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Operations with lists

Slawa

Hello

 I have two questions.
The first:
How to remove from list repeating elements? For example is
list1 = (1 2 3 4 3 2 1) how to receive list2 = (4) having removed 1, 2 and 3
And how to receive list3 = (1 2 3 4) having removed only duplicating elements but not the initial.

The second:
How from two lists to receive one containing elements which is both in the first initial list and in the second? For example:
Is list1 = (1 2 3 4 5) and list2 = (2 4 6 7) how to receive list containing 2 and 4 as they is both in list1 and in list2?

 The case is certainly considered when the size of list is equal N

Best regard

Slawa 

Parents

  • Hi Slawa,

    Your first question is how to make a list of unique elements, I cannot take credit for writing this, but here is some useful code for doing such an operation:


      procedure(uniquify(L)
    let(((hash makeTable("uniqueList" nil)) (q L) (p cdr(L)))
        hash[car(L)] = t
        while(p
          if(hash[car(p)] then
              p = cdr(p)
              setcdr(q p)
          else
              hash[car(p)] = t
              q = p
              p = cdr(p)
          ); if
        ); while
        L
    ); let
  ); procedure
  ;; this does not change the sort order of the list and is of order N, 
  ;; it uses local variables and a hash table for storage
  
  uniquify(list1)
  => (1 2 3 4)

    The other part of your first question, how to remove all repeating elements from the list leaving only elements that were never present more than once; I will have to think about how that can be done.

    Your second question is asking for the intersection (or "overlap") of two lists, this can be coded as below:

      procedure(overlap(L1 L2)
    setof(item L1 member(item L2))
  ); procedure
  
  list1 = '(1 2 3 4 5)
  list2 = '(2 4 6 7)
  overlap(list1 list2)
  => (2 4)

     (The procedure name "intersect" already exists, it is a waveform function) This may not be the most efficient code, but if the lists are 'small' then it should be sufficient.

    Hopefully this helps you!

    Regards,

    Lawrence.

Reply

  • Hi Slawa,

    Your first question is how to make a list of unique elements, I cannot take credit for writing this, but here is some useful code for doing such an operation:


      procedure(uniquify(L)
    let(((hash makeTable("uniqueList" nil)) (q L) (p cdr(L)))
        hash[car(L)] = t
        while(p
          if(hash[car(p)] then
              p = cdr(p)
              setcdr(q p)
          else
              hash[car(p)] = t
              q = p
              p = cdr(p)
          ); if
        ); while
        L
    ); let
  ); procedure
  ;; this does not change the sort order of the list and is of order N, 
  ;; it uses local variables and a hash table for storage
  
  uniquify(list1)
  => (1 2 3 4)

    The other part of your first question, how to remove all repeating elements from the list leaving only elements that were never present more than once; I will have to think about how that can be done.

    Your second question is asking for the intersection (or "overlap") of two lists, this can be coded as below:

      procedure(overlap(L1 L2)
    setof(item L1 member(item L2))
  ); procedure
  
  list1 = '(1 2 3 4 5)
  list2 = '(2 4 6 7)
  overlap(list1 list2)
  => (2 4)

     (The procedure name "intersect" already exists, it is a waveform function) This may not be the most efficient code, but if the lists are 'small' then it should be sufficient.

    Hopefully this helps you!

    Regards,

    Lawrence.

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