is a default case necessary in a switch statement

No, but it is not a bad idea to put default statements in switch statements for error- or logic-checking purposes. For instance, the following switch statement is perfectly normal:

switch (char_code) {
ase ‘Y’:
case ‘y’: printf(“You answered YES!\n”);
break;
case ‘N’:
case ‘n’: printf(“You answered NO!\n”);
break;
}

Consider, however, what would happen if an unknown character code were passed to this switch statement. The program would not print anything. It would be a good idea, therefore, to insert a default case where this condition would be taken care of:

...
default: printf(“Unknown response: %d\n”, char_code);
break;
...

Additionally, default cases come in handy for logic checking. For instance, if your switch statement handled a fixed number of conditions and you considered any value outside those conditions to be a logic error, you could insert a default case which would flag that condition. Consider the following example:

void move_cursor(int direction) {
  switch (direction) {
   case UP: cursor_up();
   break;
   case DOWN: cursor_down();
   break;
   case LEFT: cursor_left();
   break;
   case RIGHT: cursor_right();
   break;
   default: printf(“Logic error on line number %ld!!!\n”,__LINE__);
   break;
 } 
}