faster_ruby.rb

# from http://youtu.be/fGFM_UrSp70

def slow(&block)
  block.call
end

def fast
  yield         # 5x faster than passing a block as an argument
end

#####################

(1..100).map { |i| i.to_s } # slow
(1..100).map(&:to_s)        # faster by 20%

#####################

enum.map do
  # code
end.flatten(1) # 2 passes through the enum - slow

enum.flat_map do
  # code
end            # single pass through the enum - 4.5x faster

#####################

enum.inject({}) do |h, e|
  h.merge(e => e)         # hash is duplicated and modified, new (increasingly larger) object allocated each time
end

enum.inject({}) do |h, e|
  h.merge!(e => e)         # merge! modifies the hash in place, no new object allocation - 3x faster
end

enum.inject({}) do |h, e|
  h[e] = e                 # new { e => e } hash is not allocated each time for merging - 2x faster still
end

#####################

# Hash#fetch retrieves a value from a hash. If the key is not found in the hash,
# it can return a default value, which can be provided as a second argument or
# as a block

{ :foo => 'bar' }.fetch(:bar, (0..9).to_a)    # slow - the array of 10 Fixnums is allocated on every call

{ :foo => 'bar' }.fetch(:bar) { (0..9).to_a } # the block is not evaluated unless the key is not found
                                              # - 2x faster when the key is present in the hash

{ :foo => 'bar' }[:foo] || (0..9).to_a        # may be faster still; benchmark

# Update: in my hands, method #3 is 2x faster than method #2, which is 6x faster than method #1
# When the hash key is missing, #1 runs as before, #3 runs as #1, and #2 runs ~10% slower

#####################

'http://example.com'.gsub('http://', 'https://') # even if a single substitution is desired, scans the entire string
'http://example.com'.sub('http://', 'https://')  # replaces and stops; more intention-revealing if a single substitution is needed
                                                 # - 50% faster even on short target string

'slug from name'.tr(' ', '_')                    # 5x faster than same with `gsub`

#####################

a, b = 1, 2

a = 1
b = 2   # 40% faster and far more readable

#####################

begin
  foo
rescue NoMethodError
  bar
end

if respond_to?(:foo)
  foo
else
  bar
end                     # 10x faster

#####################

array.each_with_index do |element, index|
  # do something with element & index
end

index = 0
while index < array.size do
  # do something with array[index] & index
  index += 1
end                                         # 80% faster, at the cost of readability & Ruby-ness