what is more efficient in core C? switch / if statment?

I have a question on how i should write my code, basically I have a switch statement that is used to change a char to a specific char depending on what linenumber it is on. however it only changes if the linenumber is an odd number ( starting from 0, so every 2nd line itll pick a new char). currently my code is....

int linenumber;
char zone = UNKNOWN;

  for (linenumber = 0; linenumber < 21; linenumber++) {

     switch (linenumber) {
        case (1):
           zone == 'a';
           break;

        case (3):
           zone == 'b';
           break;

        case (5):
           zone == 'c';
           break;

        case (7):
           zone == 'd';
           break;

        case (9):
           zone == 'e';
           break;

        case (11):
           zone == 'f';
           break;

        case (13):
           zone == 'g';
           break;

        case (15):
           zone == 'h';
           break;

        case (17):
           zone == 'i';
           break;

        case (19):
           zone == 'j';
           break;

        if (linenumber % 2 == 0) {

        }
    }
}

however, since the switch only happens for every 2nd case, is it more efficient to write a if statement such as....

if (linenumber % 2 == 1) {
   switch.....
}

to put the switch statement inside... just wondering if an extra if statement would be more or less efficient in this situation.

Thanks in advance!