RE: simple problem with Map?!?

*To*: mathgroup at smc.vnet.net*Subject*: [mg49505] RE: [mg49480] simple problem with Map?!?*From*: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>*Date*: Thu, 22 Jul 2004 02:45:34 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

>-----Original Message----- >From: Relishguy [mailto:relishguy at pluggedin.org] To: mathgroup at smc.vnet.net >Sent: Wednesday, July 21, 2004 12:41 PM >To: mathgroup at smc.vnet.net >Subject: [mg49505] [mg49480] simple problem with Map?!? > > >I do not understand why the last evaluation ends up screwed: > >In[1]:= d0 = {30,60,180,60,30}; > >In[2]:= tt = Plus @@ d0 >Out[2]= 360 > >In[3]:= pp = N[d0 / tt] >Out[3]= {0.0833333,0.166667,0.5,0.166667,0.0833333} > >In[4]:= ppp = N[d0 / (Plus @@ d0)] >Out[4]= {0.0833333,0.166667,0.5,0.166667,0.0833333} > >In[5]:= pppp = N[# / (Plus @@ #)]& /@ d0 >Out[5]= {1.,1.,1.,1.,1.} > >Can anyone explain this simply? Do I need Hold[] or Evaluate[] >somewhere? > >TIA. > >Regards.. > > Well, what does you last expression evaluate to? A pure function is mapped over your list, resulting in a list of applications. That is, it's equal to In[13]:= Table[N[#/(Plus @@ #)] &[d0[[i]]], {i, 1, Length[d0]}] Out[13]= {1., 1., 1., 1., 1.} Let's try it e.g. for the first element In[14]:= N[30/(Plus @@ 30)] Out[14]= 1. And this is because In[15]:= Plus @@ 30 Out[15]= 30 Now "Apply[f, expr] or f @@ expr replaces the head of expr by f" But if the expression is an atom, nothing is there to be replaced, and the atom is returned (forgetting about the f). -- Hartmut Wolf