Ruby sorting based on an if statement

Question

I am trying to sort an array of words in Ruby 1.8.7 by an insane set of rules. This can be simplified to wanting to sort case-insensitive .sort_by{|a| a.downcase} then re-sort case-sensitive but only if the 2 strings are the same.

I would think you could just .downcase compare only the strings that are equal and then sort just those, sending 0 for the rest. But no, that isn't working for me.

Here's what I have:

["A", "a", "a.", "A.", "Ba", "ba"].sort_by{|a| a.downcase}.sort{|a,b| a.downcase==b.downcase ? a<=>b : 0 }

Desired output:

["A", "a", "A.", "a.", "Ba", "ba"]

Thanks for your help.

mu is too short · Accepted Answer · 2016-06-23 20:52:34Z

4

If a.downcase == b.downcase then you want to sort using a case-sensitive comparison, right? That really means that you want to sort by the pair [a.downcase, a] and arrays in Ruby compare element by element so you just want:

array.sort_by { |s| [ s.downcase, s ] }

Suppose sort_by is comparing two arrays a1 and a2 to see which one goes first. That's just an a1 <=> a2 call. If a1[0] != a2[0] then the spaceship operator won't have to look at the second elements of the arrays to break a tie because there is no tie and <=> can return +1 or -1 right away. If a1[0] == a2[0] then a1[0] <=> a2[0] is 0 and the spaceship has to look at the second elements to see how to break the tie (or leave the tie as-is if a1 == a2). That sounds like the comparison logic you're after.

edited Jun 23, 2016 at 20:52

answered Jun 23, 2016 at 20:45

mu is too short

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3 Comments

Cary Swoveland Over a year ago

Where 𝝁 says " ...and arrays in Ruby compare element by element..." that is explained more fully in the doc for Arrays#<=>. To carry this one step further, If you (the OP) wanted to sort first on s downcased, then sort ties on the length of s, longest first, then sort ties of both s.downcase and s.size by s, you could sort on the array [s.downcase, -s.size, s].

lwassink Over a year ago

Using an array to sort by multiple conditions is an idea I hadn't seen before. This is very helpful.

paulguy Over a year ago

This is a great answer to the question. Exactly the answer I needed for the question I posed, so thank you. I have to add that I'm torn because I posted this to try to fix my a.downcase==b.downcase approach, instead your answer made me perfectly rethink my question in the way I needed to get the job done, so I will happily give you the checkmark. Unfortunately, now this question feels like a duplicate to all these: google.com/#q=ruby+sort+by+multiple+site:stackoverflow.com and for that I apologize to anyone who eventually marks this question as a duplicate.

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