String concatenation in Ruby

You can do that in several ways:

1. As you shown with << but that is not the usual way

2. With string interpolation

   source = "#{ROOT_DIR}/#{project}/App.config"
   

3. with +

   source = "#{ROOT_DIR}/" + project + "/App.config"
   

The second method seems to be more efficient in term of memory/speed from what I've seen (not measured though). All three methods will throw an uninitialized constant error when ROOT_DIR is nil.

When dealing with pathnames, you may want to use File.join to avoid messing up with pathname separator.

In the end, it is a matter of taste.

The + operator is the normal concatenation choice, and is probably the fastest way to concatenate strings.

The difference between + and << is that << changes the object on its left hand side, and + doesn't.

irb(main):001:0> s = 'a'
=> "a"
irb(main):002:0> s + 'b'
=> "ab"
irb(main):003:0> s
=> "a"
irb(main):004:0> s << 'b'
=> "ab"
irb(main):005:0> s
=> "ab"

If you are just concatenating paths you can use Ruby's own File.join method.

source = File.join(ROOT_DIR, project, 'App.config')

Since this is a path I'd probably use array and join:

source = [ROOT_DIR, project, 'App.config'] * '/'

I'd prefer using Pathname:

require 'pathname' # pathname is in stdlib
Pathname(ROOT_DIR) + project + 'App.config'

about << and + from ruby docs:

+: Returns a new String containing other_str concatenated to str

<<: Concatenates the given object to str. If the object is a Fixnum between 0 and 255, it is converted to a character before concatenation.

so difference is in what becomes to first operand (<< makes changes in place, + returns new string so it is memory heavier) and what will be if first operand is Fixnum (<< will add as if it was character with code equal to that number, + will raise error)

You may use + or << operator, but in ruby .concat function is the most preferable one, as it is much faster than other operators. You can use it like.

source = "#{ROOT_DIR}/".concat(project.concat("/App.config"))

You can also use % as follows:

source = "#{ROOT_DIR}/%s/App.config" % project

This approach works with ' (single) quotation mark as well.

You can concatenate in string definition directly:

nombre_apellido = "#{customer['first_name']} #{customer['last_name']} #{order_id}"

Situation matters, for example:

# this will not work
output = ''

Users.all.each do |user|
  output + "#{user.email}\n"
end
# the output will be ''
puts output

# this will do the job
output = ''

Users.all.each do |user|
  output << "#{user.email}\n"
end
# will get the desired output
puts output

In the first example, concatenating with + operator will not update the output object,however, in the second example, the << operator will update the output object with each iteration. So, for the above type of situation, << is better.

For your particular case you could also use Array#join when constructing file path type of string:

string = [ROOT_DIR, project, 'App.config'].join('/')]

This has a pleasant side effect of automatically converting different types to string:

['foo', :bar, 1].join('/')
=>"foo/bar/1"

For Puppet:

$username = 'lala'
notify { "Hello ${username.capitalize}":
    withpath => false,
}