Question

I’m trying to create a simple hash to look up for corresponding strings to some symbols (:student_number, :first_name, …). I’m having issue retrieving data from this function though. Here’s the function

Snippet A

  def get_nice_column_name(col_symbol)
    column_names = { 
      :first_name => "Student's First Name",
      :last_name => "Student's Last Name", 
      :email => "Student's Email", 
      :given_name => "Student's Given Name"
    }
    return column_names[col_symbol]
  end

Here’s how I use it, but not working:

Snippet B

col_titles = []
params = {:first_name => 'true', 
          :last_name => 'true', 
          :email => 'true', 
          :given_name => 'true' }

params.each do |key, value|
  if ( value == 'true')
    col_titles << get_nice_column_name(key)
  end
end

When I look into col_titles, I expect [“Student’s First Name”, “Student’s Last Name”], but I actually don’t get anything, just [] empty array.

I thought that was weird so I tried printing out the object_id of the symbols (col_symbol in snippet A) and the symbols in the hash column_names, I get different object_ids. I’m wondering why they are different (they both present the same symbols). If I add this to the function get_nice_column_name in Snippet A:

    puts "col_symbol is " + col_symbol.object_id.to_s + ", while :first_name is " + (:first_name).object_id.to_s
    puts "col_symbol is " + col_symbol.object_id.to_s + ", while :last_name is " + (:last_name).object_id.to_s
    puts "col_symbol is " + col_symbol.object_id.to_s + ", while :email is " + (:email).object_id.to_s
    puts "col_symbol is " + col_symbol.object_id.to_s + ", while :given_name is " + (:given_name).object_id.to_s

I would get this in the console

col_symbol is 98351040, while :first_name is 1221688
col_symbol is 98351040, while :last_name is 580888
col_symbol is 98351040, while :email is 168888
col_symbol is 98351040, while :given_name is 1290648

983541040 doesn’t really match any of the {1221688, 580888, 168888, 1290648}. Is this why my get_nice_column_name is useless ? Because the symbols are different under the hood?

Thanks for your help guys !

Regards

Answer 1

If you are using Ruby 1.9 this is a bit more concise way to do it.

column_names = { 
  :first_name => "Student's First Name",
  :last_name  => "Student's Last Name",
  :email      => "Student's Email",
  :given_name => "Student's Given Name"
}

params = {
  :first_name => 'FirstName',
  :last_name  => 'LastName',
  :email      => 'Email',
  :given_name => 'GivenName'
}
col_titles = []

col_titles = column_names.values
data = params.values_at(*column_names.keys)

col_titles # => ["Student's First Name", "Student's Last Name", "Student's Email", "Student's Given Name"]
data # => ["FirstName", "LastName", "Email", "GivenName"]

This takes advantage of Ruby 1.9’s new Hash behavior, where Ruby remembers the order of insertion and will honor that order when retrieving the keys and values. You can do something similar with 1.8, but you’d have to define the order of the columns in an array, then use that to pull the column headings and values of the data instead of relying on column_names to set the order.