/*REXX program demonstrates an evolutionary algorithm  (using mutation).*/
parse arg  children  MR  seed .        /*get options (maybe) from  C.L. */
if children=='' then children = 10     /*# of children produced each gen*/
if MR      =='' then MR       = '4%'   /*the char Mutation Rate each gen*/
if right(MR,1)=='%' then MR=strip(MR,,'%')/100  /*expressed as %? Adjust*/
if seed\=='' then call random ,,seed   /*allow the runs to be repeatable*/
abc   = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ '  ;    Labc=length(abc)
target= 'METHINKS IT IS LIKE A WEASEL' ;    Ltar=length(target)
parent= mutate( left('',Ltar), 1)      /*gen rand str,same length as tar*/
say center('target string',Ltar,'─') "children" 'mutationRate'
say target center(children,8) center((MR*100/1)'%',12)             ;   say
say center('new string',Ltar,'─') "closeness" 'generation'

       do gen=0  until  parent==target;      close=fitness(parent)
       almost=parent
                         do  children;       child=mutate(parent,MR)
                         _=fitness(child);   if _<=close then iterate
                         close=_;            almost=child
                         say almost right(close,9) right(gen,10)
                         end   /*children*/
       parent=almost
       end   /*gen*/
exit                                   /*stick a fork in it, we're done.*/
/*───────────────────────────────────FITNESS subroutine─────────────────*/
fitness: parse arg x; hit=0;   do k=1  for Ltar
                               hit=hit+(substr(x,k,1)==substr(target,k,1))
                               end   /*k*/
return hit
/*───────────────────────────────────MUTATE subroutine──────────────────*/
mutate:  parse arg x,rate,?            /*set  ?  to a null,  x=string.  */
            do j=1  for Ltar;              r=random(1,100000)
            if .00001*r<=rate  then ?=? || substr(abc,r//Labc+1,1)
                               else ?=? || substr(x,j,1)
            end   /*j*/
return ?