Looping a list

[q742tt9]

Hello,

I am attempting to create a little program that iterates through a list deleting every third item until only one remains. Unfortunately, I feel as if I am going down a rabbit hole trying to fix my loop process. Good chance I am just thinking about it wrong.

Currently, I think I have two issues. First that my iterator needs to hold it's location after a node is erased, but mostly that once I we get to the end of the list we core dump... I can't seem to catch the iterator at the end.

Any help would be appreciated.

#include <iostream>
#include <list>

using namespace std;

void showRemains(std::list<int> &suit);

int main()   {

	int size;
	std::list<int> numb;
	std::list<int>::iterator it;
	

	std::cout << "Please input a number " << endl;
	

	while (!(std::cin >> size) || size < 0 )   {
		std::cout << "Please enter a valid number of number!" << std::endl;
		std::cin.clear();  // Clear cin stream
		std::cin.ignore(50, '\n');	// Ignore previous cin entries
	}
	
	std::cout << "Thank you. There are " << size << " numbers." << endl;
	
	for (int i=0; i<size; i++)  {
		numb.push_back(i+1);
		
	}
	

	showRemains(numb);
	
       it = numb.begin();

	while (numb.size() > 1)   {
		
		
		//std::cout << "size " << numb.size() << " it " << *it << endl;
		if (it == numb.end() ) { it = numb.begin(); }
		else { it++; }
		if (it == numb.end() ) { it = numb.begin(); }
		else { it++; }
		
		std::cout << "Number #" << *it << " removed." << endl;
		numb.erase(it);
		//std::cout << "size " << numb.size() << " it " << *it << endl;
		if (it == numb.end() ) { it = numb.begin(); }
		else { it++; }//advance to next 
		//std::cout << "size " << numb.size() << " it " << *it << endl;
		showRemains(numb);
		
	}
	
	std::cout << "Number #" << numb.front() << " is the final number!!!" << endl;
	
	
	
	

return 0;
}

void showRemains(std::list<int> &suit)  {

	std::cout << "Remaining numb" << endl;
	for (std::list<int>::iterator i=suit.begin(); i !=suit.end(); i++)   {
		std::cout << *i << " ";
	}
	std::cout << endl << endl;

return;
}

Edit: Well i figured out my first issue, I simply had set my iterator to begin() inside my while loop instead of before... I feel silly (updated the code accordingly). I am still getting a segmentation fault and am not sure why my if statements aren't bringing me back the the start.

[MiiNiPaa]

numb.erase(it);
//std::cout << "size " << numb.size() << " it " << *it << endl;
if (it == numb.end() ) { it = numb.begin(); }
else { it++; }//advance to next  

You should never use invalidated iterators. In list iterators to deleted elements are invalidated. Save iterator to another variable, increment main iterator and delete saved one.

[q742tt9]

Thanks MiiNiPaa,

That makes sense, though I am not sure I understand the method which this can be done. If I create a second iterator equal to the one to be delete, would it too not be invalid after the delete?

[MiiNiPaa]

You will use it only to delete and then won't touch it again.

[q742tt9]

If I understand correctly, it would be something like

		std::cout << "Number #" << *it << " removed." << endl;
		std::list<int>::iterator del = it;
		numb.erase(del);

But this seems to kill both it and del. So I am pretty sure I am not on the right track.

[MiiNiPaa]

MiiNiPaa wrote:

Save iterator to another variable, increment main iterator and delete saved one.

1 2 3

std::list<int>::iterator del = it; ++it numb.erase(del);

//or std::list<int>::iterator del = it++; numb.erase(del);

[cire]

or..
numb.erase(it++);

or...
it = numb.erase(it);

[q742tt9]

Thanks again, reading comprehension is a good skill to have. Things are working much better now, though I have realized that referencing numb.end() is referring to "blank space" after the last element where I am attempting to compare to the last element.

		if (it == numb.end() ) { it = numb.begin(); }
		else { it++; }

Is there a trick to checking the end of the list? I attempted to use back() however I get invalid operators with that.

[q742tt9]

Got it!

By advancing the iterator, then checking for the end I am in the right spot when I check.

		it++; 
		if (it == numb.end() ) { it = numb.begin(); }

Thanks for the help everyone. This one is in the bag!