Ruby - Initializing and updating objects

Question

This is confusing me:

class Person 
    attr_accessor :id, :name

    def initialize(init)
        init.each_pair do |key, val|
            instance_variable_set('@' + key.to_s, val)
        end
    end
end

@adam = Person.new(:id => 1, :name => "Adam")
@eve = Person.new(:id => 2)

@people = [ @adam, @eve, nil ]

print @people

@people.map! do |person|
    person ||= Person.new(:id => 3, :name => "Some default")

    if person.name.nil?
        person.name = "Eve"
    end
end 

print @people 

# outputs 
# [#<Person:0x007ff184303900 @id=1, @name="Adam">, #<Person:0x007ff184303770 @id=2>, nil]
# [nil, "Some default", "Some default"]

I'd like to fully populate @people so that I end up with something along the lines of:

[#<Person:0x007ff184303900 @id=1, @name="Adam">, #<Person:0x007ff184303770 @id=2,  @name="Eve">, #<Person:0x007ff184301111 @id=3,  @name="Some default">]

What am I doing wrong?

How on earth were you hoping to set @eve's name to "Eve"? I don't see that. — Sergio Tulentsev
– Sergio Tulentsev, Commented May 29, 2013 at 15:32
.map! will replace each element of your array with whatever the method returns (the last executed statement). Try .each instead, or do a "return person" at the end of your .map! method — dmoss18
– dmoss18, Commented May 29, 2013 at 15:35

Sergio Tulentsev · Accepted Answer · 2013-05-29 15:37:13Z

2

What map does is it calls the block you supplied, passes an element of a collection to it and grabs value returned from the block. In absence of explicit return keyword, value of a block is the value of last evaluated expression. That would be the if expression in your example. It can return either nil or a name. And those values are mapped to the original collection. Fix is easy: just make sure that you return a Person from the block (by evaluating it last, for example).

@people.map! do |person|
  person ||= Person.new(:id => 3)

  if person.name.nil?
    person.name = "Some default"
  end
  person
end

print @people

# >> [#<Person:0x007fb5f288b288 @id=1, @name="Adam">, #<Person:0x007fb5f288b0f8 @id=2, @name="Some default">, #<Person:0x007fb5f288ae28 @id=3, @name="Some default">]

answered May 29, 2013 at 15:37

Sergio Tulentsev

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4 Comments

Snowcrash Over a year ago

+1 That works for me. Really annoying when you post a complete code sample and then the only thing that needed changing was a return!

Sergio Tulentsev Over a year ago

Your code could benefit from little improvements, though. But you should pick up the feeling for idiomaticity as you gain more experience.

Snowcrash Over a year ago

pretty neat. I assume send("#{k}=", v) is operating on 'self' which would be 'init', right?

Sergio Tulentsev Over a year ago

@SnowCrash: No, it would a instance of Person. Loops like each don't introduce new scope.

dmoss18 · Accepted Answer · 2013-05-29 15:37:02Z

1

.map! replaces each element with whatever is returned from the .map! method (which in this case is person.name). Add a "return person" at the end of your .map! method. Or do a .each instead of .map

answered May 29, 2013 at 15:37

dmoss18

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