A function in Wolfram Mathematica:
SplitBy[list,f] splits list into sublists consisting of runs of successive elements that give the same value when f is applied.
( src )The function is incredibly useful when parsing, for example, a fortune file, which is present in any *NIX system, like:
the more general problem may be easier to solve -- G.Polya % The world doesn't live off jam and fancy perfumes - it lives off bread and meat and potatoes. Nothing changes. All the big fancy stuff is sloppy stuff that crashes. I don't need dancing baloney - I need stuff that works. That's not as pretty, and just as hard. -- Theo de Raadt % As a friend of mine, the computer scientist Guy Steele, once put it, “LISP is a high-level language, but you can still feel the bits sliding between your toes.” -- Daniel Hillis -- The Pattern On The Stone: The Simple Ideas That Make Computers Work %
How do you pick a random element if it may consist of several lines? Here I split the fortune file using my own reimplementation of this function in Racket:
#lang racket ; another function I didn't find: (provide pick-random) (define (pick-random lst) (list-ref lst (random (length lst)))) ; works almost like http://reference.wolfram.com/language/ref/SplitBy.html in Mathematica: (define (drop-first-elem-if l pred) (if (pred (first l)) (drop l 1) l)) (define (drop-empty-lists l) (filter (λ (x) (not (null? x))) l)) (provide split-list-into-sublists) (define (split-list-into-sublists xs pred) (if (null? xs) '() (let* ((new-list (drop-first-elem-if xs pred)) (first-chunk (takef new-list (negate pred))) (rest-of-list (dropf new-list (negate pred)))) (cons first-chunk (drop-empty-lists (split-list-into-sublists rest-of-list pred)))))) ; main program (define (pick-random-fortune-from filename) (define sliced (split-list-into-sublists (file->lines filename) (λ (n) (equal? "%" n)))) (map (λ (s) (displayln s)) (pick-random sliced))) ; print each line (void (pick-random-fortune-from "/home/i/tmp/fortunes.txt")) ; "void" to suppress printing "void"
Isn't it nice? Of course, my implementation is crude and slow. If you can suggest better or more efficient, please drop me email.
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