Re: Question on Positive[x]=True

*To*: mathgroup at smc.vnet.net*Subject*: [mg7890] Re: [mg7791] Question on Positive[x]=True*From*: dreissNOSPAM at nospam.earthlink.net (David Reiss)*Date*: Thu, 17 Jul 1997 15:35:50 -0400*Organization*: EarthLink Network, Inc.*Sender*: owner-wri-mathgroup at wolfram.com

In article <5qci9o$ejn$3 at dragonfly.wolfram.com>, seanross at worldnet.att.net wrote: > Raya Firsov-Khanin wrote: > > > > Dear Steve, > > > > I have the following question. I want to state that a variable > > x is positive. On page 1170 of the Third edition of The Mathematica book > > it is said that I can do it by making a definition > > > > Positive[x] = True > > > > However, while trying to do it I get a message : > > > > Set::write: Tag Positive in Positive[x] is Protected > > > > Any suggestions on how one can make variables positive, negative etc? > > > > Thanks a lot. > > Raya Khanin. > > University of Edinburgh > > Protected is an attribute which must be removed before modifying. > > Try > > Unprotect[Positive]; > Positive[x]=True; > Protect[Positive]; An alternative is to use an upvalue for x as in, Positive[x] ^= True -- David Reiss dreissNOSPAM at nospam.earthlink.net http://www.batnet.com/dreiss To send personal email, remove the words "nospam" and "NOSPAM" from the email address