Ruby templating: is it possible that only pass some values to variables in template?

Question

I'm learning ruby template and looking at the example provided by ruby documentation.

require 'erb'
x = 42
template = ERB.new <<-EOF
   The value of x is: <%= x %>
EOF
puts template.result(binding)

When I change the example by doing something like this:

require 'erb'
x = 42
template = ERB.new <<-EOF
   The value of x is: <%= x %>
   The value of y is <%= y %>
EOF
puts template.result(binding)

I was hoping the template will become something like this:

The value of x is: 42
The value of y is <%= y %>

It gives me the error:

Error: undefined local variable or method `y' for main:Object (NameError)

Looks like we need to pass all the values for all the variable substitutions in the template.

Question: I'm just wondering if it is possible that we can have two variable substitutions in the template but only pass one value into the template when binding the data?

@JagdeepSingh, what I was trying to say is that I have to always define the variables(x and y in this case) even I want the template result in "The value of x is: 42 The value of y is <%= y %>" ? — Francis Zhu
– Francis Zhu, Commented Jul 24, 2018 at 12:32

Jagdeep Singh · Accepted Answer · 2018-07-24 12:51:17Z

2

You can use double opening %%s to prevent it from being evaluated in erb.

This solution might look ugly but it returns exactly what you want (if it is what you want):

require 'erb'

template = ERB.new <<-EOF
  The value of x is: <% if defined?(x) %><%= x %><% else %><%%= x %><% end %>
  The value of y is: <% if defined?(y) %><%= y %><% else %><%%= y %><% end %>
EOF
 => #<ERB:0x00007feeec80c428 @safe_level=nil, @src="#coding:UTF-8\n_erbout = +''; _erbout.<<(-\"  The value of x is: \");  if defined?(x) ; _erbout.<<(( x ).to_s);  else ; _erbout.<<(-\"<%= x %>\");  end ; _erbout.<<(-\"\\n  The value of y is: \"\n);  if defined?(y) ; _erbout.<<(( y ).to_s);  else ; _erbout.<<(-\"<%= y %>\");  end ; _erbout.<<(-\"\\n\"\n); _erbout", @encoding=#<Encoding:UTF-8>, @frozen_string=nil, @filename=nil, @lineno=0> 

puts template.result(binding)
  The value of x is: <%= x %>
  The value of y is: <%= y %>
 => nil 

x = 20

puts template.result(binding)
  The value of x is: 20
  The value of y is: <%= y %>
 => nil 

y= 50

puts template.result(binding)
  The value of x is: 20
  The value of y is: 50
 => nil

I assume you can do the formatting.

answered Jul 24, 2018 at 12:51

Jagdeep Singh

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2 Comments

Sergio Tulentsev Over a year ago

Will behave as specified, but the implementation is... wordy :)

Jagdeep Singh Over a year ago

Can't help it. :D

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