From: John Tromp <tromp@...>
Date: 2002-10-09T19:31:25+09:00
Subject: strange Hash default behaviour

I wrote a ruby program to read a list of graph edges and produce
an adjacency list using a Hash. The graph edges look like
p12 -- p31
I hoped to write this as

nbs = Hash.new([])
while line = gets
  if line =~ /p(\d+) -- p(\d+)/
    u, v = $1.to_i, $2.to_i
    nbs[u] << v
    nbs[v] << u
  end
end

but unfortunately, the default value [] for the Hash is shared by all
elements, rather than copied, as I expected.

I was wondering if this was a conscious design decision, or an oversight.
I finally ended up with the much uglier

nbs = Hash.new()
while line = gets
  if line =~ /p(\d+) -- p(\d+)/
    u, v = $1.to_i, $2.to_i
    nbs[u] = [] unless nbs.has_key?(u)
    nbs[u] << v
    nbs[v] = [] unless nbs.has_key?(v)
    nbs[v] << u
  end
end

Is there another way to avoid the explicit tests?

regards,

%!PS                       %  -John Tromp (http://www.cwi.nl/~tromp/)
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