Ruby offers a lot of goodies and one of them is inject . Jay Fields wrote a good article about various ways in which you can use inject . As per the documentation if you do not specity an input value for inject then inject will treat the first element as the first value and will start with second item.

Code will make it easier to understand.

foo = %w(a b c)
output = foo.inject('') do |sum, element|
  puts "currently processing element #{element}"
  sum = sum + element
end
puts output

You will get output like this.

currently processing element a
currently processing element b
currently processing element c
abc

Now try change the code so that the empty string is not being passed to inject call.

foo = %w(a b c)
output = foo.inject do |sum, element|
  puts "currently processing element #{element}"
  sum = sum + element
end
puts output

This time the output is

currently processing element b
currently processing element c
abc

Notice that even though the final result is same, no iteration was done for the element ‘a’.

Now try running the same code with array having only one element.

foo = %w(a)
output = foo.inject do |sum, element|
  puts "currently processing element #{element}"
  sum = sum + element
end
puts output

This time the output is

a

Notice that this time code inside the loop was not executed.

Today I refactored some code where I converted a lot of each methods into inject methods. Some of the tests started failing. That was because inside each method a lot of different concerns were also being handled. When I refactored the code to use inject then code inside iteration will not get executed if the array length is 1.

I had to refactor the code further to not to handle different responsibilities in one single loop.