# Can someone explain this syntax in new?

**URL:** <https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794>\
**Category:** Ruby\
**Created:** [May 27, 2008, 9:09pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794 "2008-05-27T21:09:22Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [May 27, 2008, 9:09pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794/1 "2008-05-27T21:09:22Z")

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I found this in an old post:

"Here’s a Hash that automatically creates new hashes for each key you  
try to access (specifically 1-level deep for a ‘2-dimensional’ hash):

people = Hash.new{ |me,key| me[key] = {} }  
people[:gavin][:age] = 34  
people[:gavin][:sex] = :male  
people[:fido][:species] = :dog  
p people  
#=\> {:gavin=\>{:age=\>34, :sex=\>:male}, :fido=\>{:species=\>:dog}}

…but there was no further explanation. Could somebody explain the  
syntax in the first line? Is this overwriting new()?

Thanks,  
Robb

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<div class="post-metadata">

**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [May 27, 2008, 9:14pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794/2 "2008-05-27T21:14:58Z")

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Robb Shecter wrote:

> people = Hash.new{ |me,key| me[key] = {} }  
> […]  
> Â Is this overwriting new()?

No, it calls new and passes the above block to it.  
The block will then be stored and executed every time a key is accessed  
that  
does not exist (with the hash object and the requested key as  
arguments).

HTH,  
Sebastian

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<div class="post-metadata">

**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [May 27, 2008, 9:19pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794/3 "2008-05-27T21:19:10Z")

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On Tuesday 27 May 2008, Robb Shecter wrote:

> p people  
> #=\> {:gavin=\>{:age=\>34, :sex=\>:male}, :fido=\>{:species=\>:dog}}
> 
> …but there was no further explanation. Could somebody explain the  
> syntax in the first line? Is this overwriting new()?
> 
> Thanks,  
> Robb

If you look at the ri documentation for Hash#new, you’ll see that it can  
be  
used in three form: the first is Hash.new() which returns an hash with a  
default value of nil; the second is Hash.new(obj) which returns an hash  
with a  
default value of obj. This means that whenever you try to access an  
element  
using a key which is not in the hash, the method will return obj. The  
third  
form of new takes a block. Whenever you try to access a nonexistent key,  
the  
block is called with two arguments: the hash itself and the key. In the  
block  
you can do whatever you want, including computing a value from the key  
and  
storing it in the hash.

In the code you posted, the block simply stores a new, empty hash under  
the  
key key of the hash.

I hope this helps

Stefano

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<div class="post-metadata">

**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [May 27, 2008, 9:22pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794/4 "2008-05-27T21:22:15Z")

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Sebastian H. wrote:

> No, it calls new and passes the above block to it.  
> The block will then be stored and executed every time a key is accessed  
> that  
> does not exist (with the hash object and the requested key as  
> arguments).

Ah hah. I see; this is a particular feature of the Hash class – one of  
the three ways to invoke new(). Thanks!

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<div class="post-metadata">

**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [May 27, 2008, 9:19pm UTC](https://www.ruby-forum.com/t/can-someone-explain-this-syntax-in-new/138794/5 "2008-05-27T21:19:48Z")

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> try to access (specifically 1-level deep for a ‘2-dimensional’ hash):  
> …but there was no further explanation. Could somebody explain the  
> syntax in the first line? Is this overwriting new()?

No, this is calling new() with a block. In this case, for keys that are  
not yet in the hash, the specified block will be called with 2  
parameters: one is the hash object itself, and the second one a key  
being accessed. This gives you an opportunity to initialize a value at  
that key appropriately (in the example before, it is initialized to  
another hash - {} is a literal for a hash, equivalent to Hash.new).

Gennady.