You need to test the keys and then store the state in a variable, You only change the variable when a key was pressed. You then only test the state of the variable.

Code:
#include <acknex.h>

#define STATE_1 1
#define STATE_2 2

// Variable initialisation
var current_state = STATE_1;

void main()
{
   ...
   while(1)
   {
      if (key_...)
      {
         current_state = STATE_1 ;
      }
      else if ( key_... )
      {
         current_state = STATE_2 ;
      }
      
      ...

      switch (  current_state )
      {
         case STATE_1:
            ...
            break;
         case STATE_2:
            ...
            break;
      }
   }
   ...
}


You can separate the codes in functions, eg handle_keys() ; do_stuff () ;

Best regards.

PhysX 3 Plugin Box2D Plugin