Re: using predefined expressions in functions [newbie question]

• To: mathgroup at smc.vnet.net
• Subject: [mg100212] Re: [mg100167] using predefined expressions in functions [newbie question]
• From: "David Park" <djmpark at comcast.net>
• Date: Thu, 28 May 2009 04:29:01 -0400 (EDT)
• References: <14105722.1243411978425.JavaMail.root@n11>

```I don't see how the VectorAnalysis package is yet used in your example.

Using SetDelayed means that the right hand side of the definition is not
evaluated and the definition never sees any of the left hand side arguments
on the right hand side. The arguments are useless and never get substituted.

R1 := {x1, y1, z1}
R2 := {x2, y2, z2}
f[x1_, y1_, z1_, x2_, y2_, z2_] = R1.R2

f[1, 2, 3, 4, 5, 6]
32

Perhaps it is just an example, but I don't see how this function does much.
Isn't it almost as easy to write {1, 2, 3}.{4, 5, 6}?

I often see questions on MathGroup where definitions are written in a
non-localized manner, where parts of the definition are written in separate
statements outside the principal definition. Although this will sometimes
work, it seems to be an invitation to trouble. It is better to keep the
definition together in one localized structure. I would tend to write your
definition as:

f[x1_, y1_, z1_, x2_, y2_, z2_] := {x1, y1, z1}.{x2, y2, z2}

or as:

f[x1_, y1_, z1_, x2_, y2_, z2_] :=
With[{R1 = {x1, y1, z1}, R2 = {x2, y2, z2}},
R1.R2]

David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/

From: ChangMo [mailto:nichthierwohne at gmail.com]

Hi,

I've been trying to write a function using some predefined
expressions, but the function assignment seems to understand the
symbols in the expressions as literals. eg.,

Needs["VectorAnalysis`"]
R1 := {x1,y1,z1}
R2 := {x2,y2,z2}
f[x1_,y1_,z1_,x2_,y2_,z2_] := R1 . R2

f[1,2,3,4,5,6]

gives the output
x1 x2 + y1 y2 + z1 z2
32 ( = 1*4 + 2*5 + 3*6 )

Can anyone help me with this?  Thanks for your attention.

-ChangMo

```

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