Hash of arrays - appending to one of the arrays

Ruby
1

Hi Ive done a search on this and there have been quite a few posts which
have incresed my knowlege somewhat but still have a few questions.

Im creating a hash of arrays. The keys and values are added within a
loop. For each key there may be multiple values hence the need for an
array as a value. Values for a key are found over interations of a loop.
I therefore have to append a new found value to the existing values in
an array.

I tried doing this

something.each do |blah|
if (test something)
hash[key] = hash[key] + value
end
end

but the compliler complains about there being no method for nil class.

I read the basic way to do this would be

hash[key] = [] #create blank array
hash[key] = hash[key] + value

but as im in a loop that would require me to do a check to see if the
key exists or not. If im processing lots of informatino that would slow
me down a bit.

IS there a slicker way of doing this?

2

Hi –

On Tue, 25 Mar 2008, Adam A. wrote:

hash[key] = [] #create blank array
hash[key] = hash[key] + value

but as im in a loop that would require me to do a check to see if the
key exists or not. If im processing lots of informatino that would slow
me down a bit.

IS there a slicker way of doing this?

The most common idiom is:

(hash[key] ||= []) << value

You can also make a hash that has a default behavior, for non-existent
keys, of putting an array at that key:

irb(main):004:0> hash = Hash.new {|h,k| h[k] = [] }
=> {}
irb(main):005:0> hash[1] << 2
=> [2]
irb(main):006:0> hash
=> {1=>[2]}

David

3

The most common idiom is:

(hash[key] ||= []) << value

Thanks for the reply. Can I ask what these || mean…in this case do they
mean OR???

does anyone have any good links on using the use of arrays with hashes
in this manner?

4

On Tue, Mar 25, 2008 at 1:41 PM, Adam A. [email protected]
wrote:

The most common idiom is:

(hash[key] ||= []) << value

Thanks for the reply. Can I ask what these || mean…in this case do they
mean OR???

a ||= b means a = a || b

and it’s a common idiom to assign a value only when the lhs is nil (or
false).
A hash by default will return nil for non-existing keys so:

hash[key] ||= [] could be written as hash[key] = hash[key] || []

and means: if the key is not present in the hash, create that entry in
the hash with an empty array as the value.

does anyone have any good links on using the use of arrays with hashes
in this manner?

I don’t have a link, but if you are doing that in a lot of places,
remember you
can change the default behaviour of a hash to do the above for you:

hash = Hash.new {|h,k| h[k] = []}

from there on you can say just:

hash[key] << value

because the default value for an inexistent key will trigger the
creation of a new array assigned to that key.

Jesus.

5

Hi –

On Tue, 25 Mar 2008, Adam A. wrote:

The most common idiom is:

(hash[key] ||= []) << value

Thanks for the reply. Can I ask what these || mean…in this case do they
mean OR???

Yes. This:

a ||= b

is the same as:

a or a = b

I used to describe it like this:

a = a || b

but there are some edge cases where that doesn’t apply (having to do
with hash defaults, actually, but you don’t need to worry about it in
this case). Matz clarified it with the “or” version at RubyConf last
year.

does anyone have any good links on using the use of arrays with hashes
in this manner?

I don’t know – I just sort of use them :slight_smile:

David

6

Many thanks for that. Thats helped my code a lot. Cheers

7

Hi –

On Tue, 25 Mar 2008, Jesús Gabriel y Galán wrote:

a ||= b means a = a || b
Almost, but this:

a or a = b

covers the edge cases better.

David