Re: Q: Union and SameTest Option

*To*: mathgroup at smc.vnet.net*Subject*: [mg16141] Re: [mg16016] Q: Union and SameTest Option*From*: "Tomas Garza" <tgarza at mail.internet.com.mx>*Date*: Sat, 27 Feb 1999 03:23:09 -0500*Sender*: owner-wri-mathgroup at wolfram.com

Clemens Frey wrote: > Can anybody explain to me what's going on here ? > > In[270]:= > Union[ {{a,b,1},{a,c,1},{x,y,2},{a,b,2}}, > SameTest->(#1[[2]]===#2[[2]]&) ] > Out[270]= > {{a,b,1},{a,c,1},{x,y,2}} > > is ok since the first and the last list element have the same second > component. But what is wrong here now ? > > In[272]:= > Union[ {{a,b,1},{a,c,1},{x,y,2},{a,b,2}}, > SameTest->(#1[[3]]===#2[[3]]&) ] > Out[272]= > {{a,b,1},{a,b,2},{a,c,1},{x,y,2}} Clemens, Most intriguing, and I would venture that there is a bug somewhere. In fact, if you split your original list into two parts, perform the Union operation on each of them, and finally perform a Union on the two results, you get the right answer: In[1]:= Union[Union[ {{a,b,1},{a,c,1}},SameTest->(#1[[3]]===#2[[3]]&)], Union[ {{x,y,2},{a,b,2}},SameTest->(#1[[3]]===#2[[3]]&)]] Out[1]= {{a,b,1},{a,b,2}} Tomas Garza Mexico City