# Better solution than returning a list of 3 values?

I have a function (using SetDelayed) that currently returns 3 values in a list. Later on I use the result of this list along with [[1]], [[2]], and [[3]] to use the values. Is there a way I can give each value a "name" of some sort, and return only one value in such a way that all these values can be accessed by name? (Coming from an object-oriented programming perspective, I just want to return a single object with a few fields/accessors.)

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You're already returning a single object. It's a List[] –  belisarius Oct 26 '12 at 23:00
@belisarius Yes, technically, but that's not helpful because using [[n]] makes it hard to see what's actually going on. –  jtbandes Oct 26 '12 at 23:06
yourF[x_]:={a[x],b[x],c[x]}; {myA, myB, myC} = yourF[x] –  belisarius Oct 26 '12 at 23:12
Can you give a small example? I have a hard time imagining what it is that you want to do. belisarius' answer is the obvious one, unless you intended something else. –  The Toad Oct 26 '12 at 23:12
Relevant (dupe?) questions: here, here and even maybe here –  István Zachar Oct 27 '12 at 9:09

Here are some options:

Lists of Rules

A simple option would be to return a list of rules:

$someone = {"name" -> "Fred", "gender" -> "Male", "age" -> 25};  Fields can then be extracted thus: "name" /.$someone
(* "Fred" *)

"age" /. $someone (* 25 *)  Wrapper Patterns A variation on this theme would be to define a pattern that represents a new value type: $person = person[name_, gender_, age_];

$someoneElse = person["Fred", "Male", 25];  Extracting fields is more verbose: $someoneElse /. $person :> name (* "Fred" *)  ... but it opens the possibility of extracting values computed from multiple fields: $someoneElse /. $person :> name ~~ " (" ~~ gender ~~ ")" (* "Fred (Male)" *)  Manually Defined Wrapper Accessors We could extend the previous example by writing "accessor functions" that access components of a wrapper: personName[$person] := name

personGender[$person] := gender personAge[$person] := age

personName @ $someoneElse (* Fred *) personAge @$someoneElse
(* 25 *)


Automatically Defined Wrapper Accessors

If we were going to define many such wrapper types, it would be convenient to automate the generation of the wrapper functions:

SetAttributes[assembleName, HoldAll]
assembleName[p_Symbol, s_Symbol] :=
Context[p]~~SymbolName[p]~~StringReplace[SymbolName[s], f_~~r___ :> ToUpperCase[f]~~r] //
Symbol

defineAccessors[f:w_[Verbatim[Pattern][_, Blank[]]..]] :=
Cases[f, Verbatim[Pattern][s_, Blank[]] :> (Hold[#[f], s] &@ assembleName[w, s])] /.
Hold[l:s_[___], r_] :> (l := r; s)


For example:

defineAccessors[movie[name_, year_, quote_]]
(* {movieName, movieYear, movieQuote} *)

randomMovie[] :=
RandomChoice @ {
movie["2001: A Space Odyssey",1968,"Watch out! He's got a bone!"]
, movie["Prometheus",2012,"Here, cobra, cobra... Gimme a hug!"]
, movie["Star Wars: The Phantom Menace",1999,"I say we nuke the JJB from orbit..."]
, movie["Firefly",2002,"...Sniff..."]
}

$someMovie = randomMovie[];$someMovie // movieName
(* "2001: A Space Odyssey" *)

$someMovie // movieYear (* 1968 *)$someMovie // movieQuote
(* "Watch out! He's got a bone!" *)

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Excellent...... –  Rojo Oct 27 '12 at 6:49

Just picking up three named return values:

{city, temperature, pressure} = {"London", 18, 1005};


Or using an inert object with functions defined on itself:

res = obj["London", 18, 1005]


obj["London", 18, 1005]

obj[a___]["city"] := obj[a][[1]]
obj[a___]["temperature"] := obj[a][[2]]
obj[a___]["pressure"] := obj[a][[3]]

res["city"]


"London"

res["temperature"]


18

res["pressure"]


1005

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I'm not saying I recommend this. It's prone to leaking memory

yourFunc[] := Module[{obj},
obj["this"] = "lala";
obj["that"] = 98;
obj];


So

bla = yourFunc[];
bla["this"]
bla["that"]


"lala"

98

Perhaps a better approach is passing the output variable to the function

SetAttributes[yourFunc2, HoldAll];
yourFunc2[out1_][arg_] := (out1["bla"] = 98; out1["blo"] = 98 - arg;
out1)


So

yourFunc2[x][23]

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Why is this prone to leaking memory? What could cause a leak there? –  jtbandes Oct 26 '12 at 23:45
Because the variable that actually "stores" the data is the temporary "obj" variable. So when you later clear bla, the data remains. Unless you get smart about it. –  Rojo Oct 26 '12 at 23:46
I'm giving alternative approaches to Sjoerd's, which is probably the best. –  Rojo Oct 26 '12 at 23:47