Deleting Array (messes things up)

programming flash
1

i have an array of tanks (0-44) which march down the screen and you shoot them. when there is a collision between a bullet and tank, it deletes that tank from the array. however, for some reason when tank44 is deleted, it ****s the whole array up and makes variables such as the _y = undefined. ive tried splicing tank44 and the same error occurs. can someone explain why this would happen to an array? this project is due at midnight and i just found this error

function createTanks() {
     Tanks = new Array();
     tankNum = 0;
     
     // create three rows of 15
     for(var y=0;y<3;y++) {
          for(var x=0;x<15;x++) {
               
               // create new tank and set position
               attachMovie("tank", "tank"+tankNum, tankNum);
               tankClip = this["tank"+tankNum];
               tankClip._x = x*35+175;
               tankClip._y = y*75-30;
               
               // add to array
               Tanks.push(tankClip);
               
               tankNum++;
          }
     }
     
     // set initial direction/speed
     tankDirection = 0.5;
     
}

function moveTanks() {
     // assume no change in direction
     var newDirection = false;
     
     // loop through all Tanks
     for(var i=0;i<Tanks.length;i++) {
          
............

     
     // see whether the last one hit bottom
     trace(Tanks[i-1]._y);
     if(Tanks[i-1]._y == undefined){
          trace("ERROR!");
     }
     if (Tanks[i-1]._y > 425) {
          removeTanks();
          gotoAndStop("lose");
     }
}


function checkCollision(bullet) {
     // loop though all Tanks
     for(j=Tanks.length-1;j>=0;j--) {
          tank = Tanks[j];

          // see whether the bullet is close to the tank
          if (distance(bullet,tank) < 20) {
               // remove from array
               delete Tanks[j];
               //}
               // go to "pop" frame of tank
               tank.gotoAndPlay(2);

               // add to score
               _root.score += 100;

               //trace(tankLength);
               // if no more invaders, then game over

               if(tankLength < 1) {
                    gotoAndStop("win");
                    removeTanks();
               }

               // return true, because there was a collision
               return(true);
          }
     }

     // return false, because there was a collision
     return(false);
}

function removeTanks(){
     // remove tanks from screen & array
     for(j=Tanks.length-1;j>=0;j--) {
          delete Tanks[j];
          tempTank="tank"+j;
          eval(tempTank).removeMovieClip();
     }
     
     
     
}

function distance(clip1, clip2) {
     // find distance between two movie clips
     dx = clip1._x - clip2._x;
     dy = clip1._y - clip2._y;
     return (Math.sqrt(dx*dx+dy*dy));
}