Let say I have an array
[1, 2, 3, 4, 3, 2]
I wonder how I can get the hash { 1 => 0, 4 => 3 } (value => index). I need the index of the earlier occurrence.
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2What do you mean "I need index of earlier occurence"? if you need only UNIQUE elements, they they only occur once.Tamer Shlash– Tamer Shlash2015-09-09 14:49:35 +00:00Commented Sep 9, 2015 at 14:49
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2 Answers
Could also try this
unique = array.select { |a| array.count(a) === 1 }
indexes = unique.map { |u| array.index(u) }
Hash[unique.zip(indexes)]
My algorithm is to first generate a new array of the single elements. I do that by filtering the array for the one's where the count is equal to 1. We can use the array#count method.
I then create a second array, where I map the values of the first array, to their indexes in the original array.
I then combine the values using the array#zip function and convert to a hash with Hash.
Output
{ 1 => 0, 4 => 3 }
3 Comments
Tamer Shlash
This is very expensive, complexity of O(n^2)
Richard Hamilton
Understood, that's why I edited the post. Thanks for pointing this out
engineersmnky
As a note you could cut this down one line by using
Hash[unique.map{ |u| [u,array.index(u)]}] rather than map and then zip just zip them in the mapping. OR even a 1 liner would be Hash[array.select{|a| array.count(a) == 1 }.map{ |u| [u,array.index(u)]}]a.each_with_index.with_object({}) { |(e, i), h| h.key?(e) ? h[e] = nil : h[e] = i }
.reject! { |k, v| v.nil? }
5 Comments
sawa
For readability, perhaps you should use some other value (like
nil) instead of -1.sawa
You can also use
.each_with_index.with_object({}){|(e, i), h| to make it look better.engineersmnky
rather than overwriting duplicate values then rejecting them why not avoid inserting them at all e.g.
a.each_with_index.with_object({}) { |(e, i), h| h[e] = i if a.count(e) == 1 }
Stefan
you can put the assign first, i.e.
h[e] = h.has_key?(e) ? nil : i and instead of rejecting nil values, you could select non-nil values using select { |k, v| v }Stefan
note that
reject! returns nil if nothing was rejected, you have to use the non-bang methods (either reject or select)