Re: Scoping, named patterns, local vs global variables
- To: mathgroup at smc.vnet.net
- Subject: [mg44884] Re: Scoping, named patterns, local vs global variables
- From: drbob at bigfoot.com (Bobby R. Treat)
- Date: Fri, 5 Dec 2003 05:31:53 -0500 (EST)
- References: <bqmqus$r95$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
You're missing the difference between Set (=) and SetDelayed (:=). If you define f[x_] using Set, it uses the current value of x, because that's what Set does. If you use SetDelayed, you get the behavior you want. Bobby frankeye at cox.net (Frank Iannarilli) wrote in message news:<bqmqus$r95$1 at smc.vnet.net>... > Hi, > > > For context, refer to a 1999 group posting by Ted Ersek: > > http://groups.google.com/groups?q=set+scoping+group:comp.soft-sys.math.mathematica&hl=en&lr=&ie=UTF-8&group=comp.soft-sys.math.mathematica&selm=7kho2v%24qmt%40smc.vnet.net&rnum=1 > > Summary punchline: > > x=1; > f[x_]=x^2; > In: f[3] > Out: 1 (and not 3^2=9, since x is global) > > > I'm sure I'm not alone in periodically getting burned by this > behavior. One would think in Mathematica, the named pattern on the lhs would > behave as a locally scoped variable for the rhs, but not so if there's > a global variable of the same name lurking within the session. > > > For the long-timers, this is old news. But then, why does this actual > behavior conflict with Wolfram's own longstanding documentation in The > Book, as the following snippets demonstrate? (Perhaps revisions in > The Book are warranted in the next edition!) > > Quoting from The Book for Mathematica version 5: > > In 2.7.4 Advanced Topic: Variables in Pure Functions and Rules: > > "Module and With allow you to give a specific list of symbols whose > names you want to treat as local. In some situations, however, you > want to automatically treat certain symbol names as local. > > For example, if you use a pure function such as Function[{x}, x + a], > you want x to be treated as a "formal parameter", whose specific name > is local. The same is true of the x that appears in a rule like f[x_] > -> x^2, or a definition like f[x_] := x^2. > > Mathematica uses a uniform scheme to make sure that the names of > formal parameters which appear in constructs like pure functions and > rules are kept local, and are never confused with global names." > > ..... then further on: > > "{lhs = rhs  and  lhs := rhs , local pattern names > }" > > .... then further still: > > > "Mathematica treats transformation rules as scoping constructs, in > which the names you give to patterns are local. You can set up named > patterns either using x_, x__ and so on, or using x:patt. " > > > ====>>> Is there a proper "Mathematica way" of interpreting the above > to square with actual behavior? > > Just wondering... > Cheers!