Performance

Performance/Caller

Enabled by default Supports autocorrection
Enabled No

This cop identifies places where caller[n] can be replaced by caller(n..n).first.

Examples

# bad
caller[1]
caller.first
caller_locations[1]
caller_locations.first

# good
caller(2..2).first
caller(1..1).first
caller_locations(2..2).first
caller_locations(1..1).first

Performance/CaseWhenSplat

Enabled by default Supports autocorrection
Enabled Yes

Place when conditions that use splat at the end of the list of when branches.

Ruby has to allocate memory for the splat expansion every time that the case when statement is run. Since Ruby does not support fall through inside of case when, like some other languages do, the order of the when branches does not matter. By placing any splat expansions at the end of the list of when branches we will reduce the number of times that memory has to be allocated for the expansion.

This is not a guaranteed performance improvement. If the data being processed by the case condition is normalized in a manner that favors hitting a condition in the splat expansion, it is possible that moving the splat condition to the end will use more memory, and run slightly slower.

Examples

# bad
case foo
when *condition
  bar
when baz
  foobar
end

case foo
when *[1, 2, 3, 4]
  bar
when 5
  baz
end

# good
case foo
when baz
  foobar
when *condition
  bar
end

case foo
when 1, 2, 3, 4
  bar
when 5
  baz
end

Performance/Casecmp

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where a case-insensitive string comparison can better be implemented using casecmp.

Examples

# bad
str.downcase == 'abc'
str.upcase.eql? 'ABC'
'abc' == str.downcase
'ABC'.eql? str.upcase
str.downcase == str.downcase

# good
str.casecmp('ABC').zero?
'abc'.casecmp(str).zero?

References

Performance/CompareWithBlock

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where sort { |a, b| a.foo <=> b.foo } can be replaced by sort_by(&:foo). This cop also checks max and min methods.

Examples

# bad
array.sort { |a, b| a.foo <=> b.foo }
array.max { |a, b| a.foo <=> b.foo }
array.min { |a, b| a.foo <=> b.foo }
array.sort { |a, b| a[:foo] <=> b[:foo] }

# good
array.sort_by(&:foo)
array.sort_by { |v| v.foo }
array.sort_by do |var|
  var.foo
end
array.max_by(&:foo)
array.min_by(&:foo)
array.sort_by { |a| a[:foo] }

Performance/Count

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of count on an Enumerable that follow calls to select or reject. Querying logic can instead be passed to the count call.

ActiveRecord compatibility: ActiveRecord will ignore the block that is passed to count. Other methods, such as select, will convert the association to an array and then run the block on the array. A simple work around to make count work with a block is to call to_a.count {...}.

Example: Model.where(id: [1, 2, 3].select { |m| m.method == true }.size

becomes:

Model.where(id: [1, 2, 3]).to_a.count { |m| m.method == true }

Examples

# bad
[1, 2, 3].select { |e| e > 2 }.size
[1, 2, 3].reject { |e| e > 2 }.size
[1, 2, 3].select { |e| e > 2 }.length
[1, 2, 3].reject { |e| e > 2 }.length
[1, 2, 3].select { |e| e > 2 }.count { |e| e.odd? }
[1, 2, 3].reject { |e| e > 2 }.count { |e| e.even? }
array.select(&:value).count

# good
[1, 2, 3].count { |e| e > 2 }
[1, 2, 3].count { |e| e < 2 }
[1, 2, 3].count { |e| e > 2 && e.odd? }
[1, 2, 3].count { |e| e < 2 && e.even? }
Model.select('field AS field_one').count
Model.select(:value).count

Configurable attributes

Name Default value Configurable values
SafeMode true Boolean

Performance/Detect

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of select.first, select.last, find_all.first, and find_all.last and change them to use detect instead.

ActiveRecord compatibility: ActiveRecord does not implement a detect method and find has its own meaning. Correcting ActiveRecord methods with this cop should be considered unsafe.

Examples

# bad
[].select { |item| true }.first
[].select { |item| true }.last
[].find_all { |item| true }.first
[].find_all { |item| true }.last

# good
[].detect { |item| true }
[].reverse.detect { |item| true }

Configurable attributes

Name Default value Configurable values
SafeMode true Boolean

References

Performance/DoubleStartEndWith

Enabled by default Supports autocorrection
Enabled Yes

This cop checks for double #start_with? or #end_with? calls separated by ||. In some cases such calls can be replaced with an single #start_with?/#end_with? call.

Examples

# bad
str.start_with?("a") || str.start_with?(Some::CONST)
str.start_with?("a", "b") || str.start_with?("c")
str.end_with?(var1) || str.end_with?(var2)

# good
str.start_with?("a", Some::CONST)
str.start_with?("a", "b", "c")
str.end_with?(var1, var2)

Configurable attributes

Name Default value Configurable values
IncludeActiveSupportAliases false Boolean

Performance/EndWith

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies unnecessary use of a regex where String#end_with? would suffice.

Examples

# bad
'abc' =~ /bc\Z/
'abc'.match(/bc\Z/)

# good
'abc'.end_with?('bc')

Configurable attributes

Name Default value Configurable values
AutoCorrect false Boolean

References

Performance/FixedSize

Enabled by default Supports autocorrection
Enabled No

Do not compute the size of statically sized objects.

Performance/FlatMap

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of

Examples

# bad
[1, 2, 3, 4].map { |e| [e, e] }.flatten(1)
[1, 2, 3, 4].collect { |e| [e, e] }.flatten(1)

# good
[1, 2, 3, 4].flat_map { |e| [e, e] }
[1, 2, 3, 4].map { |e| [e, e] }.flatten
[1, 2, 3, 4].collect { |e| [e, e] }.flatten

Configurable attributes

Name Default value Configurable values
EnabledForFlattenWithoutParams false Boolean

References

Performance/HashEachMethods

Enabled by default Supports autocorrection
Enabled Yes

This cop checks for uses of each_key and each_value Hash methods.

Note: If you have an array of two-element arrays, you can put parentheses around the block arguments to indicate that you're not working with a hash, and suppress RuboCop offenses.

Examples

# bad
hash.keys.each { |k| p k }
hash.values.each { |v| p v }
hash.each { |k, _v| p k }
hash.each { |_k, v| p v }

# good
hash.each_key { |k| p k }
hash.each_value { |v| p v }

Configurable attributes

Name Default value Configurable values
AutoCorrect false Boolean

References

Performance/LstripRstrip

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where lstrip.rstrip can be replaced by strip.

Examples

# bad
'abc'.lstrip.rstrip
'abc'.rstrip.lstrip

# good
'abc'.strip

Performance/RangeInclude

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies uses of Range#include?, which iterates over each item in a Range to see if a specified item is there. In contrast, Range#cover? simply compares the target item with the beginning and end points of the Range. In a great majority of cases, this is what is wanted.

Here is an example of a case where Range#cover? may not provide the desired result:

('a'..'z').cover?('yellow') # => true

References

Performance/RedundantBlockCall

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies the use of a &block parameter and block.call where yield would do just as well.

Examples

# bad
def method(&block)
  block.call
end
def another(&func)
  func.call 1, 2, 3
end

# good
def method
  yield
end
def another
  yield 1, 2, 3
end

References

Performance/RedundantMatch

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies the use of Regexp#match or String#match, which returns #<MatchData>/nil. The return value of =~ is an integral index/nil and is more performant.

Examples

# bad
do_something if str.match(/regex/)
while regex.match('str')
  do_something
end

# good
method(str =~ /regex/)
return value unless regex =~ 'str'

Performance/RedundantMerge

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where Hash#merge! can be replaced by Hash#[]=.

Examples

hash.merge!(a: 1)
hash.merge!({'key' => 'value'})
hash.merge!(a: 1, b: 2)

Configurable attributes

Name Default value Configurable values
MaxKeyValuePairs 2 Integer

References

Performance/RedundantSortBy

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where sort_by { ... } can be replaced by sort.

Examples

# bad
array.sort_by { |x| x }
array.sort_by do |var|
  var
end

# good
array.sort

Performance/RegexpMatch

Enabled by default Supports autocorrection
Enabled Yes

In Ruby 2.4, String#match?, Regexp#match? and Symbol#match? have been added. The methods are faster than match. Because the methods avoid creating a MatchData object or saving backref. So, when MatchData is not used, use match? instead of match.

Examples

# bad
def foo
  if x =~ /re/
    do_something
  end
end

# bad
def foo
  if x.match(/re/)
    do_something
  end
end

# bad
def foo
  if /re/ === x
    do_something
  end
end

# good
def foo
  if x.match?(/re/)
    do_something
  end
end

# good
def foo
  if x =~ /re/
    do_something(Regexp.last_match)
  end
end

# good
def foo
  if x.match(/re/)
    do_something($~)
  end
end

# good
def foo
  if /re/ === x
    do_something($~)
  end
end

Performance/ReverseEach

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of reverse.each and change them to use reverse_each instead.

Examples

# bad
[].reverse.each

# good
[].reverse_each

References

Performance/Sample

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of shuffle.first, shuffle.last and shuffle[] and change them to use sample instead.

Examples

# bad
[1, 2, 3].shuffle.first
[1, 2, 3].shuffle.first(2)
[1, 2, 3].shuffle.last
[1, 2, 3].shuffle[2]
[1, 2, 3].shuffle[0, 2]    # sample(2) will do the same
[1, 2, 3].shuffle[0..2]    # sample(3) will do the same
[1, 2, 3].shuffle(random: Random.new).first

# good
[1, 2, 3].shuffle
[1, 2, 3].sample
[1, 2, 3].sample(3)
[1, 2, 3].shuffle[1, 3]    # sample(3) might return a longer Array
[1, 2, 3].shuffle[1..3]    # sample(3) might return a longer Array
[1, 2, 3].shuffle[foo, bar]
[1, 2, 3].shuffle(random: Random.new)

References

Performance/Size

Enabled by default Supports autocorrection
Enabled Yes

This cop is used to identify usages of count on an Array and Hash and change them to size.

TODO: Add advanced detection of variables that could have been assigned to an array or a hash.

Examples

# bad
[1, 2, 3].count

# bad
{a: 1, b: 2, c: 3}.count

# good
[1, 2, 3].size

# good
{a: 1, b: 2, c: 3}.size

# good
[1, 2, 3].count { |e| e > 2 }

References

Performance/StartWith

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies unnecessary use of a regex where String#start_with? would suffice.

Examples

# bad
'abc' =~ /\Aab/
'abc'.match(/\Aab/)

# good
'abc'.start_with?('ab')

Configurable attributes

Name Default value Configurable values
AutoCorrect false Boolean

References

Performance/StringReplacement

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where gsub can be replaced by tr or delete.

Examples

# bad
'abc'.gsub('b', 'd')
'abc'.gsub('a', '')
'abc'.gsub(/a/, 'd')
'abc'.gsub!('a', 'd')

# good
'abc'.gsub(/.*/, 'a')
'abc'.gsub(/a+/, 'd')
'abc'.tr('b', 'd')
'a b c'.delete(' ')

References

Performance/TimesMap

Enabled by default Supports autocorrection
Enabled Yes

This cop checks for .times.map calls. In most cases such calls can be replaced with an explicit array creation.

Examples

# bad
9.times.map do |i|
  i.to_s
end

# good
Array.new(9) do |i|
  i.to_s
end

Configurable attributes

Name Default value Configurable values
AutoCorrect false Boolean

Performance/UnfreezeString

Enabled by default Supports autocorrection
Enabled No

In Ruby 2.3 or later, use unary plus operator to unfreeze a string literal instead of String#dup and String.new. Unary plus operator is faster than String#dup.

Note: String.new (without operator) is not exactly the same as +''. These differ in encoding. String.new.encoding is always ASCII-8BIT. However, (+'').encoding is the same as script encoding(e.g. UTF-8). So, if you expect ASCII-8BIT encoding, disable this cop.

Examples

# bad
''.dup
"something".dup
String.new
String.new('')
String.new('something')

# good
+'something'
+''

Performance/UriDefaultParser

Enabled by default Supports autocorrection
Enabled Yes

This cop identifies places where URI::Parser.new can be replaced by URI::DEFAULT_PARSER.

Examples

# bad
URI::Parser.new

# good
URI::DEFAULT_PARSER