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I have a simple method that iterates through an array and returns a duplicate. (Or duplicates)
def find_dup(array)
duplicate = 0
array.each { |element| duplicate = element if array.count(element) > 1}
duplicate
end
It works, but I'd like to express this more elegantly.
The reason it is three lines is that the variable "duplicate", which the method must return, is not visible to the method if I introduce it inside the block, i.e,
def find_dup(array)
array.each { |element| duplicate = element if array.count(element) > 1}
duplicate
end
I've tried a few ways to define "duplicate" as the result of a block, but to no avail.
Any thoughts?
It's a little too much to do cleanly in a one-liner, but this is a more
efficient solution.
def find_dups(arr)
counts = Hash.new { |hash,key| hash[key] = 0 }
arr.each_with_object(counts) do |x, memo|
memo[x] += 1
end.select { |key,val| val > 1 }.keys
end
The Hash.new call instantiates a hash where the default value is 0.
each_with_object modifies this hash to track the count of each element in arr, then at the
end the filter is used to select only those having a count greater than one.
The benefit of this approach over a solution using Array#includes? or Array#count is that it only scans the array a single time. Thus it is a O(N) time instead of O(N^2).
Your method is only finding the last duplicate in the array. If you want all the duplicates, I would do something like this:
def find_dups(arr)
dups = Hash.new { |h, k| h[k] = 0 }
arr.each { |el| dups[el] += 1 }
dups.select { |k, v| v > 1 }.keys
end
If what you really want is a one-liner that isn't concerned with big-O complexity and only returns the last duplicate in the array, I would do this:
def find_last_dup(arr)
arr.reverse_each { |el| return el if arr.count(el) > 1 }
end
You can do this as one line and it flows a bit nicer. Though this would find the first instance of a duplicate whereas your code is returning the last instance of a duplicate, not sure if that's part of your requirement.
def find_dup(array)
array.group_by { |value| value }.find { |_, groups| groups.count > 1 }.first
end
Also, note that making things one line doesn't strictly mean is better. I'd find the code more readable split over more lines, but that's just my opinion.
def find_dup(array)
array.group_by { |value|
value
}.find { |_, groups|
groups.count > 1
}.first
end
Just want to add one more approach to the mix.
def find_last_dup(arr)
arr.reverse_each.detect { |x| arr.count(x) > 1 }
end
Alternatively, you can get linear time complexity in two lines.
def find_last_dup(arr)
freq = arr.each_with_object(Hash.new(0)) { |x, obj| obj[x] += 1 }
arr.reverse_each.detect { |x| freq[x] > 1 }
end
For the sake of argument, the latter approach can be reduced to one line as well, but this would be unidiomatic and confusing.
def find_last_dup(arr)
arr.each_with_object(Hash.new(0)) { |x, obj| obj[x] += 1 }
.tap do |freq| return arr.reverse_each.detect { |x| freq[x] > 1 } end
end
Given:
> a
=> [8, 5, 6, 6, 5, 8, 6, 1, 9, 7, 2, 10, 7, 7, 3, 4]
You can group the dups together:
> a.uniq.each_with_object(Hash.new(0)) {|e, h| c=a.count(e); h[e]=c if c>1}
=> {8=>2, 5=>2, 6=>3, 7=>3}
Or,
> a.group_by{ |e| e}.select{|k,v| v if v.length>1}
=> {8=>[8, 8], 5=>[5, 5], 6=>[6, 6, 6], 7=>[7, 7, 7]}
In each case, the order of the result is based on the order of the elements in a that have dups. If you just want the first:
> a.group_by{ |e| e}.select{|k,v| v if v.length>1}.first
=> [8, [8, 8]]
Or last:
> a.group_by{ |e| e}.select{|k,v| v if v.length>1}.to_a.last
=> [7, [7, 7, 7]]
If you want to 'fast forward' to the first value that has a dup, you can use drop_while:
> b=[1,2,3,4,5,4,5,6]
> b.drop_while {|e| b.count(e)==1 }[0]
=> 4
Or the last:
> b.reverse.drop_while {|e| b.count(e)==1 }[0]
=> 5
def find_duplicates(array)
array.dup.uniq.each { |element| array.delete_at(array.index(element)) }.uniq
end
The above method find_duplicates duplicated the input array and deletes the first occurrence of all the elements, leaving the array with only remaining occurrences of the duplicate elements.
Example:
array = [1, 2, 3, 4, 3, 4, 3]
=> [1, 2, 3, 4, 3, 4, 3]
find_duplicates(array)
=> [3, 4]
Having an array
a = 1, 2, 3, 4
And an enumerator:
e = a.map!
Then, by calling e.next repeatedly, array a gets nicely destroyed:
e.next #=> 1
a #=> [1, 2, 3, 4]
e.next #=> 2
a #=> [nil, 2, 3, 4]
e.next #=> 3
a #=> [nil, nil, 3, 4]
That's so hilarious! But when I try
e.next { |x| 2 * x } # => 4
I get
a #=> [nil, nil, nil, 4]
instead of desired
a #=> [nil, nil, nil, 8]
What am I misunderstanding? How to make a.map! do what I want with the elements?
My problem is, that I do not fully understand enumerators. With the previous code in place, for example, enumerator e constitutes a backdoor to a:
e.each { 42 }
a #=> [42, 42, 42, 42]
I would like to know, how to do this gradually, with values other than nil. (I can gradually fill it with nils using e.rewind and e.next several times, as I shown before.
To make map! behave as you want, you need the Enumerator#feed method, consider this
ary = *1..4
enum = ary.map!
# the `loop` method handles `StopIteration` for us
loop do
x = enum.next
enum.feed(x * 2)
end
ary
# => [2, 4, 6, 8]
From reference it seems that Enumerator#next doesn't accept a block, so that doesn't have effect of your next call. If you just want to in-place double the last element while clearing all other, do something like, consider straight approach (like a = a[0..-2].map!{|x| nil} + [a.last*2], maybe more elegant). Anyway, could you please provide us with a more detailed usecase to make sure you are doing what you really need?
a.map! accepts a block, but returns an enumerator if no block is supplied. Enumerator#next does not accept a block.
You want to use this to accomplish your goal:
a.map! {|x| x * 2}
if you want to multiply all elements in the array by 2.
For info on next, check out http://ruby-doc.org/core-2.0/Enumerator.html#method-i-next
If you want the output to be exactly [nil, nil, nil, 8] you could do something like:
func = lambda { |x|
unless x == 4
nil
else
x * 2
end
}
a.map!(&func) #> [nil, nil, nil, 8]
I have a map which either changes a value or sets it to nil. I then want to remove the nil entries from the list. The list doesn't need to be kept.
This is what I currently have:
# A simple example function, which returns a value or nil
def transform(n)
rand > 0.5 ? n * 10 : nil }
end
items.map! { |x| transform(x) } # [1, 2, 3, 4, 5] => [10, nil, 30, 40, nil]
items.reject! { |x| x.nil? } # [10, nil, 30, 40, nil] => [10, 30, 40]
I'm aware I could just do a loop and conditionally collect in another array like this:
new_items = []
items.each do |x|
x = transform(x)
new_items.append(x) unless x.nil?
end
items = new_items
But it doesn't seem that idiomatic. Is there a nice way to map a function over a list, removing/excluding the nils as you go?
You could use compact:
[1, nil, 3, nil, nil].compact
=> [1, 3]
I'd like to remind people that if you're getting an array containing nils as the output of a map block, and that block tries to conditionally return values, then you've got code smell and need to rethink your logic.
For instance, if you're doing something that does this:
[1,2,3].map{ |i|
if i % 2 == 0
i
end
}
# => [nil, 2, nil]
Then don't. Instead, prior to the map, reject the stuff you don't want or select what you do want:
[1,2,3].select{ |i| i % 2 == 0 }.map{ |i|
i
}
# => [2]
I consider using compact to clean up a mess as a last-ditch effort to get rid of things we didn't handle correctly, usually because we didn't know what was coming at us. We should always know what sort of data is being thrown around in our program; Unexpected/unknown data is bad. Anytime I see nils in an array I'm working on, I dig into why they exist, and see if I can improve the code generating the array, rather than allow Ruby to waste time and memory generating nils then sifting through the array to remove them later.
'Just my $%0.2f.' % [2.to_f/100]
Try using reduce or inject.
[1, 2, 3].reduce([]) { |memo, i|
if i % 2 == 0
memo << i
end
memo
}
I agree with the accepted answer that we shouldn't map and compact, but not for the same reasons.
I feel deep inside that map then compact is equivalent to select then map. Consider: map is a one-to-one function. If you are mapping from some set of values, and you map, then you want one value in the output set for each value in the input set. If you are having to select before-hand, then you probably don't want a map on the set. If you are having to select afterwards (or compact) then you probably don't want a map on the set. In either case you are iterating twice over the entire set, when a reduce only needs to go once.
Also, in English, you are trying to "reduce a set of integers into a set of even integers".
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]
In your case, as the block evaluates to falsey, simply:
items.filter_map { |x| process_x url }
"Ruby 2.7 adds Enumerable#filter_map" is a good read on the subject, with some performance benchmarks against some of the earlier approaches to this problem:
N = 100_000
enum = 1.upto(1_000)
Benchmark.bmbm do |x|
x.report("select + map") { N.times { enum.select { |i| i.even? }.map{ |i| i + 1 } } }
x.report("map + compact") { N.times { enum.map { |i| i + 1 if i.even? }.compact } }
x.report("filter_map") { N.times { enum.filter_map { |i| i + 1 if i.even? } } }
end
# Rehearsal -------------------------------------------------
# select + map 8.569651 0.051319 8.620970 ( 8.632449)
# map + compact 7.392666 0.133964 7.526630 ( 7.538013)
# filter_map 6.923772 0.022314 6.946086 ( 6.956135)
# --------------------------------------- total: 23.093686sec
#
# user system total real
# select + map 8.550637 0.033190 8.583827 ( 8.597627)
# map + compact 7.263667 0.131180 7.394847 ( 7.405570)
# filter_map 6.761388 0.018223 6.779611 ( 6.790559)
Definitely compact is the best approach for solving this task. However, we can achieve the same result just with a simple subtraction:
[1, nil, 3, nil, nil] - [nil]
=> [1, 3]
In your example:
items.map! { |x| process_x url } # [1, 2, 3, 4, 5] => [1, nil, 3, nil, nil]
it does not look like the values have changed other than being replaced with nil. If that is the case, then:
items.select{|x| process_x url}
will suffice.
If you wanted a looser criterion for rejection, for example, to reject empty strings as well as nil, you could use:
[1, nil, 3, 0, ''].reject(&:blank?)
=> [1, 3, 0]
If you wanted to go further and reject zero values (or apply more complex logic to the process), you could pass a block to reject:
[1, nil, 3, 0, ''].reject do |value| value.blank? || value==0 end
=> [1, 3]
[1, nil, 3, 0, '', 1000].reject do |value| value.blank? || value==0 || value>10 end
=> [1, 3]
You can use #compact method on the resulting array.
[10, nil, 30, 40, nil].compact => [10, 30, 40]
each_with_object is probably the cleanest way to go here:
new_items = items.each_with_object([]) do |x, memo|
ret = process_x(x)
memo << ret unless ret.nil?
end
In my opinion, each_with_object is better than inject/reduce in conditional cases because you don't have to worry about the return value of the block.
One more way to accomplish it will be as shown below. Here, we use Enumerable#each_with_object to collect values, and make use of Object#tap to get rid of temporary variable that is otherwise needed for nil check on result of process_x method.
items.each_with_object([]) {|x, obj| (process x).tap {|r| obj << r unless r.nil?}}
Complete example for illustration:
items = [1,2,3,4,5]
def process x
rand(10) > 5 ? nil : x
end
items.each_with_object([]) {|x, obj| (process x).tap {|r| obj << r unless r.nil?}}
Alternate approach:
By looking at the method you are calling process_x url, it is not clear what is the purpose of input x in that method. If I assume that you are going to process the value of x by passing it some url and determine which of the xs really get processed into valid non-nil results - then, may be Enumerabble.group_by is a better option than Enumerable#map.
h = items.group_by {|x| (process x).nil? ? "Bad" : "Good"}
#=> {"Bad"=>[1, 2], "Good"=>[3, 4, 5]}
h["Good"]
#=> [3,4,5]
a = [4, 3, 2, nil]
a.max_by { |v| v * 2 } => NoMethodError: undefined method `*' for nil:NilClass
How to overload max_by to ignore nil values?
You can use Array.compact to remove nils before you call max_by.
a.compact.max_by { |v| v * 2 }
Welcome to Ruby: there are so many ways to solve the problem!
A very simple solution is:
a.max_by { |v| v.to_f * 2 }
since nil coerces to float as 0. This doesn't handle negative values, but since nil is only a single instance of a class called NilClass, now as with all classes in Ruby we can open it an let it learn a littly maths:
class NilClass
# overloading * operator
def *(y)
# returning negative infinity: Ruby 1.8.7
-1.0/0.0
# returning negative infinity: Ruby 1.9.2
# -Float::INFINITY
end
end
now we have
a.max_by { |v| v * 2 }
returning 4.
Here's another one:
a.max_by { |v| v.nil? ? -Float::INFINITY : v }
#=> 4
For your example this is obviously more complicated than compact, but if you want to sort the array and keep the nil values it's a handy trick. Or if you want to sort in a strange way, like zeroes to the end:
[0,4,5,6,1,9].sort_by { |v| v.zero? ? Float::INFINITY : v }
#=> [1, 4, 5, 6, 9, 0]
Greetings!
When assigning a value to an array as in the following, how could I replace the nils by 0?
array = [1,2,3]
array[10] = 2
array # => [1, 2, 3, nil, nil, nil, nil, nil, nil, nil, 2]
If not possible when assigning, how would I do it the best way afterwards? I thought of array.map { |e| e.nil? ? 0 : e }, but well…
Thanks!
To change the array after assignment:
array.map! { |x| x || 0 }
Note that this also converts false to 0.
If you want to use zeros during assignment, it's a little messy:
i = 10
a = [1, 2, 3]
a += ([0] * (i - a.size)) << 2
# => [1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 2]
There is no built-in function to replace nil in an array, so yes, map is the way to go. If a shorter version would make you happier, you could do:
array.map {|e| e ? e : 0}
nil.to_i is 0, if all the numbers are integers then below should work. I think It is also the shortest answer here.
array.map!(&:to_i)
To change the array in place
array.map!{|x|x ?x:0}
If the array can contain false you'll need to use this instead
array.map!{|x|x.nil? ? 0:x}
a.select { |i| i }
This answer is too short so I am adding a few more words.
Another approach would be to define your own function for adding a value to the array.
class Array
def addpad(index,newval)
concat(Array.new(index-size,0)) if index > size
self[index] = newval
end
end
a = [1,2,3]
a.addpad(10,2)
a => [1,2,3,0,0,0,0,0,0,0,2]