Re: Map vs. Table

*To*: mathgroup at smc.vnet.net*Subject*: [mg75422] Re: Map vs. Table*From*: Szabolcs <szhorvat at gmail.com>*Date*: Sun, 29 Apr 2007 03:13:55 -0400 (EDT)*Organization*: University of Bergen*References*: <f0v6d5$91v$1@smc.vnet.net>

Bill Rowe wrote: > On 4/27/07 at 5:25 AM, replicatorzed at gmail.com (zac) wrote: > >> consider a set of data (y values): > >> data = Table[i*j, {i, 5}, {j, 10}] > > The same data can be generated more quickly with Outer. That is: > > In[15]:= > Outer[Times,Range@5,Range@10]==Table[i*j,{i,5},{j,10}] > Just a note about this: I would also have chosen Outer to generate data, but for some reason this doesn't give a packed array. In[5]:= data=Outer[Times,Range@1000,Range@1000]; In[6]:= PackedArrayQ@data Out[6]= False > > and > > In[16]:= > Timing[Do[Table[i*j,{i,5},{j,10}],{10000}];] > > Out[16]= > {0.947914 Second,Null} > > while > > In[17]:= > Timing[Do[Outer[Times,Range@5,Range@10],{10000}];] > > Out[17]= > {0.451616 Second,Null} > Yes, but it also depends on the size of the matrix. In[10]:= Timing[Do[Table[i j,{i,1000},{j,1000}],{20}];] Out[10]= {2.125 Second,Null} In[11]:= Timing[Do[Outer[Times,Range@1000,Range@1000],{20}];] Out[11]= {8.391 Second,Null} I hope that these deficiencies of Outer and Inner (i.e that they don't work with packed arrays) will be corrected in a future Mathematica. As Jean-Marc's reply also demonstrates, it is not always easy to predict the efficiency of different approaches. Are there any published guidelines available on how to write efficient code in Mathematica? Szabolcs