Asp Forum - Array questions

Mark Woodward

1/3/2007 11:11:00 AM

Hi,

I'm trying to learn ruby by 'borrowing' peoples code and adding comments
to help me understand what they've done. The code below is 'borrowed' from
the maze solutions on rubygarden.
(http://wiki.rubygarden.org/Ruby/page/show/NubyMaz...)

I'm trying to understand the line:
@paths = Array.new(0,Array.new(0))

From 'ri Array.new' I think the outer Array is of the form:
Array.new(size=0, obj=nil)
where obj=nil is Array.new(0) ???

If so could someone explain whats going on here? I can understand
something like Array.new(5,"A") where a new Array with 5 references to the
same string "A" is created. But why create an Array with 0 elements and
have 0 references to another new empty array? How can you have 0
references?

Aren't these equivalent?

irb(main):034:0> Array.new(0,Array.new(0))
=> []
irb(main):035:0> Array.new(0)
=> []
irb(main):036:0> Array.new
=> []

I can replace that line in the code below with any of the 2 commented
lines below it and the program still runs. So I'm wondering why the author
used that particular line. I understand no one can speak for the author
but am querying whether I'm missing something here.

--------------------------------------------------------------------------
class MazeSolver

# the constructor with the maze string as a parameter
def initialize(m)

# assign the maze to the instance variable @maze
# (only visible to a particular instance of the class
# as opposed to a class variable (@@) which is accessible
# to *any* instance of the class)
@maze = m

# find the length of the first line. ie count chars to first \n
# (indexing starts at 0 so + 1)
@rowlen = @maze.index("\n")+1

# create an array of size 0, ??
# with 0 references to another new empty array ?????
# hah?
@paths = Array.new(0,Array.new(0)) <<<<<<<<<<<<
# @paths = Array.new
# @paths = Array.new(0)

# call the findPaths method
findPaths
end

#
def getNSEW(index)

# create an empty array
ret=[]

#
table =[index-@rowlen,index+@rowlen,index+1,index-1]
table.each{|n|ret.push([n,@maze[n].chr])}
return ret
end

def findPaths(loc=@maze.index('s'),current=[])
sides = getNSEW(loc)
return 0 if current.include?(loc)
current.push(loc)
ret=0
(0..3).each do |n|
case (sides[n][1])
when "e": current.push(sides[n][0]);@paths.push(current);return 1
when "-": ret += findPaths(sides[n][0],current.dup)
end
n+=1
end
ret
end

def to_s
ret=""
@paths.each do |path|
tMaze = @maze.dup
path.each{|loc|tMaze[loc]="*"}
ret += "#{tMaze.gsub(/#/,"I").gsub(/-/," ")}\n\n\n"
end
ret
end

end

# a heredoc created string representing the maze
mazeDef = <<MAZE_STRING
#########################
#s--###----#####-#####-##
###-#---##-##--#-##-----#
###-#-#-#--##-##-##-###-#
#---#-#-#-###-##-##-##--#
#-#---#-#---#-##----##-##
#-#-#-#-#-#-#-##-########
#-#-#-#-#-#---##--------#
#####-###---###########-#
#####-#####-###########-#
#####------------------e#
#########################
MAZE_STRING

# 'print' calls the to_s method of a newly created MazeSolver
# instance where mazedef is passed to the initialize method.
print MazeSolver.new(mazeDef)

--------------------------------------------------------------------------

thanks,

--
Mark

5 Answers

Kenosis

1/3/2007 10:16:00 PM

Mark Woodward wrote:
> Hi,
>
> I'm trying to learn ruby by 'borrowing' peoples code and adding comments
> to help me understand what they've done. The code below is 'borrowed' from
> the maze solutions on rubygarden.
> (http://wiki.rubygarden.org/Ruby/page/show/NubyMaz...)
>
> I'm trying to understand the line:
> @paths = Array.new(0,Array.new(0))
>
> From 'ri Array.new' I think the outer Array is of the form:
> Array.new(size=0, obj=nil)
> where obj=nil is Array.new(0) ???
>
> If so could someone explain whats going on here? I can understand
> something like Array.new(5,"A") where a new Array with 5 references to the
> same string "A" is created. But why create an Array with 0 elements and
> have 0 references to another new empty array? How can you have 0
> references?
>
> Aren't these equivalent?
>
> irb(main):034:0> Array.new(0,Array.new(0))
> => []
> irb(main):035:0> Array.new(0)
> => []
> irb(main):036:0> Array.new
> => []
>
> I can replace that line in the code below with any of the 2 commented
> lines below it and the program still runs. So I'm wondering why the author
> used that particular line. I understand no one can speak for the author
> but am querying whether I'm missing something here.
>
>
> --------------------------------------------------------------------------
> class MazeSolver
>
>
> # the constructor with the maze string as a parameter
> def initialize(m)
>
> # assign the maze to the instance variable @maze
> # (only visible to a particular instance of the class
> # as opposed to a class variable (@@) which is accessible
> # to *any* instance of the class)
> @maze = m
>
> # find the length of the first line. ie count chars to first \n
> # (indexing starts at 0 so + 1)
> @rowlen = @maze.index("\n")+1
>
> # create an array of size 0, ??
> # with 0 references to another new empty array ?????
> # hah?
> @paths = Array.new(0,Array.new(0)) <<<<<<<<<<<<
> # @paths = Array.new
> # @paths = Array.new(0)
>
> # call the findPaths method
> findPaths
> end
>
>
> #
> def getNSEW(index)
>
> # create an empty array
> ret=[]
>
> #
> table =[index-@rowlen,index+@rowlen,index+1,index-1]
> table.each{|n|ret.push([n,@maze[n].chr])}
> return ret
> end
>
>
> def findPaths(loc=@maze.index('s'),current=[])
> sides = getNSEW(loc)
> return 0 if current.include?(loc)
> current.push(loc)
> ret=0
> (0..3).each do |n|
> case (sides[n][1])
> when "e": current.push(sides[n][0]);@paths.push(current);return 1
> when "-": ret += findPaths(sides[n][0],current.dup)
> end
> n+=1
> end
> ret
> end
>
>
> def to_s
> ret=""
> @paths.each do |path|
> tMaze = @maze.dup
> path.each{|loc|tMaze[loc]="*"}
> ret += "#{tMaze.gsub(/#/,"I").gsub(/-/," ")}\n\n\n"
> end
> ret
> end
>
>
> end
>
>
> # a heredoc created string representing the maze
> mazeDef = <<MAZE_STRING
> #########################
> #s--###----#####-#####-##
> ###-#---##-##--#-##-----#
> ###-#-#-#--##-##-##-###-#
> #---#-#-#-###-##-##-##--#
> #-#---#-#---#-##----##-##
> #-#-#-#-#-#-#-##-########
> #-#-#-#-#-#---##--------#
> #####-###---###########-#
> #####-#####-###########-#
> #####------------------e#
> #########################
> MAZE_STRING
>
>
> # 'print' calls the to_s method of a newly created MazeSolver
> # instance where mazedef is passed to the initialize method.
> print MazeSolver.new(mazeDef)
>
> --------------------------------------------------------------------------
>
>
> thanks,
>
>
> --
> Mark

All I can think of is this: the author of the maze code wanted to
ensure that the initial value of @paths was assured to be an empty
array. If he/she resorted to Array.new's default argument, which could
change then @path might not be in the state the programmer expected.
Other than that, as you surimised, the 3 ways you initialized the array
should all result in the same thing, AFAIK.

Ken

Mark Woodward

1/3/2007 10:59:00 PM

Hi Ken,

Kenosis wrote:
> All I can think of is this: the author of the maze code wanted to
> ensure that the initial value of @paths was assured to be an empty
> array. If he/she resorted to Array.new's default argument, which could
> change then @path might not be in the state the programmer expected.
> Other than that, as you surimised, the 3 ways you initialized the array
> should all result in the same thing, AFAIK.
>
> Ken

I suspected that but I couldn't understand how/why this could be done
with a size 0.

@paths = Array.new(0,Array.new(0)) to me reads as "create a new array
with nothing in it and make that nothing reference another empty
array?"
ie []

@paths = Array.new(1,Array.new(0)) This, if my logic's right, reads as
"create a new array with a single element that's an empty array.
ie [[] ]

Are you saying that even though @paths is created as an empty array, it
expects the first element it gets to be an array? If not it will throw
an error?
Sorry, I can't check this in irb ATM.

cheers,

--
Mark

Robert Klemme

1/4/2007 10:56:00 AM

On 03.01.2007 23:58, Mark Woodward wrote:
> I suspected that but I couldn't understand how/why this could be done
> with a size 0.
>
> @paths = Array.new(0,Array.new(0)) to me reads as "create a new array
> with nothing in it and make that nothing reference another empty
> array?"
> ie []

Nothing cannot really reference to anything, can it?

> @paths = Array.new(1,Array.new(0)) This, if my logic's right, reads as
> "create a new array with a single element that's an empty array.
> ie [[] ]

Even that could be more clearly done with

@paths = [[]]

> Are you saying that even though @paths is created as an empty array, it
> expects the first element it gets to be an array? If not it will throw
> an error?
> Sorry, I can't check this in irb ATM.

No, there is no such error checking done. There is also not a default
value as with a Hash - an Array always returns nil when non existing
elements are referenced.

Maybe the code was created and then modified several times leaving this
artifact. The shortest and also cleanest solution would probably be to
just do

@paths = []

Kind regards

robert

Mark Woodward

1/7/2007 10:23:00 AM

On Thu, 04 Jan 2007 11:56:09 +0100, Robert Klemme wrote:

> On 03.01.2007 23:58, Mark Woodward wrote:
>> I suspected that but I couldn't understand how/why this could be done
>> with a size 0.
>>
>> @paths = Array.new(0,Array.new(0)) to me reads as "create a new array
>> with nothing in it and make that nothing reference another empty
>> array?"
>> ie []
>
> Nothing cannot really reference to anything, can it?

Not that I'm aware of ;-).

>
>> @paths = Array.new(1,Array.new(0)) This, if my logic's right, reads as
>> "create a new array with a single element that's an empty array.
>> ie [[] ]
>
> Even that could be more clearly done with
>
> @paths = [[]]
>
>> Are you saying that even though @paths is created as an empty array, it
>> expects the first element it gets to be an array? If not it will throw
>> an error?
>> Sorry, I can't check this in irb ATM.
>
> No, there is no such error checking done. There is also not a default
> value as with a Hash - an Array always returns nil when non existing
> elements are referenced.
>
> Maybe the code was created and then modified several times leaving this
> artifact. The shortest and also cleanest solution would probably be to
> just do
>
> @paths = []

Thanks Robert, I checked this in irb when I got the chance and you're
right. There's no error checking. I can put what ever I like in
@paths = Array.new(0,Array.new(0))

just as I can
@paths = []

so I'll chalk it up as a artifact as you suggested above.

>
> Kind regards
>
> robert

cheers,

--
Mark

laraine

9/1/2012 9:23:00 PM

On Aug 24, 4:19 pm, Dufus <steveha...@gmail.com> wrote:
> >On Aug 24, 1:03 pm, "HvT" <hvtuijl- SPAM- @xs4all.nl> wrote:
>
> > The audience did like the first half of the recital more than I did. BBC's
> > engineering must have left something to be desired.
>
> Agreed. In fairness , the BBC R3 sound was not great ;but then I'm
> grateful my ticket was free and I did not have to go to Edinburgh, so
> I doth not protest too much.
>
> I have heard his playing ( live, but recorded or broadcast, not in the
> halls ) at the 2010 Chopin , 2011 Rubinstein , 2011 Tchaikovsky, 2012
> Verbier, and now here. I have several reservations, but as he plays
> better than I do , I will reserve judgment for awhile , give him some
> time yet. I suspect he may be playing a bit too much, and repetoire
> that's getting a bit jaded for him . I may also be wrong about my
> suspicions.
>
> Dufus

It might have had something to do with the
sound, but I also got the impression that
his playing was small-toned and fast.

Nevertheless, I enjoyed the program quite
a lot, esp. the Medtner fairy talses, which
sounded dreamy and sad.

On the Chopin etudes, I sometimes wished
that he would use a little more rubato
at times, but very good in general, and
the other pieces even better.

Very long technically difficult program.

C.

comp.lang.ruby

Array questions

Mark Woodward

Kenosis

Mark Woodward

Robert Klemme

Mark Woodward

laraine

x Login to ForumsZone