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

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 2

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 3

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.