Cutting and Pasting Lists (MIT/GNU Scheme 12.0.90)

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### 7.4 Cutting and Pasting Lists

SRFI 1 procedure: take x i ¶
SRFI 1 procedure: drop x i ¶

  `take` returns the first i elements of list x. `drop` returns all but the first i elements of list x.

      (take '(a b c d e)  2) => (a b)
      (drop '(a b c d e)  2) => (c d e)

  x may be any value—a proper, circular, or dotted list:

      (take '(1 2 3 . d) 2) => (1 2)
      (drop '(1 2 3 . d) 2) => (3 . d)
      (take '(1 2 3 . d) 3) => (1 2 3)
      (drop '(1 2 3 . d) 3) => d

  For a legal i, `take` and `drop` partition the list in a manner which can be inverted with `append`:

      (append (take x i) (drop x i)) = x

  `drop` is exactly equivalent to performing i `cdr` operations on x; the returned value shares a common tail with x. If the argument is a list of non-zero length, `take` is guaranteed to return a freshly-allocated list, even in the case where the entire list is taken, e.g. `(take lis (length lis))`.

obsolete procedure: list-head x i ¶
standard procedure: list-tail x i ¶

  Equivalent to `take` and `drop`, respectively. `list-head` is deprecated and should not be used. `list-tail` is defined by R7RS.

procedure: sublist list start end ¶

  Start and end must be exact integers satisfying

      0 <= start <= end <= (length list)

  `sublist` returns a newly allocated list formed from the elements of list beginning at index start (inclusive) and ending at end (exclusive).

standard procedure: append list … ¶

  Returns a list consisting of the elements of the first list followed by the elements of the other lists.

      (append '(x) '(y))                      ⇒ (x y)
      (append '(a) '(b c d))                  ⇒ (a b c d)
      (append '(a (b)) '((c)))                ⇒ (a (b) (c))
      (append)                                ⇒ ()

  The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list.

      (append '(a b) '(c . d))                ⇒ (a b c . d)
      (append '() 'a)                         ⇒ a

SRFI 1 procedure: append! list … ¶

  Returns a list that is the argument lists concatenated together. The arguments are changed rather than copied. (Compare this with `append`, which copies arguments rather than destroying them.) For example:

      (define x (list 'a 'b 'c))
      (define y (list 'd 'e 'f))
      (define z (list 'g 'h))
      (append! x y z)                         ⇒ (a b c d e f g h)
      x                                       ⇒ (a b c d e f g h)
      y                                       ⇒ (d e f g h)
      z                                       ⇒ (g h)

SRFI 1 procedure: last pair ¶
SRFI 1 procedure: last-pair pair ¶

  `last` returns the last element of the non-empty, finite list pair. `last-pair` returns the last pair in the non-empty, finite list pair.

      (last '(a b c)) => c
      (last-pair '(a b c)) => (c)

obsolete procedure: except-last-pair list ¶
obsolete procedure: except-last-pair! list ¶

  These procedures are deprecated. Instead use `drop-right` or `drop-right!`, respectively, with a second argument of `1`.

Next: Filtering Lists, Previous: Selecting List Components, Up: Lists [Contents][Index]