I think you should use an array. If you now get a weapon (let's say weapon nr. 5) you set that special array part to 1:
Code:

---

int weapons[8];

[...]

weapons[4] = 1; //i got the weapon nr. 5

---

To select it with the mousewheel you just need a number that holds the weapon number.

Code:

---

int current_weapon = 0;

[...]

int old_weapon;
while(1)
{
  current_weapon += (mickey.z/120);
  current_weapon %= 7;
  while(weapons[current_weapon]==0)
  {
    current_weapon++;
    current_weapon %= 7;
  }
  if(old_weapon!=current_weapon)
  {
    change_weapon(current_weapon);//in this function morph the weapon entity
  }
  old_weapon = current_weapon;
  wait(1);
}

---

all in all nothing is really detailed, but shoould give you the clue how to do it.