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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 have this data structure resulted from a query grouping
{
[0, "AR"]=>2,
[0, nil]=>1,
[0, "AQ"]=>6,
[1, nil]=>4,
[1, "AQ"]=>3,
[2, "BG"]=>1,
[2, nil]=>1,
}
I want to manipulate it so I end up with a structure grouped like this
{
0 => {
'AR' => 2,
'AQ' => 6,
nil => 1
},
1 => {
'AQ' => 1,
nil => 4
},
2 => {
'BG' => 1,
nil => 1
}
}
input = {
[0, "AR"]=>2,
[0, nil]=>1,
[0, "AQ"]=>6,
[1, nil]=>4,
[1, "AQ"]=>3,
[2, "BG"]=>1,
[2, nil]=>1,
}
result = {}
input.each do |k, v|
if result[k[0]]
result[k[0]].merge!({ k[1] => v })
else
result[k[0]] = { k[1] => v }
end
end
puts result
#{0=>{"AR"=>2, nil=>1, "AQ"=>6}, 1=>{nil=>4, "AQ"=>3}, 2=>{"BG"=>1, nil=>1}}
I think this is not the most succinct way, I hope some advice!
hash = {
[0, "AR"]=>2,
[0, nil]=>1,
[0, "AQ"]=>6,
[1, nil]=>4,
[1, "AQ"]=>3,
[2, "BG"]=>1,
[2, nil]=>1,
}
new_hash = {}
hash.each{|k, v| new_hash[k[0]] ||= {}; new_hash[k[0]].merge!({k[1] => v})}
puts new_hash # {0=>{"AR"=>2, nil=>1, "AQ"=>6}, 1=>{nil=>4, "AQ"=>3}, 2=>{"BG"=>1, nil=>1}}
Here is one more very similar to previous answers but with using of #each_with_object:
hash = {
[0, "AR"]=>2,
[0, nil]=>1,
[0, "AQ"]=>6,
[1, nil]=>4,
[1, "AQ"]=>3,
[2, "BG"]=>1,
[2, nil]=>1,
}
result_hash = Hash.new { |h,k| h[k] = {} }
hash.each_with_object(result_hash) do |((parrent_key, key), value), res|
res[parrent_key].merge!(key => value)
end
=> {0=>{"AR"=>2, nil=>1, "AQ"=>6}, 1=>{nil=>4, "AQ"=>3}, 2=>{"BG"=>1, nil=>1}}
I came up with an answer that doesn't require additional variable assignments in its enclosing scope (it has "referential transparency": https://en.wikipedia.org/wiki/Referential_transparency)
input
.group_by { |(arr, num)| arr.first }
.each_with_object(Hash.new) do |(key, vals), hsh|
vals.each do |((key, innerkey), innerval)|
hsh[key] ||= {}
hsh[key][innerkey] = innerval
end
hsh
end
# {0=>{"AR"=>2, nil=>1, "AQ"=>6}, 1=>{nil=>4, "AQ"=>3}, 2=>{"BG"=>1, nil=>1}}
Two high-level steps:
I noticed the output object is grouped by the first array element (here, 0/1/2). I use #group_by to create a hash with that structure.
# output of `#group_by` on first array element:
key: 0, vals: [ [[0, "AR"], 2], [[0, nil], 1], [[0, "AQ"], 6] ]
key: 1, vals: [ [[1, nil], 4], [[1, "AQ"], 3] ]
key: 2, vals: [ [[2, "BG"], 1], [[2, nil], 1] ]
I use #each_with_object to construct the nested hashes. For each vals array above, I extracted the second and third values by destructuring the arrays in the block parameter (((key, innerkey), innerval)) and then the hash assignment was straightforward.
I need to pack if there are at least two adjacent numbers which are same in the format <number : number_of_occurrences >.
This is my input:
[2,2,2,3,4,3,3,2,4,4,5]
And the expected output:
"2:3,3,4,3:2,2,4:2,5"
So far I tried:
a = [1, 1, 1, 2, 2, 3, 2, 3, 4, 4, 5]
a.each_cons(2).any? do |s , t|
if s == t
If it's equal try a counter maybe, but thats not working.
You can use Enumerable#chunk_while (if you're on Ruby >= 2.3):
a.chunk_while { |a, b| a == b }
.flat_map { |chunk| chunk.one? ? chunk.first : "#{chunk.first}:#{chunk.size}" }
.join(',')
#=> "2:3,3,4,3:2,2,4:2,5"
You can also use Enumerable#chunk (Ruby ~1.9.3, maybe earlier):
a.chunk(&:itself)
.flat_map { |_, chunk| chunk.one? ? chunk.first : "#{chunk.first}:#{chunk.size}" }
.join(',')
#=> "2:3,3,4,3:2,2,4:2,5"
You could chunk elements together when they're equal, you could also slice the array between elements that are distinct (slice_when has been added in Ruby 2.2 ):
[2, 2, 2, 3, 4, 3, 3, 2, 4, 4, 5].slice_when { |a, b| a != b }.map do |ints|
if ints.size == 1
ints[0]
else
"#{ints[0]}:#{ints.size}"
end
end.join(',')
# "2:3,3,4,3:2,2,4:2,5"
It's mostly a matter of taste, both methods can achieve perfectly similar results, just like select and reject.
arr = [2, 2, 2, 3, 4, 3, 3, 2, 4, 4, 5]
arr.drop(1).each_with_object([[arr.first, 1]]) do |e,a|
a.last.first == e ? a[-1][-1] += 1 : a << [e, 1]
end.map { |a| a.join(':') }.join(',')
#=> "2:3,3:1,4:1,3:2,2:1,4:2,5:1"
I want to create a hash from the result of a database query such that its keys are the column values except the last one, and the value are the last column value (or a default value). For example, if I have rows:
1 2 3 1
1 2 4 9
1 3 2 nil
and a default value of 111, I should get:
{
1 =>
{
2 => { 3 => 1, 4 => 9},
3 => { 2 => 111}
}
}
I want to make the method generic enough to handle an arbitrary number of columns, so the signature could be:
to_lookup(rows, default_value, value_column, *columns)
How would I go about that?
Update: forgot a comma in the output.
[Edit: after reading #cthulhu's answer, I think I may have misinterpreted the question. I assumed that consecutive rows were to be grouped, rather than all rows to be grouped. I will leave my answer for the former interpretation.]
I believe this is what you are looking for:
def hashify(arr)
return arr.first.first if arr.first.size == 1
arr.slice_when { |f,s| f.first != s.first }.
each_with_object({}) do |a,h|
key, *rest = a.transpose
h[key.first] = hashify(rest.transpose)
end
end
hashify [[1, 2, 3, 1], [1, 2, 4, 9], [1, 3, 2, nil]]
#=> {1=>{2=>{3=>1, 4=>9}, 3=>{2=>nil}}}
hashify [[1, 2, 3, 1], [1, 2, 4, 9], [2, 3, 2, nil]]
#=> {1=>{2=>{3=>1, 4=>9}}, 2=>{3=>{2=>nil}}}
Replacing nil with the default can be done before or after the construction of the hash.
Enumerable#slice_when was bestowed upon us in v2.2. For earlier versions, you could replace:
arr.slice_when { |f,s| f.first != s.first }
with
arr.chunk { |row| row.first }.map(&:last)
I simplified things by removing the ability to pass a default,
I also simplified the signature method to have only one parameter.
RSpec.describe "#to_lookup" do
def to_lookup(rows)
return rows.first.first if rows.flatten.size == 1
h = {}
rows.group_by { |e| e.first }.each_entry do |k, v|
v.each &:shift
h[k] = to_lookup(v)
end
h
end
let :input do
[
[1, 2, 3, 1],
[1, 2, 4, 9],
[1, 3, 2, 111],
]
end
let :output do
{
1 => {
2 => {3 => 1, 4 => 9},
3 => {2 => 111}
}
}
end
it { expect(to_lookup(input)).to eq(output) }
end
BTW I wonder what output do you want for following input:
1 2 3 1
1 2 3 2
EDIT: working code snippet: http://rubysandbox.com/#/snippet/566aefa80195f1000c000000
Let's say I have a Ruby array.
[1,2,3,4,4,5,6,6,7,7]
I want to find the values that occur 2 or more times.
[4,6,7]
It will help my process to first determine which items occur only once then remove those. So I'd like to solve this by first finding the items that occur once.
There are probably better ways, but this is one:
> [1,2,3,4,4,5,6,6,7,7].group_by{|i| i}.reject{|k,v| v.size == 1}.keys
=> [4, 6, 7]
Breaking it down:
> a = [1,2,3,4,4,5,6,6,7,7]
=> [1, 2, 3, 4, 4, 5, 6, 6, 7, 7]
> a1 = a.group_by{|i| i}
=> {1=>[1], 2=>[2], 3=>[3], 4=>[4, 4], 5=>[5], 6=>[6, 6], 7=>[7, 7]}
> a2 = a1.reject{|k,v| v.size == 1}
=> {4=>[4, 4], 6=>[6, 6], 7=>[7, 7]}
> a2.keys
=> [4, 6, 7]
Everyone loves a really difficult to follow one liner :)
[1,2,3,4,4,5,6,6,7,7].each_with_object(Hash.new(0)) { |o, h| h[o] += 1 }.select { |_, v| v > 1 }.keys
Add some white space and some comments
[1,2,3,4,4,5,6,6,7,7].each_with_object(Hash.new(0)) { |o, h|
h[o] += 1
}.select { |_, v|
v > 1
}.keys
Enumerate and pass in our memo hash to each iteration the Hash defaults to having 0 for any key
Increment counter for the object
Select only key value pairs where the value is greater than 1
Grab just the keys
This looks quite similar to Phillip's neat answer - in theory this should use slightly less memory as it will not have to build the intermediate arrays to perform counting
Another way:
a = [1,2,3,4,4,5,6,6,7,7]
au = a.uniq
a.reject { |i| au.delete(i) }
#=> [4, 6, 7]
If efficiency is important, you could use a set:
require 'set'
s = Set.new
a.reject { |e| s.add?(e) }
#=> [4, 6, 7]
You can use Array#select to return the elements where Array#count is greater than 1:
2.1.2 :005 > arr = [1,2,3,4,4,5,6,6,7,7]
=> [1, 2, 3, 4, 4, 5, 6, 6, 7, 7]
2.1.2 :006 > arr.select { |e| arr.count(e) > 1 }.uniq
=> [4, 6, 7]
Hope this helps