This array holds the unique camera ID assigned to the given view port, and can be set to a new camera or read to get the current camera, returning -1 if no camera is assigned. You can have up to 8 view ports active in a room (array values 0 through 7), and can assign a camera to any of them by simply setting this variable to the camera ID value (as returned by the functions camera_create() or camera_create_view()). If you have set the camera to a view port through the Room Editor, this variable will hold the camera ID for that port.
view_camera[0 ... 7];
Camera index or -1 if no camera is assigned
if view_camera == -1
view_camera = camera_create_view(0, 0, room_width, room_height);
The above code checks to see if the view port  has a camera attached to it and if it doesn't then one is created and set to it.