Stable sort_by?

ruby-talk
1

I guess you have to use sort instead of sort_by in such a case,
like:

test = [[1, 'b'], [1, 'c'], [1, 'a'], [0, 'a']]

#sort for first item, than for second in reverse order
p test.sort{|a, b|(a[0] <=> b[0]).nonzero? || (b[1] <=> a[1]) }

=>[[0, "a"], [1, "c"], [1, "b"], [1, "a"]]

cheers

Simon

···

-----Original Message-----
From: list-bounce@example.com
[mailto:list-bounce@example.com] On Behalf Of Mike Fletcher
Sent: Tuesday, December 13, 2005 3:57 PM
To: ruby-talk ML
Subject: Re: stable sort_by?

Frederick Ros wrote:
> h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

OK, this or something like it should be in the rdoc for
sort_by. I was
doing something similar and wound up writing a similar sort with
multiple if tests like the OP because I didn't remember that Array
implements a sane <=>.

Also, would there be a similar cool way to do this in one
step where you
want a mix of ascending and descending sorts on different
parts? Say I
had a hash (in YAML):

Bobby:
  age: 11
  lastname: Smith
Suzy:
  age: 13
  lastname: Jones
Ted:
  age 12
  lastname: Smith

And I wanted to sort
  - alphabetically by lastname
  - then age oldest to youngest

So in Perl I'd do something like:

my @sorted_keys = sort { $a->{lastname} cmp $b->{lastname}
                                        >> $b->{age} <=> $a->{age} }
keys %data;

In this case I could use

sorted_keys = data.sort_by { |k,v| [ v["lastname"], -1*v["age"] ] }

But what if I wanted reverse alphabetically by lastname
(Z-A)? In perl
I'd swap $a and $b in the first comparison, but I can't think of a
spiffy way to do the same using sort_by.

--
Posted via http://www.ruby-forum.com/\.

2

Kroeger, Simon (ext) wrote:

I guess you have to use sort instead of sort_by in such a case,
like:

test = [[1, 'b'], [1, 'c'], [1, 'a'], [0, 'a']]

#sort for first item, than for second in reverse order
p test.sort{|a, b|(a[0] <=> b[0]).nonzero? || (b[1] <=> a[1]) }

=>[[0, "a"], [1, "c"], [1, "b"], [1, "a"]]

Aaah, it's chaining with nonzero? (since it'll return the -1 or 1 if
that's the case) that I was missing.

Thanks.

···

--
Posted via http://www.ruby-forum.com/\.