scheme - Difference between 3 methods of detecting true in Racket -

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it seem big deal there significant differences in following 3 methods of counting number of true items?

#lang racket  (define (counttrue . vars)   (length (remove* (list #f) vars))    )  (define (counttrue2 . vars)   (define c 0)   (for ((item vars))     (when item (set! c (add1 c))))   c    )  (define (counttrue3 . vars)   (count (lambda(x) x) vars)    )

they produce identical results there reason why particular 1 should or should not chosen?

edit: on using time function, following results obtained 3 functions above , 2 each answers @chrisjester-young , @sylwester : 

"---------- counttrue ------------" cpu time: 751 real time: 751 gc time: 16 "---------- counttrue2 ------------" cpu time: 946 real time: 947 gc time: 10 "---------- counttrue3 ------------" cpu time: 456 real time: 457 gc time: 8 "---------- counttrue_chris1 ------------" cpu time: 726 real time: 727 gc time: 9 "---------- counttrue_chris2 ------------" cpu time: 595 real time: 595 gc time: 8 "---------- counttrue_sylwester1 ------------" cpu time: 543 real time: 544 gc time: 7 "---------- counttrue_sylwester2 ------------" cpu time: 515 real time: 515 gc time: 7

hence, "count lambda" method fastest.

the count version idiomatic (except i'd write (count identity items)). furthermore, set! version not idiomatic racket, , racket doesn't optimise use, can see timing tests.

here couple of alternatives timing pleasure:

  1. using for comprehensions:

    (for/sum ((x (in-list items)) #:when x) 1)

  2. manual looping:

    (let loop ((sum 0)            (items items))   (cond ((null? items) sum)         ((car items) (loop (add1 sum) (cdr items)))         (else (loop sum (cdr items)))))

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