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Re: Please Help Eliminate My Ignorance

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As you can see in tutorial/OperatorInputForms (documentation center)
SetDelayed (:=) has a higher precedence than CompundExpression  (;).

So test[p_] := x = 1; Append[p, p[[4]]];  is interpreted as (test[p_] :=
 x = 1); Append[p, p[[4]]];
Therefore the definition includes only the first part. If you want to
include the second as well you need to use parenthesis:

test[p_] := (x = 1; Append[p, p[[4]]])

Cheers -- Sjoerd

 Mathematica questions also answered at Stackoverflow:
http://stackoverflow.com/questions/tagged/mathematica

> -----Original Message-----
> From: Michael Mandelberg [mailto:mmandelberg@comcast.net]
> Sent: Tuesday, 3 May 2011 05:24
> To: 'Sjoerd C. de Vries'
> Subject: RE: Please Help Eliminate My Ignorance
>
> Thanks.  So how do I write a function that does both things?
>
> Michael
>
> -----Original Message-----
> From: Sjoerd C. de Vries [mailto:sjoerd.c.devries@gmail.com]
> Sent: Monday, May 02, 2011 4:29 PM
> To: Michael Mandelberg
> Subject: Re: Please Help Eliminate My Ignorance
>
> Hi Michael,
>
> In 1. because of SetDelayed (:=) the Part ([[ ]]) statement is not
executed
> directly. It lays dormant until test is called. In 2. Append and the Part
contained
> in it are executed immediately. Here the problem with p being not  a List
that
> can be indexed directly leads to an error message. If you call test in 1.
with a
> argument that is not a list you get the same error message.
>
>
> Cheers -- Sjoerd
>
> Mathematica questions also answered at Stackoverflow:
> http://stackoverflow.com/questions/tagged/mathematica
>
> On May 2, 12:52 pm, Michael Mandelberg <mmandelb...@comcast.net>
> wrote:
> > Why does Mathematica like the first, but not the second?
> >
> > 1.
> > Clear[x, testp, p];
> > test[p_] :=  Append[p, p[[4]]];
> >
> > 2.
> > Clear[x, testp, p];
> > test[p_] := x = 1; Append[p, p[[4]]];
> >
> > Part::partd: Part specification p[[4]] is longer than depth of object.
> >
> >
> >
> > Append::normal: Nonatomic expression expected at position 1 in
> > Append[p,p[[4]]]. >>
>
>

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Re: Please Help Eliminate My Ignorance "Sjoerd C. de Vries" <sjoerd.c.devries@gmail.com> - 2011-05-03 09:48 +0000

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