Show Markers on Google Maps According to Post ID in the WordPress

gistfile1.txt

add_filter('wpgmp_markers','wpgmp_markers',1,3 );

function wpgmp_markers($places,$map) {
  global $post;
  
  if( $post->ID == 1 ) {
  $places = array_slice($places, 1,10);
  } else if ($post->ID == 2) {
  $places = array_slice($places, 10,10);
  } else {
   $places = array_slice($places, 20,10);
  }
  return $places;

}