How do I check if a variable is defined in rails?

Question

<% if dashboard_pane_counter.remainder(3) == 0 %>
  do something
<% end>

If dasboard_pane_counter wasn't defined, how can I get this to evaluate to false rather than throw an exception?

Matt · Accepted Answer · 2011-10-18T11:06:48.527

52

<% if defined?(:dashboard_pane_counter) && dashboard_pane_counter.remainder(3) == 0  %>
  # do_something here, this assumes that dashboard_pane_counter is defined, but not nil
<% end %>

edited Oct 18 '11 at 11:06

answered Oct 18 '11 at 11:01

Matt

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symbol did not work for me. so i had to check this way `if defined?(variable_name)` – HungryCoder Jun 07 '12 at 15:30

katzmopolitan · Answer 2 · 2015-11-27T18:48:23.870

5

local_assigns can be used for that, since this question is from a few years ago, I verified that it exists in previous versions of rails

<% if local_assigns[:dashboard_pane_counter] 
                 && dashboard_pane_counter.remainder(3) == 0%>
<% end %>

It's in the notes here

http://apidock.com/rails/ActionController/Base/render

edited Nov 27 '15 at 18:48

answered Feb 20 '14 at 19:30

katzmopolitan

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Yule · Answer 3 · 2014-05-12T10:14:46.987

5

When using rails and instance variables, nil has a try method defined, so you can do:

<% if @dashboard_pane_counter.try(:remainder(3)) == 0  %>
   #do something
<% end %>

so if the instance variable is not defined, try(:anything) will return nil and therefore evaluate to false. And nil == 0 is false

edited May 12 '14 at 10:14

answered Oct 18 '11 at 11:11

Yule

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1

This will still cause an error if the variable hasn't been defined, so doesn't answer the question. – michaeltwofish May 09 '14 at 02:33
No it won't, rails has 'magic built in': `1.9.3p392 :005 > @hh => nil 1.9.3p392 :006 > @hh.try(:anything) => nil ` – Yule May 09 '14 at 09:57
So for instance variables, nil has a 'try' method – Yule May 09 '14 at 09:59
Ah, you're right, my mistake. If you edit your answer to that clear I can remove my down vote. – michaeltwofish May 11 '14 at 23:49

score 0 · Answer 4 · answered Oct 05 '18 at 01:21

Posting this answer for beginner coders like myself. This question can be answered simply using two steps (or one if using &&). It is a longer and less pretty answer but helps new coders to understand what they are doing and uses a very simple technique that is not present in any of the other answers yet. The trick is to use an instance (@) variable, it will not work with a local variable:

if @foo
  "bar"
end

If @foo is defined it will be return "bar", otherwise not (with no error). Therefore in two steps:

if @dashboard_pane_counter
  if @dashboard_plane_counter.remainder(3) == 0
    do something
  end
end

score -1 · Answer 5 · answered Oct 18 '11 at 13:50

-1

Another way, with a neat gem, is 'andand.'

https://github.com/raganwald/andand

answered Oct 18 '11 at 13:50

andrewpthorp

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score -3 · Answer 6 · answered Oct 18 '11 at 11:12

-3

Insted of

if !var.nil?

I would use

unless var.nil?

Thats much better ruby code!

answered Oct 18 '11 at 11:12

davidb

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1

4

this only applies if the variable was defined previous to this code. Try if !adallajglaksdkfaj.nil? on the first line of a "function". kablammo :) – baash05 Nov 14 '12 at 00:06

How do I check if a variable is defined in rails?

6 Answers6