...
prod=1
a=7               /*or somesuch.*/
b=3               /*  likewise. */

op='**'           /*or whatever.*/
...
    select
    when op=='+'         then  r=a+b           /*add.                   */
    when op=='-'         then  r=a-b           /*subtract.              */
    when op=='∙'         then do; r=a*b; prod=prod*r; end    /*multiply.*/
    when op=='*'         then  r=a*b           /*multiply.              */
    when op=='**'        then  r=a**b          /*power (exponentiation) */
    when op=='/'  & b\=0 then  r=a/b           /*divide.                */
    when op=='%'  & b\=0 then  r=a/b           /*interger divide.       */
    when op=='//' & b\=0 then  r=a/b           /*modulus (remainder).   */
    when op=='||'        then  r=a||b          /*concatenation.         */
    when op=='caw'       then  r=xyz(a,b)      /*call the XYZ subroutine*/
    otherwise                  r='[n/a]'       /*signify not applicable.*/
    end   /*select*/

say 'result for'   a   op   b   "="   r