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.

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

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

The 2 most obvious answers have already been give, 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 ...