From: Garance A Drosehn <drosihn@...>
Date: 2005-07-16T11:08:57+09:00
Subject: Cascading <=> comparisons

Let's say I have a hash with some values in it, and I want to
print out that hash in some sort order.  I can:

korder = vals.keys.sort { |ka, kb|
    vals[kb] <=> vals[ka]
}
korder.each { |key| printf " %3d - %s\n", vals[key], key }

That's pretty easy, but what if I want to cascade comparisons
in that sort, such that if the data-values are equal, then I want
the sort order to be based on some other comparison?  It
seems to me that I have to:

korder = vals.keys.sort { |ka, kb|
    sres = vals[kb] <=> vals[ka]
    sres = ka <=> kb    if sres == 0
    sres
}
korder.each { |key| printf " %3d - %s\n", vals[key], key }

In perl, I can just casade the '<=>' comparisons, because it
will treat the zero value as "false".  Thus, I could get away
with a one-liner somewhat similar to:

vals[kb] <=> vals[ka] or a <=> kb

but that doesn't work in ruby.  Is there some other way I could
collapse multiple <=> comparisons into a single statement?
I do not mind the multi-statement form (now that I've convinced
myself that I can't cascade them the way I could in perl), but I'm
just wondering if there's a better way to do it.

-- 
Garance Alistair Drosehn     =             drosihn@gmail.com
Senior Systems Programmer               or   gad@FreeBSD.org
Rensselaer Polytechnic Institute;             Troy, NY;  USA