Re: The uses of Condition (/;)

*To*: mathgroup at smc.vnet.net*Subject*: [mg82974] Re: The uses of Condition (/;)*From*: Szabolcs Horvát <szhorvat at gmail.com>*Date*: Mon, 5 Nov 2007 05:05:40 -0500 (EST)*References*: <fghcbl$k9n$1@smc.vnet.net> <fgk9sk$pmk$1@smc.vnet.net>

magma wrote: > Usage C is described in the Mathematica 6 documentation under Condition - More > information (and nowhere else I think) Thanks for the reply! Yes, that's what I meant by hidden: it's hidden under "More information". I thought that using /; with Module is completely different from the other two uses, therefore it would deserve an explanation at a more prominent place in the documentation. But Ingolf Dahl's and your examples showed that this is not the case---in fact, B and C are sub-cases of the same kind of usage. My confusion came from not realizing that /; has a higher precedence than ;. Szabolcs > in the form > > lhs:=Module[{vars},rhs/;test] > > They say that "this usage allows local variables to be shared between > test and rhs " > On the same Documentation page , under Scope, there is a concrete > example of usage C. > > f[x_] := Module[{u}, u^2 /; ((u = x - 1) > 0)] > > here it is seen that the pattern (u) is a local variable. > In your examples you were using x, a global variable, so no difference > was apparent. > In C, condition is evaluated before definition, for example (please > make sure you have Remove[f] before typing this: > > f[x_] := Module[{u}, (Print [u]; u^2) /; ((u = x - 1) > 0)] > f[3] > 2 > 4 > u only acquires a value after cond is evaluated and then def (Print > [u]; u^2) will be evaluated. > A and B are very similar, but A is considered a bit faster, because > evaluation stops immediately if the cond is not safisfied