Re: Forcing Argument Evaluation
- To: mathgroup at smc.vnet.net
- Subject: [mg49891] Re: Forcing Argument Evaluation
- From: "Peltio" <peltio at twilight.zone>
- Date: Thu, 5 Aug 2004 09:21:26 -0400 (EDT)
- References: <ceqtgk$js9$1@smc.vnet.net>
- Reply-to: "Peltio" <peltioNOSP at Mdespammed.com.invalid>
- Sender: owner-wri-mathgroup at wolfram.com
"Scott Guthery" wrote
>f[x_] := x^2; a = {f,f/2};
a[[1]][2] gives 4
a[[2]][2] gives f/2[2]
>I know I'm missing something fundamental.
The source of the problem, here, is that in Mathematica any expression can
have a head that is itself an expression .
expr = head[arg_sequence]
The head of the first expression is f and so head[2] matches f[x_].
The head of the second expression is f/2 and head[2] is Times[etc...], hence
no match for f[x_]. The evaluation of the head does not carry anywhere,
since it is done before the evaluation of the argument and of the expression
as a whole.
Of course, as you surely already know, you can operate on lists as you
wished by defining each function in a 'stand alone' manner:
a = { #^2 &, #^2/2 &};
a[[1]][2] gives 4
a[[1]][2] gives 2
You would also want to take a look at the older posts in the Mathgroup on
how to implement operator algebra in Mathematica.
Cheers,
Peltio
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