How to bind WASD to arduino robot?

I want W-A-S-D input movement for my arduino brain Zumo bot to control it from my keyboard. Heres my code so far. I am not exactly sure how to go about doing this but I want to use Switch/Case to do it.

%%Bookkeeping
close all; clear all; clc;
%%Script settings
comPort = 'COM3';
%%Setup
ser = arduino(comPort);
%%Loop(s)
rdir = 7;
rspeed = 9;
ldir = 8;
lspeed = 10;
time = 1;
ser.pinMode(rdir,'output');
ser.pinMode(rspeed

    if true
      % code
    end,'output');
  ser.pinMode(ldir,'output');
  ser.pinMode(lspeed,'output');

user_input = ;

switch user_input
    case 'w'
        ser.digitalWrite(rdir,0);
        ser.digitalWrite(ldir,0);
        ser.analogWrite(rspeed,255);
        ser.analogWrite(lspeed,255);
    case 's'
        ser.digitalWrite(rdir,1);
        ser.digitalWrite(ldir,1);
        ser.analogWrite(rspeed,255);
        ser.analogWrite(lspeed,255);
    case 'a'
        ser.digitalWrite(rdir,0);
        ser.digitalWrite(ldir,1);
        ser.analogWrite(rspeed,100);
        ser.analogWrite(lspeed,100);
    case 'd'
        ser.digitalWrite(rdir,1);
        ser.digitalWrite(ldir,0);
        ser.analogWrite(rspeed,100);
        ser.analogWrite(lspeed,100);
    otherwise
        ser.digitalWrite(rdir,0);
        ser.digitalWrite(ldir,0);
        ser.analogWrite(rspeed,0);
        ser.analogWrite(lspeed,0);
end