# Array questions

**URL:** <https://www.ruby-forum.com/t/array-questions/81054>\
**Category:** Ruby\
**Created:** [January 3, 2007, 12:16pm UTC](https://www.ruby-forum.com/t/array-questions/81054 "2007-01-03T12:16:01Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 3, 2007, 12:16pm UTC](https://www.ruby-forum.com/t/array-questions/81054/1 "2007-01-03T12:16:01Z")

</div>

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/NubyMazeSolutions](http://wiki.rubygarden.org/Ruby/page/show/NubyMazeSolutions))

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,

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 3, 2007, 4:12pm UTC](https://www.ruby-forum.com/t/array-questions/81054/2 "2007-01-03T16:12:44Z")

</div>

On 1/3/07, Mark W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hi,

Aren’t these equivalent?

Yes they are!  
I guess that the author wanted to demonstrate what kind of objects will  
be  
stored in the Array later, but it is not very readable and concise,  
looking  
at the code he might have written something like this

Array.new(0){ Array.new(2){ 0 } }

which still is [] of course but  
which might be interpreted as an empty Array that should only have  
arrays of  
two distinct objects that are integers.  
I guess this style might not be everyone’s cup of tea, I personally  
would  
like the “auto-documentation” idea which might be the origin of the code  
but  
not quite successfully I guess ☹

Please note to almost always use the block form as  
arry=Array.new(n,‘a’) creates n times the same reference to “a” and  
arry.last.sub!(“a”,“b”) changes arry into [“b”, “b”,…, “b”]

HTH  
Robert

i

> –  
> Mark

–  
“The real romance is out ahead and yet to come. The computer revolution  
hasn’t started yet. Don’t be misled by the enormous flow of money into  
bad  
defacto standards for unsophisticated buyers using poor adaptations of  
incomplete ideas.”

- Alan Kay

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 3, 2007, 11:20pm UTC](https://www.ruby-forum.com/t/array-questions/81054/3 "2007-01-03T23:20:46Z")

</div>

Mark W. wrote:

> From ‘ri Array.new’ I think the outer Array is of the form:
> 
> but am querying whether I’m missing something here.  
> # (only visible to a particular instance of the class  
> # hah?  
> def getNSEW(index)
> 
> ```
> n+=1
> ret += "#{tMaze.gsub(/#/,"I").gsub(/-/," ")}\n\n\n"
> 
> ```
> 
> #########################  
> #########################  
> 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

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 4, 2007, 12:02pm UTC](https://www.ruby-forum.com/t/array-questions/81054/4 "2007-01-04T12:02:23Z")

</div>

On 03.01.2007 23:58, Mark W. 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
```

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 4, 2007, 12:00am UTC](https://www.ruby-forum.com/t/array-questions/81054/5 "2007-01-04T00:00:59Z")

</div>

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,

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 7, 2007, 11:32am UTC](https://www.ruby-forum.com/t/array-questions/81054/6 "2007-01-07T11:32:19Z")

</div>

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

> On 03.01.2007 23:58, Mark W. 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 ;-).

> > expects the first element it gets to be an array? If not it will throw
> 
> @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,

---

<div class="post-metadata">

**Author:** ![marksbrain](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marksbrain](https://www.ruby-forum.com/u/marksbrain)\
**Post date:** [January 6, 2007, 7:30pm UTC](https://www.ruby-forum.com/t/array-questions/81054/7 "2007-01-06T19:30:39Z")

</div>

On 1/4/07, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> On 03.01.2007 23:58, Mark W. wrote:  
>   
> Maybe the code was created and then modified several times leaving this  
> artifact.

Robert  
it simply is newbie code if I understood correctly.  
Robert

The shortest and also cleanest solution would probably be to

> just do
> 
> @paths = []

Kind regards

> ```
> robert
> 
> ```

–  
“The real romance is out ahead and yet to come. The computer revolution  
hasn’t started yet. Don’t be misled by the enormous flow of money into  
bad  
defacto standards for unsophisticated buyers using poor adaptations of  
incomplete ideas.”

- Alan Kay