Flatten a list

Write a function to flatten the nesting recursively in an arbitrary list of values. Your program should work on the equivalent of this list:

  [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []]

Where the correct result would be the list:

   [1, 2, 3, 4, 5, 6, 7, 8]

C.f. Tree traversal

Common Lisp

<lang lisp>(defun flatten (tree)

 (let ((result '()))
   (labels ((scan (item)
              (if (listp item)
                (map nil #'scan item)
                (push item result))))
     (scan tree))
   (nreverse result)))</lang>

While this is a common homework problem in Common Lisp, it should be noted that it is typically a mistake to use it in a realistic program; it is almost always easier to rewrite the program such that it does not generate the deeply-nested lists in the first place. In particular, note that flattening means that none of the values can be a list, and therefore they can not be nil.

Python

Function flatten is recursive: <lang python>>>> def flatten(lst): return sum( ([x] if type(x) is not list else flatten(x) for x in lst), [] )

>>> lst = [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []] >>> flatten(lst) [1, 2, 3, 4, 5, 6, 7, 8]</lang>

Function flat is iterative and flattens the list in-place: <lang python>>>> def flat(lst): i=0 while i<len(lst): while True: try: lst[i:i+1] = lst[i] except (TypeError, IndexError): break i += 1

>>> lst = [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []] >>> flat(lst) >>> lst [1, 2, 3, 4, 5, 6, 7, 8]</lang>

Ruby

flatten is a built-in method of Arrays <lang ruby>flat = [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []].flatten p flat # => [1, 2, 3, 4, 5, 6, 7, 8]</lang>