Question

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

void compute(int num)
{
list<int> L;
list<int>::iterator i;
list<int>::iterator i2;
int p;
cout<<"Enter the number of numbers\n";
cin>>p;
int a;
for(int k=1;k<=p;k++)
{
    cin>>a;
    L.push_back(k);
}
cout<<endl;
for(i=L.begin() ; i!=L.end() ; ++i)
{
    cout<<*i<<endl;
}

long int k=1;

for(i=L.begin() ; i!=L.end() ; ++i )
{
    if(k%2!=0) //This is where I try and delete values in odd positions
    {
        i2=L.erase(i);
    }
    k++;
}

for(i=L.begin() ; i!=L.end() ; ++i )
{
    cout<<*i<<endl;
}

}

int main()
{
//  int testcases, sailors;
//cin>>testcases;

//for(int i=1 ; i<=testcases ; i++)
{
//  cin>>sailors;
}
//for(int i=1;i<=testcases;i++)
{
//  int num;
    //cin>>num;
    //compute(num);
}
compute(0);
return 0;

}

I am trying to erase elements using L.erase() function in Lists. But I get an error saying
“Debug assertion failed! ……Expression:list iterator not incrementable”
but we CAN increment iterator right?

Answer 1

erase invalidates the iterator that was passed in as parameter – since the element at the position the iterator was pointing to was just erased! And on that same iterator, an increment is attempted in the next for loop in your code! That’s why it fails.

However, erase it will return an iterator pointing to the new position, which we can use; a loop where you erase something from an STL container should therefore look something like the following; I show it with the type you use, list, but you could just as well use e.g. vector:

list<int> L;
// ...
list<int>::iterator it=L.begin();
while (it!=L.end())
{
    if(eraseCondition)
    {
        it=L.erase(it);
    }
    else
    {
        ++it;
    }
}

Or, if possible, it’s even better to use std::remove_if:

container.erase(std::remove_if(L.begin(), L.end(), predicate), L.end());

In your case that will be hard – if not impossible – to use since the predicate would need state information (the information whether the index is odd or even). So I’d recommend going with a loop structure as mentioned above; just keep in mind the remove_if for the general case of removing all elements where a certain predicate returns true!