Re: Scala beginner question

On Wed, Mar 2, 2016, 4:39 PM Kevin Wright <[email protected]> wrote:

> Numerous better ideas…
>
> 1. You’re making checks against two different regexes that can be combined
> into one.
> 2. You’re attempting to match “.” in one of those expressions.  This will
> match any character unless you escape it to a literal with “\.”
> 3. Anything used in a pattern match should begin with an uppercase letter
>
> *Taking points 1,2,3 into consideration:*
>
> val UnwantedChar = """[0-9]|\.|;|,|"|:""".r
>
> 4. You don’t need to supply the empty parameter block when calling
> `toLowerCase`.  By convention, this syntax is only used for methods with
> side effects, not for simple getters or conversions
> 5. You’re explicitly converting a String to a StringOps - This is already
> done for you via the magic of implicits
> 6. When pattern matching against a regex, it destructures the string
> according to any capture groups you’ve defined “(…)” in the regex. You
> haven’t specified any, so the “(_*)”s in your matches are useless
> 7. A succession of case statements wrapped in braces defines a
> `PartialFunction` (which is a subclass of `Function`); so you don’t need to
> write “(c) => c match”
>
> This gives:
>
> private def transformContent(content: String): String = {
>   content.toLowerCase filter {
>     case UnwantedChar => false
>

Without parentheses it won't use the extractor, it will check if it == the
regex.


>     case _            => true
>   }
> }
>
> 8. Using filter here is inefficient.  Regexes already have a method for
> locating and replacing all matches in a string
>
> private def transformContent(content: String): String =
>   UnwantedChar.replaceAllIn(content.toLowerCase, "")
>
>
> 9. Iterators are a good way to deal with lazy and potentially infinite
> lists.  `takeWhile` can then be used to catch the terminal null condition.
>
> import scala.io.StdIn
> val inputLines = Iterator continually StdIn.readLine() takeWhile (_ !=
> null)
> // note the () … because readLine() *is* side-effecting
> for(line <- inputLines) {
>   …
> }
>
> 10. The `transformContent` method is now so short that it’s easier to
> write the content inline
> 11. Try to avoid side-effects in the middle of other blocks of code,
> they’re better at the boundary of your app
>
> Putting it all together:
>
> import scala.io.StdIn
> val UnwantedChar = """[0-9]|\.|;|,|"|:""".r
> val inputLines = Iterator continually StdIn.readLine() takeWhile (_ !=
> null)
> val filteredInputLines = inputLines map {line =>
>   UnwantedChar.replaceAllIn(line.toLowerCase, "")
> }
> filteredInputLines foreach println
>
>
> On 2 March 2016 at 16:39, Pietro <[email protected]> wrote:
>
>> Hi everybody,
>>
>> I am not even sure this is the right place to ask this beginner
>> questions nevertheless I do prefer gmane.* to StackOverflow and I wanted
>> to give to it a try; feel free to point me to the right newsgroup if
>> this isn't the most proper place.
>>
>> I have recently started to write a simple project in Scala, I got stuck
>> in solving a couple of reasonalble simple problems; as many others I am
>> from a imperative Java/C background and I am striving to change mindset.
>>
>> The following function should wipe out all the digits and punctuation
>> characters present in the given string "content", anyway the match
>> statement never matches where I expect and the string does not get
>> modified.
>>
>> Is there a big mistake I haven't noticed ?
>>
>>   val digitRE       = """[0-9]""".r
>>   val punctuationRE = """.|;|,|"|:""".r
>>
>>   private def transformContent(content : String) : String = {
>>     val _ret = new StringOps(content)
>>     _ret.toLowerCase().filter(
>>       (c) => {
>>         c match {
>>           case digitRE(_*)       => false
>>           case punctuationRE(_*) => false
>>           case _                 => true
>>         }
>>       })
>>   }
>>
>> Then comes my next question, I would like to write a loop which reads
>> from the console user's input and stops when the the user enters a null
>> value, that is, they presses ENTER without having entered any characters
>> before it.
>>
>> This is one of my several attempt to achieve it, it obviously does
>> not work properly.
>>
>>     while ( (path = readLine()) != null)
>>       println(path)
>>   }
>>
>> The only working solution I have got so far is to
>> throw an exception, something like :
>>
>> try {
>>     while (true) {
>>             if (whatever)
>>                      throw AllDone
>>     }
>>
>> }catch (AllDone) {
>>       ...
>> }
>>
>> Any better ideas ?
>>
>> --
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