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Groups > comp.soft-sys.math.mathematica > #3831

Re: Replace, test question

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Here are two syntactic forms with essentially the same meaning in this  
context (and which give the desired output):

{58, 61, 15, 66, 10, 2, 24, 81, 45, 84} /. x_?PrimeQ -> P

{58, 61, 15, 66, 10, 2, 24, 81, 45, 84} /. x_ /; PrimeQ[x] -> P

You seem to have a hybrid of these, which itself has a different meaning:  
for each x, PrimeQ is evaluated; since PrimeQ on its own evaluates to  
itself, the pattern does not match anywhere.

On Mon, 18 Jul 2011 11:15:10 +0100, PA32R <tio540s1@gmail.com> wrote:

> I'm very new to Mathematica and trying to work my way through it. I
> don't understand why:
> {58, 61, 15, 66, 10, 2, 24, 81, 45, 84} /. x_ /; PrimeQ -> P
> returns:
> {58, 61, 15, 66, 10, 2, 24, 81, 45, 84}
>
> I would expect it to take the list, replace everything in the list
> that matches any pattern and passes the PrimeQ test, i.e., is prime,
> with P to yield:
> {58, P, 15, 66, 10, P, 14, 81, 45, 84}
>
> Can someone explain my misunderstanding?
>
> Thanks.

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Thread

Replace, test question PA32R <tio540s1@gmail.com> - 2011-07-18 10:15 +0000
  Re: Replace, test question Peter Breitfeld <phbrf@t-online.de> - 2011-07-19 11:06 +0000
  Re: Replace, test question "Oleksandr Rasputinov" <oleksandr_rasputinov@hmamail.com> - 2011-07-19 11:11 +0000
  Re: Replace, test question Yves Klett <yves.klett@googlemail.com> - 2011-07-19 11:12 +0000
    Re: Replace, test question PA32R <tio540s1@gmail.com> - 2011-07-20 10:39 +0000

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