matlab switch语句

matlab switch语句strmonth = menu('please choose a month','Janurary','February','March','April','May','June','July','August','September','October','November','December');switch strmonthcase 'Janurary'a = 31;case 'February'a = 28;case 'March'a = 31;case 'April'a = 30; case 'May'a = 31;case'June'a = 30;case 'July'a = 31;case 'August'a = 31;case 'September'a = 30;case 'October'a =31;case 'November'a =30;case 'December'a =31;enddisp(a)；选择,2月,出错：Undefined function or variable 'a'.Error in ==> month at 30disp(a);这是为什么.