Weird behaviour with hashes in Ruby

Question

I have a method that tries to parse a query into a a hash.

CONTACT_SEARCH_FIELDS = ['LastUpdate','Name','RecAdd','PhoneNumber','Tag_Group','FirstName','LastName','FamilyName','FamilyHead','ClientStatus','ContactType','ClientSource','TaxId']
CONTACT_SEARCH_OPERANDS = ['=','>','<','!=','Like','BeginsWith','IsEmpty']

def search (query, page = 1)
    body = [{}]*query.length
    query.each_with_index do |expr, i|
        body[i]["Field"] = CONTACT_SEARCH_FIELDS.index expr[0]
        body[i]["Operand"] = CONTACT_SEARCH_OPERANDS.index expr[1]
        body[i]["Value"] = expr[2]
    end
    return body
end

The method is called like this

search([["FirstName", "=", "John"], ["LastName", "=", "Smith"]])

The problem is that running this gives a very weird output.

search([["FirstName", "=", "John"], ["LastName", "=", "Smith"]])
=> [{"Operand"=>0, "Value"=>"Smith", "Field"=>6}, {"Operand"=>0, "Value"=>"Smith", "Field"=>6}]

I did some debugging and the problem is that all the hashes in the array are get set on every iteration.

I dont understand what is the reason behind this. I would also welcome any shorter or better versions of this code.

Arup Rakshit · Accepted Answer · 2014-02-25 08:26:25Z

2

Change the line

body = [{}]*query.length

The above means, you are creating an Array, whose elements are same Hash objects.

Example :

a = [{}]*3 # => [{}, {}, {}]
a.map(&:object_id) # => [18499356, 18499356, 18499356]
a[0]["a"] = 2
a # => [{"a"=>2}, {"a"=>2}, {"a"=>2}]

to

body = Array.new(query.length) { {} }

But the above means, you are creating an Array, whose elements are different Hash objects.

Example :

a = Array.new(3) { {} } # => [{}, {}, {}]
a.map(&:object_id) # => [17643864, 17643852, 17643840]
a[0]["a"] = 2
a # => [{"a"=>2}, {}, {}]

edited Feb 25, 2014 at 8:26

answered Feb 25, 2014 at 8:20

Arup Rakshit

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1 Comment

Zero Fiber Over a year ago

Does the first definition create n hashes which point to the same address in memory?

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