# Select random elements from list

## Problem

I want to be able to take random elements from a list, avoiding duplicate elements when possible.

For example, if I ask for 4 random elements from the list `(a b c d)` I don't want to get `(d a b a)`.

## Solution

``````(define (pick elements how-many)
(if (< how-many 0)
(error "Cannot create a list of negative length."
how-many)
(letrec* ((list-length (length elements))
(new-index (lambda (iteration-list-length)
(random-integer (if (zero? iteration-list-length)
list-length
iteration-list-length))))
(do-pick
(lambda (pick-from unchosen result index how-many)
(cond ((zero? how-many)
result)
((null? pick-from)
(do-pick (if (null? unchosen)
elements
unchosen)
'()
result
index
how-many))
((zero? index)
(do-pick (cdr pick-from)
unchosen
(cons (car pick-from)
result)
(new-index (+ (length (cdr pick-from))
(length unchosen)))
(- how-many 1)))
(else
(do-pick (cdr pick-from)
(cons (car pick-from)
unchosen)
result
(- index 1)
how-many))))))
(do-pick elements '() '() (random-integer list-length) how-many))))
``````

Credit: Tim Van den Langenbergh

## Usage

Since the procedure uses randomness, the elements chosen will vary.

``````(pick (iota 100) 100)
;; => (79 28 18 40 90 29 30 66 80 36 23 34 42 84 25 35 88 54 15 92 69 8 2
;;     16 95 27 19 74 91 77 68 3 83 57 26 86 89 60 53 47 21 85 72 0 73 96
;;     93 58 32 10 ...)

(pick '(A C G T) 5)
;; => (T G A T C)

(+ 1 (car (pick (iota 6) 1)))
;; => 1
``````

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