In Ruby, can you perform string interpolation on data read from a file?

Question

In Ruby you can reference variables inside strings and they are interpolated at runtime.

For example if you declare a variable foo equals "Ted" and you declare a string "Hello, #{foo}" it interpolates to "Hello, Ted".

I've not been able to figure out how to perform the magic "#{}" interpolation on data read from a file.

In pseudo code it might look something like this:

interpolated_string = File.new('myfile.txt').read.interpolate

But that last interpolate method doesn't exist.

Answer 1

Instead of interpolating, you could use erb. This blog gives simple example of ERB usage,

require 'erb'
name = "Rasmus"
template_string = "My name is <%= name %>"
template = ERB.new template_string
puts template.result # prints "My name is Rasmus"

Kernel#eval could be used, too. But most of the time you want to use a simple template system like erb.

Answer 2

I think this might be the easiest and safest way to do what you want in Ruby 1.9.x (sprintf doesn't support reference by name in 1.8.x): use Kernel.sprintf feature of "reference by name". Example:

>> mystring = "There are %{thing1}s and %{thing2}s here."
 => "There are %{thing1}s and %{thing2}s here."

>> vars = {:thing1 => "trees", :thing2 => "houses"}
 => {:thing1=>"trees", :thing2=>"houses"}

>> mystring % vars
 => "There are trees and houses here." 

Answer 3

Well, I second stesch's answer of using erb in this situation. But you can use eval like this. If data.txt has contents:

he #{foo} he

Then you can load and interpolate like this:

str = File.read("data.txt")
foo = 3
result = eval("\"" + str + "\"")

And result will be:

"he 3 he"

Answer 4

You can read the file into a string using IO.read(filename), and then use the result as a format string (http://www.ruby-doc.org/core-2.0/String.html#method-i-25):

myfile.txt:

My name is %{firstname} %{lastname} and I am here to talk about %{subject} today.

fill_in_name.rb:

sentence = IO.read('myfile.txt') % {
  :firstname => 'Joe',
  :lastname => 'Schmoe',
  :subject => 'file interpolation'
}
puts sentence

result of running "ruby fill_in_name.rb" at the terminal:

My name is Joe Schmoe and I am here to talk about file interpolation today.

Answer 5

Ruby Facets provides a String#interpolate method:

Interpolate. Provides a means of extenally using Ruby string interpolation mechinism.

try = "hello"
str = "\#{try}!!!"
String.interpolate{ str }    #=> "hello!!!"

NOTE: The block neccessary in order to get then binding of the caller.

Answer 6

The 2 most obvious answers have already been given, but if they don't to it for some reason, there's the format operator:

>> x = 1
=> 1
>> File.read('temp') % ["#{x}", 'saddle']
=> "The number of horses is 1, where each horse has a saddle\n"

where instead of the #{} magic you have the older (but time-tested) %s magic ...

Answer 7

Using daniel-lucraft's answer as my base (as he seems to be the only one that answered the question) I decided to solve this problem in a robust manner. Below you will find the code for this solution.

# encoding: utf-8

class String
  INTERPOLATE_DELIMETER_LIST = [ '"', "'", "\x02", "\x03", "\x7F", '|', '+', '-' ]
  def interpolate(data = {})
    binding = Kernel.binding

    data.each do |k, v|
      binding.local_variable_set(k, v)
    end

    delemeter = nil
    INTERPOLATE_DELIMETER_LIST.each do |k|
      next if self.include? k
      delemeter = k
      break
    end
    raise ArgumentError, "String contains all the reserved characters" unless delemeter
    e = s = delemeter
    string = "%Q#{s}" + self + "#{e}"
    binding.eval string
  end
end

output =
begin
  File.read("data.txt").interpolate(foo: 3)
rescue NameError => error
  puts error
rescue ArgumentError => error
  puts error
end

p output

for the input

he #{foo} he

you get the output

 "he 3 he"

The input

"he #{bad} he\n"

will raise a NameError exception. And the input

"\"'\u0002\u0003\u007F|+-"

will raise and ArgumentError exception complaining that the input contained all available delimiter characters.

Answer 8

Might as well throw my own solution into the mix.

irb(main):001:0> str = '#{13*3} Music'
=> "\#{13*3} Music"
irb(main):002:0> str.gsub(/\#\{(.*?)\}/) { |match| eval($1) }
=> "39 Music"

The weakness is that the expression you want to evaluate may have further { } in it, so the regex should probably be improved.