What does %i or %I do in ruby?

It can be hard to find the official Ruby documentation (it's here). At the time of writing the current version is 2.5.1, and the documentation for the %i construct is found in the documentation for Ruby's literals.

There are some surprising (to me at least!) variants of Ruby's % construct. There are the often used %i %q %r %s %w %x forms, each with an uppercase version to enable interpolation. (see the Ruby literals docs for explanations.

But you can use many types of delimiters, not just []. You can use any kind of bracket () {} [] <>, and you can use (quoting from the ruby docs) "most other non-alphanumeric characters for percent string delimiters such as “%”, “|”, “^”, etc."

So %i% bish bash bosh % works the same as %i[bish bash bosh]

%i[ ] # Non-interpolated Array of symbols, separated by whitespace
%I[ ] # Interpolated Array of symbols, separated by whitespace

The second link from my search results http://ruby.zigzo.com/2014/08/21/rubys-notation/

Examples in IRB:

%i[ test ]
# => [:test]
str = "other"
%I[ test_#{str} ]
# => [:test_other] 

It's like %w and %W which work similar to ' and ":

x = :test

# %w won't interpolate #{...} style strings, leaving as literal
%w[ #{x} x ]
# => ["\#{x}", "x"]

# %w will interpolate #{...} style strings, converting to string
%W[ #{x} x ]
# => [ "test", "x"]

Now the same thing with %i and %I:

# %i won't interpolate #{...} style strings, leaving as literal, symbolized
%i[ #{x} x ]
# => [:"\#{x}", :x ]

# %w will interpolate #{...} style strings, converting to symbols
%I[ #{x} x ]
# => [ :test, :x ]