gistfile1.rb

my_array = %w[test one two three]

# Inject takes the value of the block and passes it along
# This often causes un-intended errors
my_array.inject({}) do |transformed, word|
  puts transformed
  transformed[word] = word.capitalize
end
# Output:
# {}
# Test
# IndexError: string not matched
# from (pry):6:in `[]='

# What was really wanted was:
my_array.inject({}) do |transformed, word|
  puts transformed
  transformed[word] = word.capitalize
  transformed
end
# Output:
# {}
# {"test"=>"Test"}
# {"test"=>"Test", "one"=>"One"}
# {"test"=>"Test", "one"=>"One", "two"=>"Two"}
=> {"test"=>"Test", "one"=>"One", "two"=>"Two", "three"=>"Three"}


# On the other hand, each_with_object does exactly as you expect
# It ignores the return value of the block and only passes the
# initial object along

my_array.each_with_object({}) do |word, transformed|
  puts transformed
  transformed[word] = word.capitalize
  "this is ignored"
end
# Output:
# {}
# {"test"=>"Test"}
# {"test"=>"Test", "one"=>"One"}
# {"test"=>"Test", "one"=>"One", "two"=>"Two"}
=> {"test"=>"Test", "one"=>"One", "two"=>"Two", "three"=>"Three"}