excercise  recursive programming with list 2016

1)  given a simple list of numbers, write a function to change every "2" in input list to "0" in output list. for example  input ( 1 2 3 2 4 ), output ( 1 0 3 0 4 )

2)  do the same as 1) but input is a complex list. for example  input ( (1 2 ) 3 ((2)) 4)  output ( (1 0) 3 ((0)) 4 )

solution

1) 

a simple duplication of a list

dup(L)
  if L == NIL ret NIL
  ret cons( head(L), dup(tail(L)) )

then modify it to change "2" to "0"

dupx(L)
  if L == NIL ret NIL
  else if  head(L) == number(2)  
           ret cons( number(0), dupx(tail(L)) )
  else ret cons( head(L), dupx(tail(L)) )