The good thing about these exercises is that you know about new selectors, I didn't know there was an #allButLast and #allButLastDo:. Regards, Esteban A. Maringolo On Fri, May 17, 2019 at 5:44 AM Sven Van Caekenberghe <sven@stfx.eu> wrote:
I would make that
^ String new: gifts size * 10 streamContents: [ :stream | gifts allButLastDo: [:each | stream nextPutAll: each; nextPutAll: ', ']. stream nextPutAll: 'and '; nextPutAll: gifts last ]
Your iteration is well done, clear and to the point.
On 17 May 2019, at 02:44, Richard O'Keefe <raoknz@gmail.com> wrote:
stream := WriteStream on: (String new: gifts size * "estimated size per gift" 10). "The number after #new: is the *initial* allocation; the stream will reallocate it as needed." gifts allButLastDo: [:each | stream nextPutAll: each; nextPutAll: ', ']. stream nextPutAll: 'and '; nextPutAll: gifts last. ^stream contents Using #allButLastDo: saves making a copy of (most of) gifts. How could you find out about these things?
You want to join some strings, so you look for a "join" method in String and you will find
join: aCollection ^ self class new: (aCollection size * self size) streamContents: [:stream | aCollection do: [:each | stream nextPutAll: each asString] separatedBy: [stream nextPutAll: self]]
You might wonder if there is already something close to what you want, so you might enter "commas" into Spotter. If you did that, you would find Collection>>asCommaStringAnd so that the whole thing is very nearly gifts asCommaStringAnd
Just as the concatenation selector #, works with most kinds of sequences, so building sequences up using a WriteStream works with most kinds of sequences.
Let's work through a little example. We are just going to build up the string 'Quick' one character at a time. s := ''. s := s , 'Q'. "creates a new string holding 'Q'" s := s , 'u'. "creates a new string holding 'Qu'" s := s , 'i'. "creates a new string holding 'Qui'" s := s , 'c'. "creates a new string holding 'Quic'" s := s , 'k'. "creates a new string holding 'Quick'"
You see that building a string of n characters will actually create n new strings, all but the last of which will be thrown away, taking O(n**2) time.
Now let's use a stream. w := WriteStream on: (String new: 4). "Yes, I know that's too small." w nextPutAll: 'Q'. "The stream now holds 'Q...' in its buffer." w nextPutAll: 'u'. "The stream now holds 'Qu..' in its buffer." w nextPutAll: 'i'. "The stream now holds 'Qui.' in its buffer." w nextPutAll: 'c'. "The stream now holds 'Quic' in its buffer." s nextPutAll: 'k'. "There is no room left in the buffer, so the stream allocates a new buffer twice the size and copies the old one into it. Now it has 'Quic....' and it has room. The stream now holds 'Quick...' in its buffer." s := w contents. "We are asking for the defined elements of the buffer. This means s := buffer copyFrom: 1 to: w position."
You see that building a string of n characters this way requires a minimum of two strings, the buffer and the final result. The buffer may be periodically resized, but growing by doubling means the average cost is still O(n).
Let's time these to get an idea. Time millisecondsToRun: [ |s| s := ''. 1 to: 10000 do: [:i | s := s , i printString]. s size] => 124 Time millisecondsToRun: [ |w| w := WriteStream on: (String new: 10000). 1 to: 10000 do: [:i | w nextPutAll: i printString]. w contents size] => 7
This is exactly the reason that Java has both String and StringBuilder. The tragedy of Java (well, not the only one) is that they had the example of Smalltalk before them, showing very very clearly that the best way to handle object to text conversion is to use #printOn: as the primitive, not #printString, and they *still* went ahead and did the worse thing. (Ruby has even less excuse for privileging to_s.)
There are quite a few books about Smalltalk available as free PDFs from the Pharo web site, a wonderful resource. The Blue Book (Smalltalk-80 The Language and its Implementation) describes streams in Chapter 12.
On Fri, 17 May 2019 at 07:21, Roelof Wobben <r.wobben@home.nl> wrote: Hello,
Im testing all my solutions with critiz and can solve almost all problems,
Only this one I cannot figure out.
I have this code (gifts allButLast inject: '' into: [ :str :each | str , each , ', ' ]) , 'and ' , gifts last ]
and critiz says I should use a stream .
How can I make this work ?
Roelof