# Passing params to Map method

**URL:** <https://www.ruby-forum.com/t/passing-params-to-map-method/132828>\
**Category:** Ruby\
**Created:** [March 25, 2008, 4:25am UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828 "2008-03-25T04:25:37Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 25, 2008, 4:25am UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828/1 "2008-03-25T04:25:37Z")

</div>

Hi and thanks in advance for your help.  
I have been pounding every Ruby and Rails book I can find, desperately  
attempting to move beyond newbie status.My latest foray has taken me  
into the native 2.0.2 Rails::Info module. I am reading what I think is  
native ruby code but I can’t seem to figure out what is being passed  
to map and find. Could someone please explain (name, ) in the  
following code?

Thanks again.

module Rails  
module Info  
mattr\_accessor :properties

```
class << (@@properties = [])
  def names
    map {|(name, )| name}
  end

  def value_for(property_name)
    find {|(name, )| name == property_name}.last rescue nil
  end
end
```

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<div class="post-metadata">

**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 25, 2008, 4:58am UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828/2 "2008-03-25T04:58:00Z")

</div>

Hi –

On Tue, 25 Mar 2008, Ruby F. wrote:

> module Rails  
> end  
> end

find and map are not being passed any arguments; rather, they’re being  
given code blocks, and those blocks are being passed arguments.  
(name,) is block parameter syntax for grabbing one element of an array  
and putting it in name.

David

---

<div class="post-metadata">

**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 26, 2008, 4:47am UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828/3 "2008-03-26T04:47:14Z")

</div>

Thank you David,  
I stand corrected (actually I’m sitting)

So… I have been re-reading the best darn Ruby/Rails book on the  
planet (Ruby for Rails of course! which I already read cover to cover  
once) and I am still a bit confused but it’s getting better.  
Maybe someone could be so kind as to critique my comments below about  
what I think is happening. I have been studying this little bit code  
for about 6 … no 8, hours and hopefully, I have most of it correct.  
(personally I find it irritating when people ask questions without  
putting any personal effort into finding the answers themselves)

module Rails  
module Info  
mattr\_accessor :properties #module level property  
assessor (getter and setter)  
class \<\< (@@properties = []) #Open up Array class def to  
add methods (class \<\< object), object == an Array  
def names #define  
Array.properties.names method  
map {|(name, )| name} #read code block to return  
array comprised of first half of [name, value]  
end

```
  def value_for(property_name) #define

```

Array.properties.value\_for(“name param”) method  
find {|(name, )| name == property\_name}.last rescue nil  
#return value part of [name, value] for sent name param  
end  
end #close Array class def

```
class << self #:nodoc: #open "Info" class def to add

```

methods  
def property(name, value = nil) #define property “setter”  
except the yield makes it a “getter”  
value ||= yield #if value is nil, yield name  
to code block in names and value\_for  
properties \<\< [name, value] if value #push [name, value]  
array into @@properties array  
rescue Exception #ignore error?  
end  
…

What I think is happening is the new “Info.properties” accessor is  
cleverly leveraging self.property as a method to both set a new  
property into @@properties and to return (yield) just the name when  
the parameter passed to the code block is just the first half of the  
array (name, ). Just understanding that after 8 months and thousands  
of hours and reading thousands of pages of books is really exciting  
for me.

I am a little iffy on the “class \<\< (@@properties = [])” opening up  
the Array object, but it fits the rule.(class \<\< object)

if that is true, it is pretty cool code. (it’s pretty cool even if I  
am wrong) Ruby has quite a learning curve, especially for a VB guy.

Thanks again.

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<div class="post-metadata">

**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 27, 2008, 2:21pm UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828/4 "2008-03-27T14:21:35Z")

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On Mar 24, 11:57 pm, “David A. Black” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> …  
> find and map are not being passed any arguments; rather, they’re being  
> given code blocks, and those blocks are being passed arguments.  
> (name,) is block parameter syntax for grabbing one element of an array  
> and putting it in name.

From your description “block parameter syntax”, someone might conclude  
that this is something specific to blocks, but it’s more generally the  
syntax for destructuring an array. For example:

(x,) = [7,8,9] # x =\> 7  
(x,y,z) = [7,8,9]

or even

x, = [7,8,9]

In the OP’s example, I think map {|a| a[0] } is clearer, but if the  
parameter is used multiple times, e.g. map {|a,| “#{a}-#{a}” }, then  
assigning to ‘a’ initially is more concise.

---

<div class="post-metadata">

**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 26, 2008, 1:13pm UTC](https://www.ruby-forum.com/t/passing-params-to-map-method/132828/5 "2008-03-26T13:13:40Z")

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2008/3/26, Ruby F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> What I think is happening is the new “Info.properties” accessor is  
> cleverly leveraging self.property as a method to both set a new  
> property into @@properties and to return (yield) just the name when  
> the parameter passed to the code block is just the first half of the  
> array (name, ). Just understanding that after 8 months and thousands  
> of hours and reading thousands of pages of books is really exciting  
> for me.

Yep. Methods #names and #values\_for are defined for the one instance  
of Array only that is referenced by @@properties. So these are not  
general methods. But they rely on methods defined in Array and  
Enumerable to do their job. Basically they seem to rely on the fact  
that @@properties contains arrays with exactly two elements - the  
first one used as property name and the second one as property value.  
During iteration (i.e. map, find) on the array only the first is  
assigned a block parameter (small optimization because the second one  
is not needed). That’s it basically. This is the usual pattern  
matching that Ruby applies to multiple assignments:

irb(main):001:0\> (a,(b,c),d)=1,2,3,4,5  
=\> [1, 2, 3, 4, 5]  
irb(main):002:0\> a  
=\> 1  
irb(main):003:0\> b  
=\> 2  
irb(main):004:0\> c  
=\> nil  
irb(main):005:0\> d  
=\> 3  
irb(main):006:0\> (a,(b,c),d)=1,[2,3],4,5  
=\> [1, [2, 3], 4, 5]  
irb(main):007:0\> a  
=\> 1  
irb(main):008:0\> b  
=\> 2  
irb(main):009:0\> c  
=\> 3  
irb(main):010:0\> d  
=\> 4

What makes this a bit hard to read is that assignment to @@properties  
and the class\<\<@@properties are lumped into one statement.

> I am a little iffy on the “class \<\< (@@properties = [])” opening up  
> the Array object, but it fits the rule.(class \<\< object)
> 
> if that is true, it is pretty cool code. (it’s pretty cool even if I  
> am wrong) Ruby has quite a learning curve, especially for a VB guy.

Yeah, it seems VB is a bad place to start learning to program. 🙂

Kind regards

robert