Loop interval changing unexpectedly - c++

Question

I am writing a loop to remove every third element from an array until there is only one element left.
here is the code...
int elimcnt = 1;//counts how many elements looped through
int cnt = 0;//counts how many elements deleted for printing purposes
for (int i = 0; v.size() > 1; i++, elimcnt++) {
if (i == v.size()) {//reset i to the beginning when it hits the end
i = 0;
}
if (elimcnt%in.M == 0 && elimcnt != 0) {//in.M is elimination index which is 3
v.erase(v.begin() + (elimcnt%v.size()) - 1);
cnt++;
if (cnt%in.K == 0) {//in.K is how often you will print which is after 7 deletes
print_vector(v, cnt);
}
}
}
what actually happens when i run it is that it will correctly delete the first element but after that it deletes every 4th element from there on out.
Here is an example input...
A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3
B4 B5 B6 B7 B8 B9 C1 C2 C3 C4 C5 C6
C7 C8 C9 D1 D2 D3 D4 D5 D6 D7 D8 D9
E1 E2 E3 E4 E5
What is supposed to be outputted...
A1 A2 A4 A5 A7 A8 B1 B2 B4 B5 B7 B8
C1 C2 C4 C5 C6 C7 C8 C9 D1 D2 D3 D4
D5 D6 D7 D8 D9 E1 E2 E3 E4 E5
This is what is actually outputted...
A1 A2 A4 A5 A6 A8 A9 B1 B3 B4 B5 B7
B8 B9 C2 C3 C4 C6 C7 C8 D1 D2 D3 D4
D5 D6 D7 D8 D9 E1 E2 E3 E4 E5
I cant seem to figure out what is causing the code to do this so any help will be greatly appreciated.

Answer 1

The problem is in the expression used in the statement
v.erase(v.begin() + (elimcnt%v.size()) - 1);
^^^^^^^^^^^^^^^^^^^^^
Consider a sequence of numbers
1, 2, 3, 4, 5, 6
For the first traversing of the sequence You need to delete 3 and 6
After deleting 3 you will get
1, 2, 4, 5, 6
and the variable elimcnt after the deleting will be incremented and will be equal to 4. However the size of the sequence is now equal to 5. So when elimcnt will be equal to 6 then the expression elimcnt%v.size()) - 1 will be equal to 0 and the element 1 will be deleted.
I could suggest a more safe approach using iterators.
for example
size_t elimcnt = 0;//counts how many elements looped through
size_t cnt = 0;
for (auto it = v.begin(); v.size() > 1; it == v.end() ? it = v.begin() : it )
{
if (++elimcnt % in.M == 0)
{
it = v.erase(it);
if (++cnt % in.K == 0)
{
print_vector(v, cnt);
}
}
else
{
++it;
}
}