Why does the x value change in this program?

Question

I have created this code, and when I run it, don't get any errors until the arrow leaves the screen (ie: (*I)->x>maxx), after which the O will randomly teleport (Well, I'm guessing its not random, but I'm trying to find a pattern to it).

EDIT: the random teleportation don't seem to occur if I move up, and if I move down, the O is teleported directly to the bottom. Also, a glitch has occured where the O becomes a '>'. (I am trying to figure out how that happens)

EDIT: the transform-into-'>' glitch occurs if the O is at the bottom right of the screen (player.x=9;player.y=9) and the sequence "wqs" is entered.

EDIT: I've removed the class declarations because I am fairly sure that the error is within the _move()s and check().

EDIT: The transform glitch appears to occur when 'wq' is typed, then any other character is entered (ie "skiping" the next move)

EDIT: The tranform glitch occurs when player.x=9; player.y=8; and then 'q' is pressed, the next move the player tranforms into a '>'

This is the code:

#include<vector>
#include<iostream>
#include<string>
using namespace std;
const int maxx = 10, maxy = 10; //two constants that show the size of the sector
char sector[maxx][maxy]; //array of characters used to display the sector
prgm player(0, 0, 'O'); //player definition at x0,y0,and displayed with 'O'
const int vsize = 1; //size of the enemy array (ie: how many enemies there will be
X1 a(9, 5, 'X', 10); //enemy "a", has a move function that moves it back and forth
virus * viral_data[vsize] = {&a}; //array of enemies used to set the sector
vector<antivirus*> antiviral_data; //vector of pointers to "antivirus" the weapon used
vector<antivirus*>::iterator I; //iterator for previous vector

void display() //function to display the sector
{
    for(int i = 0; i < maxy; i++) 
    {
        for(int j = 0; j < maxx; j++)
        {
            cout<<sector[j][i];
        }
        cout<<endl;
    }
    return;
}

void p_move() //function to get players input, then move the player or create "antivirus"
{
    char dir;
    cin>>dir;
    switch(dir)
    {
    case 'w':
        player.y--;
        break;
    case 'a':
        player.x--;
        break;
    case 's':
        player.y++;
        break;
    case 'd':
        player.x++;
        break;
    case 'q':
        antiviral_data.push_back(new aX1(player.x, player.y, '>')); //creates a new aX1 at the players position
        break;
    }
    return;
}

void v_move() //uses the enemies move
{
    for(int i = 0; i < vsize; i++)
    {
        viral_data[i]->move();
    }
    return;
}

void a_move() //uses the weapon (ie: moves the weapon forward)
{
    for(I = antiviral_data.begin(); I < antiviral_data.end(); I++)
    {
        (*I)->move();
    }
    return;
}

void set() //sets the sector array (char)
{
    for(int i = 0; i < maxy; i++)
    {
        for(int j = 0; j < maxx; j++)
        {
            sector[j][i] = ' '; makes the entire sector blank
        }
    }
    sector[player.x][player.y]=player.sym; //sets the sector at the player's position to 'O'
    for(int i = 0; i < vsize; i++)
    {
        sector[viral_data[i]->x][viral_data[i]->y] = viral_data[i]->sym; //sets the sector at each enemy's position to be 'X'
    }
    for(I = antiviral_data.begin(); I < antiviral_data.end(); I++)
    {
        sector[(*I)->x][(*I)->y] = (*I)->sym; //sets the sector at each weapon's position to be '>'
    }
    return;
}

void check() //prevents the player from moving off the screen, erases bullet if it moves of the screen (to prevent access to non-allocated memory)
{
    if(player.x < 0)
    {
        player.x = 0;
    }
    if(player.y < 0)
    {
        player.y = 0;
    }
    if(player.x > (maxx-1))
    {
        player.x = (maxx-1);
    }
    if(player.y > (maxy-1))
    {
        player.y = (maxy-1);
    }
     //PROBLEM APPEARS TO OCCUR HERE
    for(I = antiviral_data.begin(); I! = antiviral_data.end();)
    {
        if((*I)->x > maxx)
        {
            I = antiviral_data.erase(I);
        }
        else
        {
            I++;
        }
    }
     //*-*-*-*-*-*-*-*-*-*-*-*-*-*-*

    return;
}
int main()
{
    while(true)
    {
        set(); //set sector
        display(); //display sector
        p_move(); //player's move
        v_move(); //enemy's move
        a_move(); //bullet's move
        check();//check moves
    }
    return 0;
}

Answer 1

In check(), the test

((*I)->x > maxx)

should be

((*I)->x >= maxx)

. This is an off-by-one error that lets the > get one square off the screen. When the display routine tries to display it, it clobbers the display symbol for the X.