CODEWITHSUNDEEP
lispcommon-lisp
Alex A
Alex A

Reputation: 19

Return a list of characters from a mixed list

I am trying to write a Lisp function to return a list of characters (no repeats) from a list (with ints, characters, etc). I'm still a beginner with Lisp and am having trouble starting. Our prof mentioned using atom but I can't figure out what she meant. Here is the question:

"Write a lisp function that accepts a list as the input argument (the list is mixed up integers, decimals, characters and nested lists) and creates a list including all the characters in the original list without any duplication. Sample program output is shown below:

  1. ‘((z f) (b a 5 3.5) 6 (7) (a) c) -> (z f b a c)
  2. ‘( (n) 2 (6 h 7.8) (w f) (n) (c) n) -> (h w f c n) "

Upvotes: 1

Views: 879

Answers (1)

Svante
Svante

Reputation: 51501

What your assignment calls “characters” are actually symbols with a name of length 1. It seems that you can just mentally replace the word “characters” with “symbols” and work with this.

An atom is anything that is not a cons—any non-empty list consists of a chain of conses. For example, symbols, numbers, strings, and nil are atoms.

A cons (actually a cons cell) is a simple datastructure that can hold two things. In a list, the first thing of each cons is some list element, and the second either a pointer to the next cons or nil. You can also have lists as list elements; then also the first thing would be a pointer to a list. This would then formally be a tree. The accessor function for the first thing of a cons is called car or first, the accessor function for the other thing is called cdr or rest. Car and cdr are a bit archaic, and mainly used when you see the cons cell as a tree node, while first and rest are more modern, and mainly used when you see the cons cell as a list chain link.

You can test whether a thing is an atom with the function atom. If it is not an atom, it is a list with at least one element.

Your assignment has a few parts:

  • Walk the tree to look at each element. This can be done through recursion, or through looping in one direction and recursing in the other.
  • Keep a list of symbols that you already found.
  • If the element you look at is a symbol (with a name of length 1…), then check whether it is new, if yes, add it to your list.
  • Finally return that list.

One useful idiom is to use push or pushnew, which put new elements at the front of the list, and at the end reverse it.

Upvotes: 2

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