From: matt@... (matt neuburg)
Date: 2009-03-28T11:46:38+09:00
Subject: Re: Sorting decimal index numbers?

Aaron Vegh <aaron@vegh.ca> wrote:

> Hi there,
> I'm working with a list of items that contain decimal-based index
> numbers. I'm sorry I don't know the term for these kinds of numbers, and
> I'm having trouble finding out how to sort them properly. To wit:
> 
> 1.1
> 1.10
> 1.2
> 1.3
> 1.4
> 1.5
> 1.6
> 1.7
> 1.8
> 1.9
> 
> When the index numbers are float or strings, they sort like this. But I
> want "1.10" to be the last number. Any idea how to put these guys in the
> right order?

These are not numbers; they're strings. Or at least, they need to be; if
they weren't, 1.1 and 1.10 would be indistinguishable. So let's assume
they *are* strings. Then what you're asking to do is to compare the two
halves of each string as an integer. Take 1.1 and 2.1, for instance;
they compare as 1 and 2 would compare as integers. Very well, now take
1.10 and 1.2, for instance. 1 and 1 are the same, so 10 and 2 need to
compare as the integers 10 and 2. So:

arr = %w{
1.1
1.10
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
}
arr = arr.sort do |x,y|
  x1, x2 = x.split(".")
  y1, y2 = y.split(".")
  if x1 != y1
    x1.to_i <=> y1.to_i
  else
    x2.to_i <=> y2.to_i
  end
end
p arr

If there are a lot of these, however, it would be much better to do a
keyed sort using sort_by. How to create keys depends on what you know
about the data. If we knew for a fact that the second half was always
between 1 and 99, it would be easy:

arr = arr.sort_by do |x|
  x1, x2 = x.split(".")
  x1.to_i * 100 + x2.to_i
end

If the second half can be any length, though, you'll have to determine
the maximum length first, and change that "100" multiplier accordingly.

m.


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