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

This blog is part of our Ruby 2.4 series.

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

1
2string = "Good"
3string.concat(" morning")
4#=> "Good morning"
5
6array = ['a', 'b', 'c']
7array.concat(['d'])
8#=> ["a", "b", "c", "d"]
9

String#prepend

1
2string = "Morning"
3string.prepend("Good ")
4#=> "Good morning"
5

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

1
2string = "Good"
3string.concat(" morning", " to", " you")
4#=> ArgumentError: wrong number of arguments (given 3, expected 1)
5

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

1
2string = "Good"
3string.concat(" morning", " to", " you")
4#=> "Good morning to you"
5
6array = ['a', 'b']
7array.concat(['c'], ['d'])
8#=> ["a", "b", "c", "d"]
9

String#prepend

1
2string = "you"
3string.prepend("Good ", "morning ", "to ")
4#=> "Good morning to you"
5

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

1
2"Good".concat
3#=> "Good"
4

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.

1
2str = "Ruby"
3str << str
4str
5#=> "RubyRuby"
6
7str = "Ruby"
8str.concat str
9str
10#=> "RubyRuby"
11
12str = "Ruby"
13str << str << str
14#=> "RubyRubyRubyRuby"
15
16str = "Ruby"
17str.concat str, str
18str
19#=> "RubyRubyRuby"
20

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.

Abhishek Jain in Ruby 2.4
October 28, 2016
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