaining beyond any elements removed,
or end() if no such element exists. So when you erase,
don't increment 'it', assign it the return value from
'erase()'.

Basically, this is what I want.
I will have a huge list of some object.
I want to traverse it all the way and delete
objects that meet a certain criteria.
any help will be much appreciated.

#include<iostre am>
#include<list>
#include<string >

int main()
{
std::list<std:: string> l;
l.push_back("fi rst");
l.push_back("se cond");
l.push_back("de leteme");
l.push_back("fo urth");
l.push_back("de leteme");
l.push_back("si xth");

std::cout << l.size() << '\n'; // prints 6

for(std::list<s td::string>::it erator it = l.begin(); it!=l.end(); it++)
if((*it).compar e("deleteme") == 0)
it = l.erase(it);

std::cout << l.size() << '\n'; // prints 4
return 0;
}

Also look up 'std::list::rem ove_if()'

-Mike

[Mike Wahler]

"Mike Wahler" <mk******@mkwah ler.net> wrote in message
news:TR******** *********@newsr ead1.news.pas.e arthlink.net...

for(std::list<s td::string>::it erator it = l.begin(); it!=l.end(); it++) if((*it).compar e("deleteme") == 0)
it = l.erase(it);

This isn't quite right. It produces a valid iterator,
but the 'for' loop increments it, which will skip
over the item after the erased one. See Alf's post
for a proper way to do this.

Sorry for the mistake.

-Mike

[David Hilsee]

"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. .

* Christoff Pale:

suppose I have a list of strings;
I have to iterate of the strings and delete certain entries.
I and not sure how to do this?

for example:
#include<list>
#include<string >
#include<iostre am>
int main(){
list<string> l;
l.push_back("fi rst");
l.push_back("se cond");
l.push_back("de leteme");
l.push_back("fo urth");
l.push_back("de leteme");
l.push_back("si xth");
// I want to erase entries that say "deleteme"

I can get and iterator, but that screws up things
list<string>::i terator it=l.begin();
for( ; it!=l.end(); it++){
if( (*it).compare(" deleteme")==0){
l.erase(it);
}
}
}

This gives me segmentation fault

#include <algorithm>
#include <iostream>
#include <list>
#include <string>

int main()
{
typedef std::list<std:: string> StringList;
typedef StringList::ite rator Iterator;

StringList l;
l.push_back( "first" );
l.push_back( "second" );
l.push_back( "deleteme" );
l.push_back( "fourth" );
l.push_back( "deleteme" );
l.push_back( "sixth" );

for( Iterator it = l.begin(); it != l.end(); )
{
it->compare( "deleteme" ) == 0? it = l.erase( it ) : ++it;
}

std::copy(
l.begin(), l.end(),
std::ostream_it erator<std::str ing>( std::cout, "\n" )
);
}

Whoah, that's giving me Perl flashbacks! Is there something wrong with
if/else? ;-)

--
David Hilsee

[Alf P. Steinbach]

* Mike Wahler:

for(std::list<s td::string>::it erator it = l.begin(); it!=l.end(); it++)
if((*it).compar e("deleteme") == 0)
it = l.erase(it);

What happens here if there are two "deleteme" items in sequence, or
if there is a "deleteme" item at the end of the list?

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alf P. Steinbach]

* Mike Wahler:

"Mike Wahler" <mk******@mkwah ler.net> wrote in message
news:TR******** *********@newsr ead1.news.pas.e arthlink.net...

for(std::list<s td::string>::it erator it = l.begin(); it!=l.end();

it++)

if((*it).compar e("deleteme") == 0)
it = l.erase(it);

This isn't quite right. It produces a valid iterator,
but the 'for' loop increments it, which will skip
over the item after the erased one. See Alf's post
for a proper way to do this.

I'm sorry for already responding to your previous posting before
reading this. Grr. When shall I learn to read _everything_ first?

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Mike Wahler]

"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. .

* Mike Wahler:

"Mike Wahler" <mk******@mkwah ler.net> wrote in message
news:TR******** *********@newsr ead1.news.pas.e arthlink.net...

for(std::list<s td::string>::it erator it = l.begin(); it!=l.end();

it++)

if((*it).compar e("deleteme") == 0)
it = l.erase(it);

This isn't quite right. It produces a valid iterator,
but the 'for' loop increments it, which will skip
over the item after the erased one. See Alf's post
for a proper way to do this.

I'm sorry for already responding to your previous posting before
reading this. Grr. When shall I learn to read _everything_ first?

No problem. I sometimes do the same thing myself.

-Mike

[Heinz Ozwirk]

"Alf P. Steinbach"

* Christoff Pale: [...] for( Iterator it = l.begin(); it != l.end(); )
{
it->compare( "deleteme" ) == 0? it = l.erase( it ) : ++it;
it->compare( "deleteme" ) == 0? it = l.erase( it/*!*/++/*!*/ ) : ++it;
}

Heinz

[Alf P. Steinbach]

* Heinz Ozwirk:

"Alf P. Steinbach"

* Christoff Pale:

[...]

for( Iterator it =3D l.begin(); it !=3D l.end(); )
{
it->compare( "deleteme" ) =3D=3D 0? it =3D l.erase( it ) : =

++it;
=20
it->compare( "deleteme" ) =3D=3D 0? it =3D l.erase( it/*!*/++/*!*/ ) =
: ++it;

}

If (and here I'm guessing) that is supposed to mean
it = l.erase( it++ )
then that's wrong.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?