Lists STL

[armen_shlang]

small question about LIST stl...
My code looks something like this...

#include <list>
#include <iostream>
using namespace std;


void main()
{
	list<int> randomNums;
	int number20 = 0;
	while( number20 < 5 )
	{
		(number20++);
		randomNums.push_back((2*number20) + 1);
		randomNums.push_back(2*number20);
	}
	list<int>::iterator iter = randomNums.begin();

	randomNums.erase(randomNums.end());

	for( list<int>::iterator iter = randomNums.begin() ; iter != randomNums.end() ; iter++)
	{
		cout << (*iter) << endl;
	}

}

And i get en error when trying to use the erase command with the randomNums.end(); which makes sense.
>>>HOW CAN I ERASE THE LAST ELEMENT FROM THE LIST?

also when I try to erase using this code:
[code]
randomNums.erase(iter++);
[/coode]
it won't erase the second entry.
>>WHY DOES LIST FUNCTION LIKE THIS??

[JohnW@Wessex]

randomNums.end() returns the iterator to one past the last element in the list.
If you want to erase the last element then you you need to subtract 1 from the end() iterator or use rbegin() to get the iterator.

Also randomNums.erase(iter++) increments the iterator after setting the parameter for the function.

[armen_shlang]

I see --- so "--iter" would fix the problem!!!
also I was reading this book and I noticed this example which I've never seen it before... what does it signify?

list<char> list1 = make< list<char> >("remembering");

whats the make for? In my last example I didn;t use it.

[laserlight]

If you want to erase the last element then you you need to subtract 1 from the end() iterator or use rbegin() to get the iterator.

list also has pop_back(), so it makes sense to use it instead of erase() for the last element.

also I was reading this book and I noticed this example which I've never seen it before... what does it signify?

I do not think make() is in the C++ standard library, so what it does depends on the implementation.

[armen_shlang]

Well if your specifically want to delete the last element yes, but if it's a general loop that deletes lets say blocks every time a ball hits them, then u need to use erase.
Thx all.

[potatoCode]

Well if your specifically want to delete the last element yes, but if it's a general loop that deletes lets say blocks every time a ball hits them, then u need to use erase.
Thx all.

Hello there,

you didn't quite understand what laserlight meant.
in the example you gave us,
1) your approach to delete the element in the list is encumbered in comparison with what laserlight suggested.
2) you have to be very careful when you use an iterator to delete an element - especially in a loop you're thinking about - thus, your approach is more error-prone.

hope this helps.

[code_carnage]

I agree with potatocode deleting in a loop is error prone as it might invalidate your iterator. Got to be careful.

[JohnW@Wessex]

According to 'Effective STL', for associative containers (set, map) you should use

container.erase(itr++);

For sequence containers (vector, string deque) you should use

itr = container.erase(itr);

For 'list' you can use either.

[jlou]

Yes, but deleting in a loop is more complicated because you usually have a condition and you want the iterator to increment when that condition is not met. People often use for loops which include an increment of the iterator for each pass through the loop, but that causes problems if you use either of the above choices.

[JohnW@Wessex]

, but that causes problems if you use either of the above choices.

Yes, I didn't show the full extract from the book, just the erase lines. The lines showed how to have a valid iterator after erasing.

The expanded code is...

Associative containers

itr = container.begin();

while (itr != container.end()
{
	if (condition)
	{
		container.erase(itr++);
	}
	else
	{
		++itr;
	}
}

Sequence containers

itr = container.begin();

while (itr != container.end()
{
	if (condition)
	{
		itr = container.erase(itr);
	}
	else
	{
		++itr;
	}
}