问题
I would like to sort some data and I would like to use symbol
回答1:
I believe Racket's sort can do what you want, with the appropriate less-than and #:key parameter. For example:
> (sort '((strawberry 5 zambia) (banana 2 sweden))
symbol<?
#:key first)
=>
'((banana 2 sweden) (strawberry 5 zambia))
See the sort docs for more info.
回答2:
In R6RS Scheme you have list-sort (in R6RS) and it takes a comparison procedure as first argument and the list of data as the second.
You don't show how your fruit-list looks like but if it's close to what you want as output you can do:
#!r6rs
(import (rnrs)
(only (srfi :1) fold))
(define (make-cmp cmp accessor)
(lambda (x y)
(cmp (accessor x) (accessor y))))
(define (compose . lst)
(define cmds (reverse lst))
(lambda (x)
(fold (lambda (proc arg) (proc arg))
x
cmds)))
(define (sorted-via obj what)
(define <fruitname? (make-cmp string<? (compose symbol->string car)))
(define <location? (make-cmp string<? (compose symbol->string caddr)))
(define <number? (make-cmp < cadr))
(cons 'fruits
(list-sort (case what
((fruitname) <fruitname?)
((location) <location?)
((number) <number?)
(else (raise 'invalid-sort-field)))
(cdr obj))))
R7RS-small, like R5RS, doesn't have a sort procedure so it's back to SRFI-95 Sorting and merging which looks similar to #!racket:
#!r7rs
(import (scheme)
(only (srfi :1) fold)
(srfi :95))
(define (sorted-via obj what)
(cons 'fruits
(sort (cdr obj)
(if (eq? what 'number) < string<?)
(case what
((fruitname) (compose symbol->string car))
((location) (compose symbol->string caddr))
((number) cadr)
(else (raise 'invalid-sort-field))))))
And if you want to use #!racket:
#!racket
(define (sorted-via obj what)
(cons 'fruits
(sort (cdr obj)
(if (eq? what 'number) < symbol<?)
#:key (case what
((fruitname) car)
((location) caddr)
((number) cadr)
(else (raise 'invalid-sort-field))))))
来源：https://stackoverflow.com/questions/23316092/sorting-in-ascending-order-in-scheme-programming