is this valid smalltalk

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is this valid smalltalk

Roelof Wobben
Hello,

For a challenge of Exercism I need to check if some parenthes and
brackets are balanced.

so for example

()  = true
([])  = true

but (])  is not true because the bracket has no opening bracket.

Now I wonder if I can premature end a #do:  like this

collection do: [:element | (condition with element) ifFalse: [^false]]

in pseudo code

is this a valid way to end the do ?
or is there a better way ?

Roelof

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Re: is this valid smalltalk

K K Subbu
On 04/04/19 4:46 PM, Roelof Wobben wrote:

> Hello,
>
> For a challenge of Exercism I need to check if some parenthes and
> brackets are balanced.
>
> so for example
>
> ()  = true
> ([])  = true
>
> but (])  is not true because the bracket has no opening bracket.
>
> Now I wonder if I can premature end a #do:  like this
>
> collection do: [:element | (condition with element) ifFalse: [^false]]

Have you looked at anySatisfy:

  #('the' 'quick' 'brown' 'fox') anySatisfy: [:s | s beginsWith: 'x' ]

Regards .. Subbu

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Re: is this valid smalltalk

Roelof Wobben
Op 4-4-2019 om 13:22 schreef K K Subbu:

> On 04/04/19 4:46 PM, Roelof Wobben wrote:
>> Hello,
>>
>> For a challenge of Exercism I need to check if some parenthes and
>> brackets are balanced.
>>
>> so for example
>>
>> ()  = true
>> ([])  = true
>>
>> but (])  is not true because the bracket has no opening bracket.
>>
>> Now I wonder if I can premature end a #do:  like this
>>
>> collection do: [:element | (condition with element) ifFalse: [^false]]
>
> Have you looked at anySatisfy:
>
>  #('the' 'quick' 'brown' 'fox') anySatisfy: [:s | s beginsWith: 'x' ]
>
> Regards .. Subbu
>
yep,  i also  took a look at that one


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Re: is this valid smalltalk

Herby Vojčík
In reply to this post by Roelof Wobben
On 4. 4. 2019 13:16, Roelof Wobben wrote:

> Hello,
>
> For a challenge of Exercism I need to check if some parenthes and
> brackets are balanced.
>
> so for example
>
> ()  = true
> ([])  = true
>
> but (])  is not true because the bracket has no opening bracket.
>
> Now I wonder if I can premature end a #do:  like this
>
> collection do: [:element | (condition with element) ifFalse: [^false]]
>
> in pseudo code
>
> is this a valid way to end the do ?

Not sure I understand your question properly, but yes, you can do this,
it is called "non-local return" and is a known pattern in Smalltalk.

> or is there a better way ?

   (collection
     detect: [:element | (condition with element) not]
     ifNone: [sentinel]) == sentinel ifFalse: [ ^false ]

for example does not force you to employ it, as well as

   (collection allSatisfy: [:element | (condition with element)])
     ifFalse: [ ^false ]

>
> Roelof

Herby

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Re: is this valid smalltalk

Richard O'Keefe
In reply to this post by Roelof Wobben
For a change this is an exercism problem I know something about.
I did look at the exercism web site, and did all the SML exercises,
but could not find any for Smalltalk or Pharo.

The exercise is in fact about using a stack.
   stack <- empty
   for each character of the string
      if it is one of ( [ { push it on the stack
      if it is one of ) ] }
         if the stack is empty or the top element does not
            correspond to this character, return false
      if it is not one of ( ) [ ] { } just ignore it
   return (the stack is empty)

BOTH of the "return" constructs in this pseudo-code become
^ something
in Smalltalk.  "Long returns", where "^" occurs inside a block,
are well defined, very useful, and universally accepted in Smalltalk.

There is no analogue of "^" for returning from a block.

By the way, this has nothing to do with #do:.  It's about
returning from the method through any number of blocks.
For example, you will sometimes see code like
   x := aDictionary at: aKey ifAbsent: [^false].


On Fri, 5 Apr 2019 at 00:16, Roelof Wobben <[hidden email]> wrote:
Hello,

For a challenge of Exercism I need to check if some parenthes and
brackets are balanced.

so for example

()  = true
([])  = true

but (])  is not true because the bracket has no opening bracket.

Now I wonder if I can premature end a #do:  like this

collection do: [:element | (condition with element) ifFalse: [^false]]

in pseudo code

is this a valid way to end the do ?
or is there a better way ?

Roelof

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Re: is this valid smalltalk

Roelof Wobben
I already thought to use this flow.

but I only wondered if I could use the ^false this way and appearently it is allowed.

so I can begin with this one.

The Pharo track is still hidden but I saw on the github page that in a short time it will be public with some 20 - 25 challenges.

Roelof


Op 4-4-2019 om 14:36 schreef Richard O'Keefe:
For a change this is an exercism problem I know something about.
I did look at the exercism web site, and did all the SML exercises,
but could not find any for Smalltalk or Pharo.

The exercise is in fact about using a stack.
   stack <- empty
   for each character of the string
      if it is one of ( [ { push it on the stack
      if it is one of ) ] }
         if the stack is empty or the top element does not
            correspond to this character, return false
      if it is not one of ( ) [ ] { } just ignore it
   return (the stack is empty)

BOTH of the "return" constructs in this pseudo-code become
^ something
in Smalltalk.  "Long returns", where "^" occurs inside a block,
are well defined, very useful, and universally accepted in Smalltalk.

There is no analogue of "^" for returning from a block.

By the way, this has nothing to do with #do:.  It's about
returning from the method through any number of blocks.
For example, you will sometimes see code like
   x := aDictionary at: aKey ifAbsent: [^false].


On Fri, 5 Apr 2019 at 00:16, Roelof Wobben <[hidden email]> wrote:
Hello,

For a challenge of Exercism I need to check if some parenthes and
brackets are balanced.

so for example

()  = true
([])  = true

but (])  is not true because the bracket has no opening bracket.

Now I wonder if I can premature end a #do:  like this

collection do: [:element | (condition with element) ifFalse: [^false]]

in pseudo code

is this a valid way to end the do ?
or is there a better way ?

Roelof