Using the following example:
array = [1,20]
new_array = []
i = array[0]
while i < array[1]
new_array.push(i)
i+= 2
end
#new_array = [1,3,5,7,9,11,13,15,17,19]
Is there a more elegant way to write this loop without have to write an empty array (new_array) and an external variable loop counter (i)? I was thinking something along the lines of new_array.map{|x| } but instead of iterating through each element, it continually adds a number until it hits a certain limit.
Assuming your goal is to create an array of odd numbers up to a limit, you can use a range with a step.
limit = 20
array = (1..limit).step(2).to_a
EDIT
If you want to be able to descend as well as ascend you can use step.
#Ascending
start = 1
limit = 20
array = start.step(limit, 2).to_a
#Descending
start = 20
limit = 1
array = start.step(limit, -2).to_a
For the sake of having an alternative, you could also select (Enumerable#select) odds numbers (Integer#odds?) out of your Range:
(1..20).select(&:odd?)
#=> [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
My answer is addressed to the question stated in the title, not to the particular example presenteed.
Suppose the user is asked to enter a sequence of strings, with an empty string signifying that the user is finished. These strings are to be saved in an array which is to be returned.
This is a typical way of writing the code using a while loop:
def gettem
arr = []
until (s = gets.chomp).empty? do
arr << s
end
arr
end
One could instead use Kernel#loop and the keyword break, which some (including me) prefer to while and until loops.
def gettem
arr = []
loop do
s = gets.chomp
break if s.empty?
arr << s
end
arr
end
A third way, suggested by #Aleksei Matiushkin in his answer here (which I had not seen before) is the following:
def gettem
loop.with_object([]) do |_,arr|
s = gets.chomp
break arr if s.empty?
arr << s
end
end
This uses the form of loop that returns an enumerator (see the doc). If I run this and enter "dog", "cat" and "\n", the return value is ["dog", "cat"], as desired.
This approach has three of advantages over the other approaches:
the variable arr is confined to the block, away from prying eyes;
fewer lines of code are needed; and
the return value can be chained, as illustrated below.
def gettem
loop.with_object([]) do |_,arr|
s = gets.chomp
break arr if s.empty?
arr << s
end.then { |arr| [arr.size, arr] }
end
When executing this method and entering "dog", "cat" and "\n", the array [2, ["dog", "cat"]] is returned.
I've used an underscore for the first block variable (which always has a value of nil) to signify that it is not used in the block calculation.
I am iterating over an array, and I'm wondering if there's a shorthand to refer to the receiver of #each (or #each_with_index) method from within the iteration.
self returns main.
You should be able to just reference it:
my_thing.each {|one_thing| puts my_thing }
This is pretty similar to the answer I gave here https://stackoverflow.com/a/45421168/2981429 but slightly different.
First off, you can create a scope with self bound to the array, and then execute the each in that scope:
[1].instance_exec do
# in this scope, self is the array
# thus we can use just 'each' because the self is inferred
each do |x|
# note that since 'class' is a special keyword,
# it needs to be explicitly namespaced on self
puts self.class, x
end
end
# => prints Array, 1
You can create a utility function to do this, if you want:
def bound_each(enumerable, &blk)
enumerable.instance_exec { each &blk }
end
bound_each([1]) { |x| puts self.class, x }
# prints Array, 1
You can call your each method within an Object#tap block and reference the original receiver like that.
[1, 2, 3].tap { |i| i.each { |j| p i.dup << j } }
# [1, 2, 3, 1]
# [1, 2, 3, 2]
# [1, 2, 3, 3]
#=> [1, 2, 3]
Here the receiving object is [1, 2, 3] and is passed to the block-variable i which we can use locally or in nested scopes such as each's block.
Avoid modifying the receiving object else you may end up with undesired results such as an infinite array. Using dup could allay this possibility.
This is an interesting question. As far as I know it's not possible – the closest I can come up with would be to use inject (or reduce) and explicitly pass the receiver as an argument. A bit pointless, but there might be a use-case for it that I'm not seeing:
a = [1,2,3]
a.inject(a) do |this, element|
this == a #=> true
this.include?(element) #=> true
this
end
Apart from looking a bit redundant, you have to be very sure to return this at the end of each iteration, as the return value will become this in the next iteration. For that reason (and the fact that you could just reference your collection in an each block, as in David's answer) I don't recommend using this.
Edit - as Simple Lime pointed out in the comments – I missed the obvious Enumerator#with_object, which has the same (rather pointless) effect, but without the drawback of having to return this at the end of each iteration. For example:
a = [1,2,3]
a.map.with_object(a) do |element, this|
this == a #=> true, for each iteration
end
I still don't recommend that you use this though.
What is the idiomatic Ruby way to write this code?
Given an array, I would like to iterate through each element of that array, but skip the first one. I want to do this without allocating a new array.
Here are two ways I've come up with, but neither feels particularly elegant.
This works but seems way too verbose:
arr.each_with_index do |elem, i|
next if i.zero? # skip the first
...
end
This works but allocates a new array:
arr[1..-1].each { ... }
Edit/clarification: I'd like to avoid allocating a second array. Originally I said I wanted to avoid "copying" the array, which was confusing.
Using the internal enumerator is certainly more intuitive, and you can do this fairly elegantly like so:
class Array
def each_after(n)
each_with_index do |elem, i|
yield elem if i >= n
end
end
end
And now:
arr.each_after(1) do |elem|
...
end
I want to do this without creating a copy of the array.
1) Internal iterator:
arr = [1, 2, 3]
start_index = 1
(start_index...arr.size).each do |i|
puts arr[i]
end
--output:--
2
3
2) External iterator:
arr = [1, 2, 3]
e = arr.each
e.next
loop do
puts e.next
end
--output:--
2
3
OK, maybe this is bad form to answer my own question. But I've been racking my brain on this and poring over the Enumerable docs, and I think I've found a good solution:
arr.lazy.drop(1).each { ... }
Here's proof that it works :-)
>> [1,2,3].lazy.drop(1).each { |e| puts e }
2
3
Concise: yes. Idiomatic Ruby… maybe? What do you think?
I'm looking for something similar to #detect in enumerables, but not quite. This is what enumerable does:
[1, 2, 3].detect {|i| i > 1 } #=> 2
it returns the first instance of the array which matches the condition. Now, my purpose is to return the value of the block. Concern is not exactly the conditions, but for instance, the first which is not nil. Something like this:
[var1, var2, var3].wanted_detect {|var| another_function(var) }
in which the function would return the first result of another_function call which isn't nil.
Mapping the values of applying the method on the variables and then using detect is not an option. This one would ideally have to work in lazy enumerators, for which the early mapping of all possible values is a no-go
[var1, var2, var3].lazy.map { |var| another_function(var) }.reject(&:nil?).first
If you don't have access to Enumerable#lazy, it is easy enough to implement what you want:
module Enumerable
def wanted_detect
self.each do |obj|
val = yield obj
return val if val
end
end
end
Demo:
[1, 2, 3, 4].wanted_detect { |x| x*x if x > 2 }
# => 9
EDIT: Sorry, I missed the last paragraph till falsetru pointed it out.
Thanks for the comments, falsetru.
I have a Ruby array containing some string values. I need to:
Find all elements that match some predicate
Run the matching elements through a transformation
Return the results as an array
Right now my solution looks like this:
def example
matchingLines = #lines.select{ |line| ... }
results = matchingLines.map{ |line| ... }
return results.uniq.sort
end
Is there an Array or Enumerable method that combines select and map into a single logical statement?
I usually use map and compact together along with my selection criteria as a postfix if. compact gets rid of the nils.
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}
=> [3, 3, 3, nil, nil, nil]
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}.compact
=> [3, 3, 3]
Ruby 2.7+
There is now!
Ruby 2.7 is introducing filter_map for this exact purpose. It's idiomatic and performant, and I'd expect it to become the norm very soon.
For example:
numbers = [1, 2, 5, 8, 10, 13]
enum.filter_map { |i| i * 2 if i.even? }
# => [4, 16, 20]
Here's a good read on the subject.
Hope that's useful to someone!
You can use reduce for this, which requires only one pass:
[1,1,1,2,3,4].reduce([]) { |a, n| a.push(n*3) if n==1; a }
=> [3, 3, 3]
In other words, initialize the state to be what you want (in our case, an empty list to fill: []), then always make sure to return this value with modifications for each element in the original list (in our case, the modified element pushed to the list).
This is the most efficient since it only loops over the list with one pass (map + select or compact requires two passes).
In your case:
def example
results = #lines.reduce([]) do |lines, line|
lines.push( ...(line) ) if ...
lines
end
return results.uniq.sort
end
Another different way of approaching this is using the new (relative to this question) Enumerator::Lazy:
def example
#lines.lazy
.select { |line| line.property == requirement }
.map { |line| transforming_method(line) }
.uniq
.sort
end
The .lazy method returns a lazy enumerator. Calling .select or .map on a lazy enumerator returns another lazy enumerator. Only once you call .uniq does it actually force the enumerator and return an array. So what effectively happens is your .select and .map calls are combined into one - you only iterate over #lines once to do both .select and .map.
My instinct is that Adam's reduce method will be a little faster, but I think this is far more readable.
The primary consequence of this is that no intermediate array objects are created for each subsequent method call. In a normal #lines.select.map situation, select returns an array which is then modified by map, again returning an array. By comparison, the lazy evaluation only creates an array once. This is useful when your initial collection object is large. It also empowers you to work with infinite enumerators - e.g. random_number_generator.lazy.select(&:odd?).take(10).
If you have a select that can use the case operator (===), grep is a good alternative:
p [1,2,'not_a_number',3].grep(Integer){|x| -x } #=> [-1, -2, -3]
p ['1','2','not_a_number','3'].grep(/\D/, &:upcase) #=> ["NOT_A_NUMBER"]
If we need more complex logic we can create lambdas:
my_favourite_numbers = [1,4,6]
is_a_favourite_number = -> x { my_favourite_numbers.include? x }
make_awesome = -> x { "***#{x}***" }
my_data = [1,2,3,4]
p my_data.grep(is_a_favourite_number, &make_awesome) #=> ["***1***", "***4***"]
I'm not sure there is one. The Enumerable module, which adds select and map, doesn't show one.
You'd be required to pass in two blocks to the select_and_transform method, which would be a bit unintuitive IMHO.
Obviously, you could just chain them together, which is more readable:
transformed_list = lines.select{|line| ...}.map{|line| ... }
Simple Answer:
If you have n records, and you want to select and map based on condition then
records.map { |record| record.attribute if condition }.compact
Here, attribute is whatever you want from the record and condition you can put any check.
compact is to flush the unnecessary nil's which came out of that if condition
No, but you can do it like this:
lines.map { |line| do_some_action if check_some_property }.reject(&:nil?)
Or even better:
lines.inject([]) { |all, line| all << line if check_some_property; all }
I think that this way is more readable, because splits the filter conditions and mapped value while remaining clear that the actions are connected:
results = #lines.select { |line|
line.should_include?
}.map do |line|
line.value_to_map
end
And, in your specific case, eliminate the result variable all together:
def example
#lines.select { |line|
line.should_include?
}.map { |line|
line.value_to_map
}.uniq.sort
end
def example
#lines.select {|line| ... }.map {|line| ... }.uniq.sort
end
In Ruby 1.9 and 1.8.7, you can also chain and wrap iterators by simply not passing a block to them:
enum.select.map {|bla| ... }
But it's not really possible in this case, since the types of the block return values of select and map don't match up. It makes more sense for something like this:
enum.inject.with_index {|(acc, el), idx| ... }
AFAICS, the best you can do is the first example.
Here's a small example:
%w[a b 1 2 c d].map.select {|e| if /[0-9]/ =~ e then false else e.upcase end }
# => ["a", "b", "c", "d"]
%w[a b 1 2 c d].select.map {|e| if /[0-9]/ =~ e then false else e.upcase end }
# => ["A", "B", false, false, "C", "D"]
But what you really want is ["A", "B", "C", "D"].
You should try using my library Rearmed Ruby in which I have added the method Enumerable#select_map. Heres an example:
items = [{version: "1.1"}, {version: nil}, {version: false}]
items.select_map{|x| x[:version]} #=> [{version: "1.1"}]
# or without enumerable monkey patch
Rearmed.select_map(items){|x| x[:version]}
If you want to not create two different arrays, you can use compact! but be careful about it.
array = [1,1,1,2,3,4]
new_array = map{|n| n*3 if n==1}
new_array.compact!
Interestingly, compact! does an in place removal of nil. The return value of compact! is the same array if there were changes but nil if there were no nils.
array = [1,1,1,2,3,4]
new_array = map{|n| n*3 if n==1}.tap { |array| array.compact! }
Would be a one liner.
Your version:
def example
matchingLines = #lines.select{ |line| ... }
results = matchingLines.map{ |line| ... }
return results.uniq.sort
end
My version:
def example
results = {}
#lines.each{ |line| results[line] = true if ... }
return results.keys.sort
end
This will do 1 iteration (except the sort), and has the added bonus of keeping uniqueness (if you don't care about uniq, then just make results an array and results.push(line) if ...
Here is a example. It is not the same as your problem, but may be what you want, or can give a clue to your solution:
def example
lines.each do |x|
new_value = do_transform(x)
if new_value == some_thing
return new_value # here jump out example method directly.
else
next # continue next iterate.
end
end
end