Re: Replace, test question

*To*: mathgroup at smc.vnet.net*Subject*: [mg120350] Re: Replace, test question*From*: "Oleksandr Rasputinov" <oleksandr_rasputinov at hmamail.com>*Date*: Tue, 19 Jul 2011 06:59:51 -0400 (EDT)*References*: <j0113e$qad$1@smc.vnet.net>

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 at 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.