Hash#inject argument syntax

Ruby
1

The code below works but I do not understand the “(key, value)”
parenthesis syntax. I have not seen it before. Could someone
explain?

–Brian

(from Rails’ active_support)
class Hash
def symbolize_keys
inject({}) do |options, (key, value)|
options[key.to_sym] = value
options
end
end
end
(above code from Rails’ active_support)

2

Brian B. wrote:

The code below works but I do not understand the “(key, value)”
parenthesis syntax. I have not seen it before. Could someone
explain?

–Brian

(from Rails’ active_support)
class Hash
def symbolize_keys
inject({}) do |options, (key, value)|

Hash#each, as you may know, actually produces an Array
of Arrays (key-value pairs). Here what is being passed
is actually one of those pairs–the parenthesised syntax
uses Ruby’s assignment rules and splats the Array to
two distinct variables. Consider it the same as:

key, value = [‘key’, ‘value’]

options[key.to_sym] = value
options

end
end
end
(above code from Rails’ active_support)

3

On 30.08.2006 02:23, Eero S. wrote:

inject({}) do |options, (key, value)|

Hash#each, as you may know, actually produces an Array
of Arrays (key-value pairs). Here what is being passed
is actually one of those pairs–the parenthesised syntax
uses Ruby’s assignment rules and splats the Array to
two distinct variables. Consider it the same as:

key, value = [‘key’, ‘value’]

The mechanism at work here is a more general pattern matching which also
is done for regular assignments:

(a,(b,c),d,(e,(f,g))) = 1,[2,3],4,[5,[6,7]]
=> [1, [2, 3], 4, [5, [6, 7]]]

a
=> 1

b
=> 2

c
=> 3

d
=> 4

e
=> 5

f
=> 6

g
=> 7

Works also with the star operator:

(a,(b,c),d,(e,(f,*g))) = 1,[2,3],4,[5,[6,7,8]]
=> [1, [2, 3], 4, [5, [6, 7, 8]]]

g
=> [7, 8]

Kind regards

robert