I have an array of mixed elements, e.g. integers and strings:
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
and I want to apply a sort but only to specific elements. In the above example, the elements to-be-sorted are the integers (but it could be anything). However, even though they should be sorted, they have to stay in their "integer spots". And the strings have to remain in their exact positions.
The sort should work like this:
[3, "foo", 2, 5, "bar", 1, "baz", 4] # before
[1, "foo", 2, 3, "bar", 4, "baz", 5] # after
"foo" is still at index 1, "bar" is at index 4 and "baz" is at index 6.
I could partition the array into integers and non-integers along with their positions:
a, b = ary.each_with_index.partition { |e, i| e.is_a?(Integer) }
a #=> [[3, 0], [2, 2], [5, 3], [1, 5], [4, 7]]
b #=> [["foo", 1], ["bar", 4], ["baz", 6]]
sort the integers:
result = a.map(&:first).sort
#=>[1, 2, 3, 4, 5]
And re-insert the non-integers at their original positions:
b.each { |e, i| result.insert(i, e) }
result
#=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
But this seems rather clumsy. I particular dislike having to deconstruct and rebuild the array one string at a time. Is there a more elegant or more direct approach?
Possible solution
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
integers = ary.select(&->(el) { el.is_a?(Integer) }).sort
ary.map { |n| n.is_a?(Integer) ? integers.shift : n }
# => [1, "foo", 2, 3, "bar", 4, "baz", 5]
I am not proficient with Ruby. Though I'd like to take a shot at what I could come up with.
The idea is as evoked in my comment.
get the indices of the integers
sort the values of the indices
insert the sorted values back into array
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
=> [3, "foo", 2, 5, "bar", 1, "baz", 4]
indices = ary.map.with_index { |item,idx| idx if item.is_a?(Integer) }.compact
=> [0, 2, 3, 5, 7]
values = ary.values_at(*indices).sort
=> [1, 2, 3, 4, 5]
indices.zip(values).each { |idx, val| ary[idx]=val}
=> [[0, 1], [2, 2], [3, 3], [5, 4], [7, 5]]
ary
=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
I have assumed that, as in the example, if arr = ary.dup and arr is modified ary is not mutated. If that is not the case one must work with a deep copy of ary.
A helper:
def sort_object?(e)
e.class == Integer
end
sort_object?(3)
#=> true
sort_object?(-3e2)
#=> true
sort_object?("foo")
#=> false
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
obj_to_idx = ary.zip((0..ary.size-1).to_a).to_h
#=> {3=>0, "foo"=>1, 2=>2, 5=>3, "bar"=>4, 1=>5, "baz"=>6, 4=>7}
to_sort = ary.select { |k| sort_object?(k) }
#=> [3, 2, 5, 1, 4]
sorted = to_sort.sort
#=> [1, 2, 3, 4, 5]
new_idx_to_orig_idx = to_sort.map { |n| obj_to_idx[n] }
.zip(sorted.map { |n| obj_to_idx[n] })
.to_h
#=> {0=>5, 2=>2, 3=>0, 5=>7, 7=>3}
new_idx_to_orig_idx.each_with_object(ary.dup) do |(new_idx,orig_idx),a|
a[new_idx] = ary[orig_idx]
end
#=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
Some of these statements may of course be chained if desired.
In-Place Re-Assignment at Designated Array Indices
You could certainly make this shorter, and perhaps even skip converting things to and from Hash objects, but the intermediate steps there are to show my thought process and make the intent more explicit and debugging a bit easier. Consider the following:
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
ints = ary.each_with_index.select { |elem, idx| [elem, idx] if elem.kind_of? Integer }.to_h
ordered_ints = ints.keys.sort.zip(ints.values).to_h
ints.keys.sort.zip(ints.values).each { |elem, idx| ary[idx] = elem }
ary
#=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
The idea here is that we:
Select just the items of the type we want to sort, along with their index within the current Array using #each_with_index. If you don't like #kind_of? you could replace it with #respond_to? or any other selection criteria that makes sense to you.
Sort Array values selected, and then #zip them up along with the index locations we're going to modify.
Re-assign the sorted elements for each index that needs to be replaced.
With this approach, all unselected elements remain in their original index locations within the ary Array. We're only modifying the values of specific Array indices with the re-ordered items.
I will throw my hat in this ring as well.
It appears the other answers rely on the assumption of uniqueness so I took a different route by building a positional transliteration Hash
(Update: took it a little further than necessary)
def sort_only(ary, sort_klass: Integer)
raise ArgumentError unless sort_klass.is_a?(Class) && sort_klass.respond_to?(:<=>)
# Construct a Hash of [Object,Position] => Object
translator = ary
.each_with_index
.with_object({}) {|a,obj| obj[a] = a.first}
.tap do |t|
# select the keys where the value (original element) is a sort_klass
t.filter_map {|k,v| k if v.is_a?(sort_klass)}
.then do |h|
# sort them and then remap these keys to point at the sorted value
h.sort.each_with_index do |(k,_),idx|
t[h[idx]] = k
end
end
end
# loop through the original array with its position index and use
# the transliteration Hash to place them in the correct order
ary.map.with_index {|a,i| translator[[a,i]]}
end
Example: (Working Example)
sort_only([3, "foo", 2, 5, "bar", 1, "baz", 4])
#=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
sort_only([3, 3, "foo", 2, 5, "bar", 1, "baz",12.0,5,Object, 4,"foo", 7, -1, "qux"])
#=> [-1, 1, "foo", 2, 3, "bar", 3, "baz", 12.0, 4, Object, 5, "foo", 5, 7, "qux"]
sort_only([3, "foo", 2, 5,"qux", "bar", 1, "baz", 4], sort_klass: String)
#=> [3, "bar", 2, 5, "baz", "foo", 1, "qux", 4]
For Reference the Second Example produces the following transliteration:
{[3, 0]=>-1, [3, 1]=>1, ["foo", 2]=>"foo", [2, 3]=>2, [5, 4]=>3,
["bar", 5]=>"bar", [1, 6]=>3, ["baz", 7]=>"baz", [12.0, 8]=>12.0,
[5, 9]=>4, [Object, 10]=>Object, [4, 11]=>5, ["foo", 12]=>"foo",
[7, 13]=>5, [-1, 14]=>7, ["qux", 15]=>"qux"}
Below code simply creates two new arrays: numbers and others. numbers are Integer instances from original array, later sorted in descending order. others looks like original array but number spots replaced with nil. at the end push that sorted numbers to nil spots
arr.inject([[], []]) do |acc, el|
el.is_a?(Integer) ? (acc[0]<<el; acc[1]<<nil) : acc[1]<<el; acc
end.tap do |titself|
numbers = titself[0].sort_by {|e| -e }
titself[1].each_with_index do |u, i|
titself[1][i] = numbers.pop unless u
end
end.last
I'm going to answer my own question here with yet another approach to the problem. This solution delegates most work to Ruby's built-in methods and avoids explicit blocks / loops as far as possible.
Starting with the input array:
ary = [3, "foo", 2, 5, "bar", 1, "baz", 4]
You could build a hash of position => element pairs:
hash = ary.each_index.zip(ary).to_h
#=> {0=>3, 1=>"foo", 2=>2, 3=>5, 4=>"bar", 5=>1, 6=>"baz", 7=>4}
extract the pairs having integer value: (or whatever you want to sort)
ints_hash = hash.select { |k, v| v.is_a?(Integer) }
#=> {0=>3, 2=>2, 3=>5, 5=>1, 7=>4}
sort their values: (any way you want)
sorted_ints = ints_hash.values.sort
#=> [1, 2, 3, 4, 5]
build a new mapping for the sorted values:
sorted_ints_hash = ints_hash.keys.zip(sorted_ints).to_h
#=> {0=>1, 2=>2, 3=>3, 5=>4, 7=>5}
update the position hash:
hash.merge!(sorted_ints_hash)
#=> {0=>1, 1=>"foo", 2=>2, 3=>3, 4=>"bar", 5=>4, 6=>"baz", 7=>5}
And voilà:
hash.values
#=> [1, "foo", 2, 3, "bar", 4, "baz", 5]
I find a lot of reference about removing duplicates in ruby but I cannot find how to create duplicate.
If I have an array like [1,2,3] how can I map it to an array with dubbed items? [1,1,2,2,3,3]
Is there a method?
Try this one
[1, 2, 3].flat_map { |i| [i, i] }
=> [1, 1, 2, 2, 3, 3]
Here's yet another way, creating the array directly with Array#new :
array = [1, 2, 3]
repetitions = 2
p Array.new(array.size * repetitions) { |i| array[i / repetitions] }
# [1, 1, 2, 2, 3, 3]
According to fruity, #ursus's answer, #ilya's first two answers and mine have comparable performance. transpose.flatten is slower than any of the others.
#Ursus answer is the most clean, there are possible solutions:
a = [1, 2, 3]
a.zip(a).flatten
#=> [1, 1, 2, 2, 3, 3]
Or
a.inject([]) {|a, e| a << e << e} # a.inject([]) {|a, e| n.times {a << e}; a}
=> [1, 1, 2, 2, 3, 3]
Or
[a, a].transpose.flatten # ([a] * n).transpose.flatten
=> [1, 1, 2, 2, 3, 3]
Try this:
[1, 2, 3] * 2
=> [1, 2, 3, 1, 2, 3]
You might want it sorted:
([1, 2, 3] * 2).sort
=> [1, 1, 2, 2, 3, 3]
I am having trouble figuring the best approach to shift some values to another key. Every value most go to the next key, and the last key's values need to be completely removed. For example
hash1 = { a: [1, 2, 3], b: [4, 5, 6], c: [7, 8, 9] }
desired_hash = hash1.some_method
desired_hash === { a: [], b: [1, 2, 3], c: [4, 5, 6] }
My thought is to rename the hash keys but was not sure if this was the best approach.
hash1 = { a: [1, 2, 3], b: [4, 5, 6], c: [7, 8, 9] }
keys = hash1.keys
=> [:a, :b, :c]
values = hash1.values
=> [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
keys.zip(values.unshift([])).to_h
=> {:a=>[], :b=>[1, 2, 3], :c=>[4, 5, 6]}
pv = []
hash1.each_with_object({}) do |(k,v),h|
h[k] = pv
pv = v
end
#=> {:a=>[], :b=>[1, 2, 3], :c=>[4, 5, 6]}
Assuming the hash keys are "in correct order" already.
You can try this:
def shift(input)
output = {}
keys = input.keys
0.upto(keys.size - 1) do |index|
current = keys[index]
if index > 0
previous = keys[index - 1]
output[current] = input[previous]
else
output[current] = []
end
end
output
end
input = { a: [1, 2, 3], b: [4, 5, 6], c: [7, 8, 9] }
p shift(input)
it's not very elegant, there is probably a much nicer solution but it's a starting point.
I couldn't find a way to build an array such as
[ [1,2,3] , [1,2,3] , [1,2,3] , [1,2,3] , [1,2,3] ]
given [1,2,3] and the number 5. I guess there are some kind of operators on arrays, such as product of mult, but none in the doc does it. Please tell me. I missed something very simple.
Array.new(5, [1, 2, 3]) or Array.new(5) { [1, 2, 3] }
Array.new(size, default_object) creates an array with an initial size, filled with the default object you specify. Keep in mind that if you mutate any of the nested arrays, you'll mutate all of them, since each element is a reference to the same object.
array = Array.new(5, [1, 2, 3])
array.first << 4
array # => [[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]
Array.new(size) { default_object } lets you create an array with separate objects.
array = Array.new(5) { [1, 2, 3] }
array.first << 4
array #=> [[1, 2, 3, 4], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]]
Look up at the very top of the page you linked to, under the section entitled "Creating Arrays" for some more ways to create arrays.
Why not just use:
[[1, 2, 3]] * 5
# => [[1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]]
The documentation for Array.* says:
...returns a new array built by concatenating the int copies of self.
Typically we'd want to create a repeating single-level array:
[1, 2, 3] * 2
# => [1, 2, 3, 1, 2, 3]
but there's nothing to say we can't use a sub-array like I did above.
It looks like mutating one of the subarrays mutates all of them, but that may be what someone wants.
It's like Array.new(5, [1,2,3]):
foo = [[1, 2, 3]] * 2
foo[0][0] = 4
foo # => [[4, 2, 3], [4, 2, 3]]
foo = Array.new(2, [1,2,3])
foo[0][0] = 4
foo # => [[4, 2, 3], [4, 2, 3]]
A work-around, if that's not the behavior wanted is:
foo = ([[1, 2, 3]] * 2).map { |a| [*a] }
foo[0][0] = 4
foo # => [[4, 2, 3], [1, 2, 3]]
But, at that point it's not as convenient, so I'd use the default Array.new(n) {…} behavior.