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Please Help Eliminate My Ignorance

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  Please Help Eliminate My Ignorance Michael Mandelberg <mmandelberg@comcast.net> - 2011-05-02 10:52 +0000
    Re: Please Help Eliminate My Ignorance "Sjoerd C. de Vries" <sjoerd.c.devries@gmail.com> - 2011-05-03 09:46 +0000

#2028 — Please Help Eliminate My Ignorance

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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#2053

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


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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