Re: Re: dictionaries?
- To: mathgroup at smc.vnet.net
- Subject: [mg20538] Re: [mg20508] Re: dictionaries?
- From: Daniel Reeves <dreeves at eecs.umich.edu>
- Date: Sat, 30 Oct 1999 00:13:55 -0400
- Sender: owner-wri-mathgroup at wolfram.com
since the topic came up, just thought I'd share this (a way to get a list
of the keys for a dictionary):
keys[hash_] :=
DownValues[hash] /. {RuleDelayed -> (ReleaseHold[#] &), hash -> (# &)}
-- -- -- -- -- -- -- -- -- -- -- --
Daniel Reeves http://ai.eecs.umich.edu/people/dreeves/
"Beware of bugs in the above code; I have only proved it correct, not
tried it."
-- Donald Knuth
----- FROM Hans Staugaard ON 99.10.27 02:05 (Today) -----
> Joe Strout wrote:
> >
> > In Python, there's an extremely handy datatype called a "dictionary"
> > which implements a many-to-1 mapping between keys (which can be any
> > hashable type) and values (which can be anything). E.g.:
> >
> > > d = {"one":1, "two":2}
> > > d["one"]
> > 1
> > > d["three"]
> > (key not found)
> >
> > I'm looking for something similar in Mathematica -- it's a great way to
> > organize lists of parameters, without having to worry about the order
> > in which they are listed.
> >
> > I've searched the online help and flipped through the Mathematica book,
> > but I can't find anything like this. Have I missed something?
> >
> > Thanks,
> > -- Joe
> >
> > --
> > ,------------------------------------------------------------------.
> > | Joseph J. Strout Biocomputing -- The Salk Institute |
> > | joe at strout.net http://www.strout.net |
> > `------------------------------------------------------------------'
> > Check out the Mac Web Directory! http://www.strout.net/macweb.cgi
>
> You could just say
>
> d["one"]=1;
> d["two"]=2;
> ...
>
> or if you have a list like this
>
> dl={"one",1,"two",2,"three",3}
>
> you could do like this
>
> Set[d[#1],#2]&@@@Partition[dl,2]
>
> If you want the function to alert you when there is no key found, you
> could, in both cases, add:
>
> d::nokey="Key not found"
> d[_]:=Message[d::nokey]
>
> Hope that helps
>
> Hans
>