list substitution problems

match=member(Px listA)
listB=
  if(eq(match listA) then
    constar(Px1 Px2 cdr(listA))
  else
    rplaca(match Px1)
    rplacd(match cons(Px2 cdr(match)))
    listA
  )

You also should consider whether a list is the right data structure for your needs - I presume whatever you are doing needs to be ordered, otherwise you could have just stuck Px2 on the beginning. Or used a table.

Regards,

Andrew.

Thanks, Andrew.

Actually, listA is the points of a path object. It should remain their original order.

Further more, if i want to break listA into 2 seperate sublists

listA1=list(P1 P2 P3 ... Px-1)

listA2=list(Px ... Pn)

What should I do to get listA1 and listA2?

Can I just break the pointer (or called link) between Px-1 and Px?

Thanks again for your quick reply.

Something like this:

match=nil
foreach(map rem listA
  when(cadr(rem)==Px match=rem)
)
listA2=cdr(match)
rplacd(match nil)
listA1=listA

Note this doesn't handle the case where the match is at the beginning of the list - but that's cheap to check for and handle, as I did in the previous response. Potentially you could make the foreach stop as soon as it finds a match by doing:

prog(()
  foreach(map rem listA
    when(cadr(rem)==Px
      match=rem
      return()
    )
  )
)

The surrounding prog allows you to use return() to exit the foreach early.

Regards,

Andrew.