String concatenation in Ruby

Question

I am looking for a more elegant way of concatenating strings in Ruby.

I have the following line:

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

Is there a nicer way of doing this?

And for that matter what is the difference between << and +?

Answer 1

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.

Answer 2

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"

Answer 3

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

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

Answer 4

from http://greyblake.com/blog/2012/09/02/ruby-perfomance-tricks/

Using << aka concat is far more efficient than +=, as the latter creates a temporal object and overrides the first object with the new object.

require 'benchmark'

N = 1000
BASIC_LENGTH = 10

5.times do |factor|
  length = BASIC_LENGTH * (10 ** factor)
  puts "_" * 60 + "\nLENGTH: #{length}"

  Benchmark.bm(10, '+= VS <<') do |x|
    concat_report = x.report("+=")  do
      str1 = ""
      str2 = "s" * length
      N.times { str1 += str2 }
    end

    modify_report = x.report("<<")  do
      str1 = "s"
      str2 = "s" * length
      N.times { str1 << str2 }
    end

    [concat_report / modify_report]
  end
end

output:

____________________________________________________________
LENGTH: 10
                 user     system      total        real
+=           0.000000   0.000000   0.000000 (  0.004671)
<<           0.000000   0.000000   0.000000 (  0.000176)
+= VS <<          NaN        NaN        NaN ( 26.508796)
____________________________________________________________
LENGTH: 100
                 user     system      total        real
+=           0.020000   0.000000   0.020000 (  0.022995)
<<           0.000000   0.000000   0.000000 (  0.000226)
+= VS <<          Inf        NaN        NaN (101.845829)
____________________________________________________________
LENGTH: 1000
                 user     system      total        real
+=           0.270000   0.120000   0.390000 (  0.390888)
<<           0.000000   0.000000   0.000000 (  0.001730)
+= VS <<          Inf        Inf        NaN (225.920077)
____________________________________________________________
LENGTH: 10000
                 user     system      total        real
+=           3.660000   1.570000   5.230000 (  5.233861)
<<           0.000000   0.010000   0.010000 (  0.015099)
+= VS <<          Inf 157.000000        NaN (346.629692)
____________________________________________________________
LENGTH: 100000
                 user     system      total        real
+=          31.270000  16.990000  48.260000 ( 48.328511)
<<           0.050000   0.050000   0.100000 (  0.105993)
+= VS <<   625.400000 339.800000        NaN (455.961373)

Answer 5

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

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

Answer 6

Here's another benchmark inspired by this gist. It compares concatenation (+), appending (<<) and interpolation (#{}) for dynamic and predefined strings.

require 'benchmark'

# we will need the CAPTION and FORMAT constants:
include Benchmark

count = 100_000


puts "Dynamic strings"

Benchmark.benchmark(CAPTION, 7, FORMAT) do |bm|
  bm.report("concat") { count.times { 11.to_s +  '/' +  12.to_s } }
  bm.report("append") { count.times { 11.to_s << '/' << 12.to_s } }
  bm.report("interp") { count.times { "#{11}/#{12}" } }
end


puts "\nPredefined strings"

s11 = "11"
s12 = "12"
Benchmark.benchmark(CAPTION, 7, FORMAT) do |bm|
  bm.report("concat") { count.times { s11 +  '/' +  s12 } }
  bm.report("append") { count.times { s11 << '/' << s12 } }
  bm.report("interp") { count.times { "#{s11}/#{s12}"   } }
end

output:

Dynamic strings
              user     system      total        real
concat    0.050000   0.000000   0.050000 (  0.047770)
append    0.040000   0.000000   0.040000 (  0.042724)
interp    0.050000   0.000000   0.050000 (  0.051736)

Predefined strings
              user     system      total        real
concat    0.030000   0.000000   0.030000 (  0.024888)
append    0.020000   0.000000   0.020000 (  0.023373)
interp    3.160000   0.160000   3.320000 (  3.311253)

Conclusion: interpolation in MRI is heavy.

Answer 7

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)

Answer 8

Let me show to you all my experience with that.

I had an query that returned 32k of records, for each record I called a method to format that database record into a formated string and than concatenate that into a String that at the end of all this process wil turn into a file in disk.

My problem was that by the record goes, around 24k, the process of concatenating the String turned on a pain.

I was doing that using the regular '+' operator.

When I changed to the '<<' was like magic. Was really fast.

So, I remembered my old times - sort of 1998 - when I was using Java and concatenating String using '+' and changed from String to StringBuffer (and now we, Java developer have the StringBuilder).

I believe that the process of + / << in Ruby world is the same as + / StringBuilder.append in the Java world.

The first reallocate the entire object in memory and the other just point to a new address.

Answer 9

Concatenation you say? How about #concat method then?

a = 'foo'
a.object_id #=> some number
a.concat 'bar' #=> foobar
a.object_id #=> same as before -- string a remains the same object

In all fairness, concat is aliased as <<.

Answer 10

Here are more ways to do this:

"String1" + "String2"

"#{String1} #{String2}"

String1<<String2

And so on ...

Answer 11

You can also use % as follows:

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

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

Answer 12

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"))

Answer 13

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.

Answer 14

You can concatenate in string definition directly:

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

Answer 15

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"

Answer 16

For Puppet:

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