Defining a Struct during initialize

Question

I have a class that reads a dynamic CSV file, and would like to create a struct based on the headers of the CSV.

It goes like this in pseudo-code:

class SomeClass
  def initialize(csvfile)
    @csv = CSV.open(csvfile, options.....)
    .....
    headers = @csv.headers  # => [:line, :error, :user, ........]   <==this is dynamic
    RECORD = Struct.new(headers)
    load_data
  end

  def load_data
    @records = []
    @csv.each do |r|
      @records << RECORD.new(r.fields)
    end
  end
end

The issue is that a constant can not (should not) be dynamically defined. So, what should be the proper way to address this?

Not a dup actually. The issue is with the proper use of struct and not with the constant. — Carlos Troncoso
– Carlos Troncoso, Commented Jul 18, 2014 at 2:16
"The issue is that a constant can not (should not) be dynamically defined. So, what should be the proper way to address this?" — Ajedi32
– Ajedi32, Commented Jul 18, 2014 at 13:16
Regardless of whether you are talking about a Struct, a Hash, or a String, you can't dynamically assign to a constant. The important part of this question asks how to address the fact that a constant can not be dynamically defined, which is why I see it as a duplicate. Well, that's my reasoning anyway... — Ajedi32
– Ajedi32, Commented Jul 18, 2014 at 13:21

sawa · Accepted Answer · 2014-07-17 09:52:23Z

3

You don't have to use a constant. Use an instance variable.

class SomeClass
  def initialize(csvfile)
    @csv = CSV.open(csvfile, options...)
    ...
    headers = @csv.headers
    @record = Struct.new(headers)
    load_data
  end
  def load_data
    @records = []
    @csv.each do |r|
      @records << @record.new(r.fields)
    end
  end
end

edited Jul 17, 2014 at 9:52

answered Jul 17, 2014 at 9:31

sawa

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1 Comment

Carlos Troncoso Over a year ago

OK, perfect, I've should have specified I knew that. The issue here is that I WANT it to be a constant, which will be initialized just once. Said that, what I would like is a constant that can be initialized dynamically upon initialization.

Community · Accepted Answer · 2017-05-23 10:26:14Z

@sawa's answer is correct. I will add something more here.

Constants are used to store values which DO NOT change. If you change a constant in your program it is probably a bug. Ruby is trying to be helpful and say "Hold on! I think something strange is going on here. You said you wanted this to be a constant but now it's being changed!"

In Ruby, you are never allowed to put a constant assignment, inside a method definitions due to the reason as I just said above. If you try to do constant assignment inside a method, you will get Syntax Error. As per your need, you can use any variables, but not constant variables, inside a method.

If you do

Y = 1
#then
Y = 2
# warning: already initialized constant Y
# warning: previous definition of Y was here

But if you try,

def meth arg
  C = arg
end
# then 
# SyntaxError: dynamic constant assignment
# C = arg
#   ^

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