Sort a string array based on a corresponding integer array

Question

Say I'm given a string array, each string has a corresponding weight. So for example:

str_arr = ["james", "blake", "rob"]
weights = [4, 7, 1]

str_arr[i] corresponds to weights[i].

How would I go about sorting str_arr in ascending order with respect to weights? For example:

(after sorting) weights would be [1, 4, 7] which corresponds to str_arr = ["rob", "james", "blake"]

How do I get ["rob", "james", "blake"]?

Most sorting comparators deal with modifying the way the sorting on the current array works, and I looked into sorting with respect to the existing order of a different array--but I'm not sure how to sort the integer array and simultaneously do the same with the str_arr based on the output of the integer array...Is there a Rubyist way of doing this?

The Ruby way would be using a Hash instead of arrays {james: 4, blake: 7, ..} , sort on the weight value and use the the .keys method to have your array — peter
– peter, Commented Mar 17, 2017 at 23:20

Stefan · Accepted Answer · 2017-03-17 23:36:15Z

2

You could use sort_by and with_index:

str_arr.sort_by.with_index { |_, i| weights[i] }
#=> ["rob", "james", "blake"]

answered Mar 17, 2017 at 23:36

Stefan

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Cary Swoveland Over a year ago

I've seen it (used at least once by @ArupRakshit, if memory serves) and was trying to remember it... Yes, the way to go.

Sagar Pandya · Accepted Answer · 2017-03-17 23:19:17Z

1

You can do the following with parallel assignment:

str_arr, weights = str_arr.zip(weights).sort_by(&:last).transpose
#=> [["rob", "james", "blake"], [1, 4, 7]]

str_arr #=> ["rob", "james", "blake"] 
weights #=> [1, 4, 7]

Key methods: Array#zip, Enumerable#sort_by and Array#transpose.

answered Mar 17, 2017 at 23:19

Sagar Pandya

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3 Comments

segue_segway Over a year ago

This wouldn't work in the case that one of the weights was missing, correct?

Eric Duminil Over a year ago

Reverse the order, and you don't need sort_by : weights.zip(str_arr).sort.transpose

Sagar Pandya Over a year ago

@Sunny not sure what you mean. How would an array with a missing weight look?

Cary Swoveland · Accepted Answer · 2017-03-17 23:30:16Z

1

str_arr.values_at(*weights.size.times.sort_by { |i| weights[i] })
  #=> ["rob", "james", "blake"]

The two steps are as follows.

a = weights.size.times.sort_by { |i| weights[i] }
  #=> [2, 0, 1]
str_arr.values_at(*a)
  #=> ["rob", "james", "blake"]

See Array#values_at and Enumerable#sort_by.

answered Mar 17, 2017 at 23:30

Cary Swoveland

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Eric Duminil Over a year ago

O(n*log(n)) is boring. Here's a nice O(n!) solution : str_arr.permutation.to_a[weights.permutation.to_a.index(weights.sort)] ;)

Eric Duminil · Accepted Answer · 2017-03-17 23:31:28Z

1

If you have two arrays that are 100% linked to each other, you should use a Hash :

str_arr = ["james", "blake", "rob"]
weights = [4, 7, 1]

weights = str_arr.zip(weights).to_h
# {"james"=>4, "blake"=>7, "rob"=>1}

You can just sort_by value then :

str_arr = ["james", "blake", "rob"]
weights = [4, 7, 1]

weights = str_arr.zip(weights).to_h

p weights.sort_by{|k, v| v}.map(&:first)
#=> ["rob", "james", "blake"]

answered Mar 17, 2017 at 23:31

Eric Duminil

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