Hi Rodolfo, Thank you for the help. ( I assume add1 is
(define (add1 x) (+ x 1)) ) On Tue, May 31, 2011 at 10:02 PM, Rodolfo Carvalho <rhcarva...@gmail.com>wrote: > [Moving the thread to the users mailing list] > > Yingjian, > > Good to see you persevering and getting it done! Congratulations. > > Before I reasoned about your code, just looking at its shape was enough to > imagine it was not properly indented. > You can use and abuse of DrRacket for your on good. > > Using the shortcut CTRL+I let's you reindent all of your code, which gives > this: > > #lang racket > (define (count-matches s l) > (define (addone s l x) > (cond > [(empty? l) x] > [(equal? s (first l)) (addone s (rest l) (+ 1 x))] > [else (addone s (rest l) x)])) > (addone s l 0)) > > > The change in indentation makes clear that you're defining an auxiliary > function and the result of calling count-matches is that of calling (addone > s l 0). > > BTW "addone" doesn't really sound to me to describe what the function is > doing -- it not always "adds one". > > > Look how a similar implementation is possible without an inner auxiliary > function: > > #lang racket > (define (count-matches s l) > (cond > [(empty? l) 0] > [(equal? s (first l)) (add1 (count-matches s (rest l)))] > [else (count-matches s (rest l))])) > > (count-matches 'x '()) ; should be 0 > (count-matches 'x '(a b x)) ; should be 1 > (count-matches 'x '(x b x)) ; should be 2 > > > []'s > > Rodolfo Carvalho > > > > On Wed, Jun 1, 2011 at 01:37, Yingjian Ma <yingjian.ma1...@gmail.com>wrote: > >> I finished it. The purpose of the function is to count the occurrance of >> a letter in a list. Ex: >> >> (count-matches 'x '()) should be 0 >> (count-matches 'x '(a b x)) should be 1 >> (count-matches 'x '(x b x)) should be 2 >> >> The keywords I can use are limited. Thank you all for the help. Another >> I need to write is to remove duplicated items from the list. >> >> It seems that under cond, one condition can only take one expression. >> What can I do if I want to do two statements? >> >> Here is the code. >> >> (define (count-matches s l) >> (define (addone s l x) >> (cond >> [(empty? l) x] >> [(equal? s (first l)) (addone s (rest l) (+ 1 x))] >> [else (addone s (rest l) x)])) >> (addone s l 0)) >> >> >> >
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