Elisp: Sequence. Split, Partition, Group by

Split, Partition

seq-split

(seq-split SEQUENCE LENGTH)

split SEQUENCE into sub lists, each with length LENGTH, except the last one which may be shorter.

(seq-split (number-sequence 1 7) 2)
;; ((1 2) (3 4) (5 6) (7))

;; second arg cannot be negative
(seq-split (number-sequence 1 7) -1)
;; Debugger entered--Lisp error: (error "Sub-sequence length must be larger than zero")

seq-partition

seq-split and seq-partition seems to be the same.

They are implemented in different ways, but end result seems same, except when second arg is negative, seq-split errors out but seq-partition return nil.

Also, seq-split is defined by defun. seq-partition is defined via cl-defgeneric. The seq-split seems a bug, because all function names starting with "seq-" is meant to be generic that works on future new data type of sequence.

(seq-partition (number-sequence 1 7 ) 2)
;; ((1 2) (3 4) (5 6) (7))

Group into association list

seq-group-by

(seq-group-by FUNCTION SEQUENCE)

• Group items into a Association List.
• The return value of FUNCTION is used as the key. (if different items result in the same key, they are grouped together.)

;; group a list of 1 to 20 by even and odd
(setq xx
      (seq-group-by
       (lambda (x)
         (if (eq (mod x 2) 0) "even" "odd"))
       (number-sequence 1 20)))
;; (("odd" 1 3 5 7 9 11 13 15 17 19) ("even" 2 4 6 8 10 12 14 16 18 20))

(assoc "even" xx)
;; ("even" 2 4 6 8 10 12 14 16 18 20)