String#concat, Array#concat & String#prepend Ruby 2.4

In Ruby, we use #concat to append a string to another string or an element to the array. We can also use #prepend to add a string at the beginning of a string.

Ruby 2.3

String#concat and Array#concat

string = "Good"
string.concat(" morning")
#=> "Good morning"

array = ['a', 'b', 'c']
array.concat(['d'])
#=> ["a", "b", "c", "d"]

String#prepend

string = "Morning"
string.prepend("Good ")
#=> "Good morning"

Before Ruby 2.4, we could pass only one argument to these methods. So we could not add multiple items in one shot.

string = "Good"
string.concat(" morning", " to", " you")
#=> ArgumentError: wrong number of arguments (given 3, expected 1)

Changes with Ruby 2.4

In Ruby 2.4, we can pass multiple arguments and Ruby processes each argument one by one.

String#concat and Array#concat

string = "Good"
string.concat(" morning", " to", " you")
#=> "Good morning to you"

array = ['a', 'b']
array.concat(['c'], ['d'])
#=> ["a", "b", "c", "d"]

String#prepend

string = "you"
string.prepend("Good ", "morning ", "to ")
#=> "Good morning to you"

These methods work even when no argument is passed unlike in previous versions of Ruby.

"Good".concat
#=> "Good"

Difference between concat and shovel << operator

Though shovel << operator can be used interchangeably with concat when we are calling it once, there is a difference in the behavior when calling it multiple times.

str = "Ruby"
str << str
str
#=> "RubyRuby"

str = "Ruby"
str.concat str
str
#=> "RubyRuby"

str = "Ruby"
str << str << str
#=> "RubyRubyRubyRuby"

str = "Ruby"
str.concat str, str
str
#=> "RubyRubyRuby"

So concat behaves as appending present content to the caller twice. Whereas calling << twice is just sequence of binary operations. So the argument for the second call is output of the first << operation.