# What is \*self?

**URL:** <https://www.ruby-forum.com/t/what-is-self/77167>\
**Category:** Ruby\
**Created:** [November 24, 2006, 9:47pm UTC](https://www.ruby-forum.com/t/what-is-self/77167 "2006-11-24T21:47:05Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![josefK](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ea666f/32.png) [@josefK](https://www.ruby-forum.com/u/josefK)\
**Post date:** [November 24, 2006, 9:47pm UTC](https://www.ruby-forum.com/t/what-is-self/77167/1 "2006-11-24T21:47:05Z")

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I encountered this added method to Array (I don’t recall  
the author - sorry ,)  
It converts an Array to a Hash by providing a block to compute the  
values given the array elements as keys:

class Array  
def to\_h(&block)  
Hash[\*self.collect{|v| [v,block.call(v)]}.flatten]  
end  
end

#exp  
a=[1,2,3]  
h=a.to\_h{|e| 2\*\*e}  
h.each{|k,v| puts “key=#{k} val=#{v}”}

#-\>key=1 val=2 key=2 val=4 key=3 val=8

Without the ‘\*’ on self it generates an error:  
array2hash.rb:3:in `[]': odd number of arguments for Hash (ArgumentError) from array2hash.rb:3:in`to\_h’  
from array2hash.rb:8

What does the ‘\*’ do to correct the error?

Does it push self up a level to Array somehow?

Does this notation generalize?

Thanks,

Mark

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**Author:** ![josefK](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ea666f/32.png) [@josefK](https://www.ruby-forum.com/u/josefK)\
**Post date:** [November 24, 2006, 9:49pm UTC](https://www.ruby-forum.com/t/what-is-self/77167/2 "2006-11-24T21:49:06Z")

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josefK wrote:

> 

“\*” is the “splat” operator when used as a prefix. It takes the  
following array, and turns into a list of parameters. Without it the  
argument to Hash[] becomes a single Array, while it expects a list of  
keys and values.

And yes, it does generalize. Try  
p [1,2,3]  
p \*[1,2,3]  
in irb to get a better idea of the result. The first prints a single  
array, the second prints _three separate values_

Vidar

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<div class="post-metadata">

**Author:** ![josefK](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ea666f/32.png) [@josefK](https://www.ruby-forum.com/u/josefK)\
**Post date:** [November 24, 2006, 9:51pm UTC](https://www.ruby-forum.com/t/what-is-self/77167/3 "2006-11-24T21:51:06Z")

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Vidar H. wrote:

> > end  
> > p [1,2,3]  
> > p \*[1,2,3]  
> > in irb to get a better idea of the result. The first prints a single  
> > array, the second prints _three separate values_
> 
> Vidar

OK - I see - it does not affect ‘self’ just the output []  
I thought it was a way to modify ‘self’

I guess ‘super’ and ‘super.super…’ is the generalization  
I mistakenly thought was happening.

Thanks much,

Mark

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<div class="post-metadata">

**Author:** ![josefK](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ea666f/32.png) [@josefK](https://www.ruby-forum.com/u/josefK)\
**Post date:** [November 24, 2006, 10:35pm UTC](https://www.ruby-forum.com/t/what-is-self/77167/4 "2006-11-24T22:35:23Z")

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On 11/24/06, josefK [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I encountered this added method to Array (I don’t recall  
> the author - sorry ,)  
> It converts an Array to a Hash by providing a block to compute the  
> values given the array elements as keys:
> 
> class Array  
> def to\_h(&block)  
> Hash[\*self.collect{|v| [v,block.call(v)]}.flatten]  
> end  
> end

Vidar explained the general concept of the splat fairly well, but the  
following clarification of order of operations should help understand  
this particular application:

# assuming self = [1, 2, 3] and block = proc{ |x| x\*\*2 }

self.  
collect{|v| [v,block.call(v)]}. # =\> [[1, 1], [2, 4], [3, 9]]  
flatten # =\> [1, 1, 2, 4, 3, 9]

Without the star, the full Method body would be (equivalently):

Hash[[1, 1, 2, 4, 3, 9]]

This complains about an odd number of elements because it’s receiving  
only one element – the array. Adding the “splat” in essence removes  
the innermost pair of square brackets:

Hash[1, 1, 2, 4, 3, 9]

Hash#[] can then use the list as key/value pairs.

As I said, Vidar explained the operation of the splat well, I just  
wanted to point out _which_ array the star was operating on – the  
result of the expression “self.collect{ … }.flatten”, not to self  
itself (no pun intended).

Jacob F.