Re: Returning a local array

*To*: mathgroup at smc.vnet.net*Subject*: [mg84226] Re: Returning a local array*From*: Art <grenander at gmail.com>*Date*: Fri, 14 Dec 2007 07:04:07 -0500 (EST)*References*: <fjqj9l$6l9$1@smc.vnet.net> <fjr2ra$53j$1@smc.vnet.net>

Hello, thanks for your reply, On Dec 13, 2:51 am, Albert Retey <a... at arcor.net> wrote: > Hi, > > > I would like to return a local array. The below seems like the wrong > > way to do it: > > > f[] := Module[{h}, h["a"] = 1; h["b"] = 2; h]; > > g = f[]; > > > I would like g not to be associated with the local variable h, but > > equivalent to defining it as follows: > > > g["a"] = 1; g["b"] = 2; > > What is wrong with this approach? While the actual rules are defined as > Downvalues for h (or, since it is local something like h$456) > setting g to this h makes things like: > > g["a"] > > work as (probably) expected. What is your specific problem with this > approach? The only thing you would probably need is an explicit delete > to remove the local symbol when not needed anymore, but this is only > necessary if you really need to avoid "memory leaks"... For the purely practical reason that ?g display all values for the array. If g is a reference to the local variable, these values are not displayed. I would be surprised if the local variable is not garbage collected when dereferenced. Is there really a 'memory leak' in this case? > > Another thing you could do is to supply the symbol as an argument like: > > f[v_]:=(v["a"]=1;v["b"]=2;v) > > and then use: > > f[g] > > If you don't like the syntax, you can make that approach look like the > other by using UpValues: > > f/:(v_=f[]):=(v["a"]=1;v["b"]=2;v) > Thanks, I used your first suggestion in addition to SetAttributed[f, HoldFirst] as Jens-Peer suggested. I am assuming this is how functions such as AppendTo work. > > Is it better to work with rules instead? It can become awkward and I > > would like to avoid it. > > > f[] := Module[{h}, h={"a" -> 1, "b" -> 2}; h]; > > g = f[]; > > "a" /. g > > This is probably a way which is more often seen in Mathematica code. > Anyway there are counterexamples and your above strategy for "misusing" > the pattern matcher to construct data structures is used elsewhere, too, > and I think I have seen it used even within parts of Mathematica's own > standard libraries... > > Note that you could also define a supporting function which makes the > access look more familiar: > > get[rules_,attr_]:=attr/.rules; > > get[g,"a"] I looked at Maeder's Classes.m which is a more elaborate illustration of how to do what you suggested. I don't see this kind of code used very much. > > > The specific problem is I am reading the header for a binary file that > > has 20 or so parameters describing the binary file. I thought this > > would be the analog of a structure in other languages and a good way > > to put these parameters in a variable. > > actually I think of this as a lookup table, dictionary, hashtable or > whatever it is called in other languages and use constructs like that > all the time. > > hth, > > albert Thanks, Art.