Re: speed (resumed)

*To*: mathgroup at smc.vnet.net*Subject*: [mg15789] Re: speed (resumed)*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Sun, 7 Feb 1999 02:04:06 -0500 (EST)*References*: <79e8qt$9ea@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Tobias, Yes Function[x, expr[x]] has more to do than expr[#] & : in addition to replacement via formal parameters it has also to check on scoping . However, I wonder if the differences that you find between Map[f,g] and f/@g are perhaps one-off since I get the following on a Toshiba Tecra 233 MgHz, 128 MgB RAM data= Table[ {Timing[Map[(#*2)&,Table[i,{i,10000}]]][[1,1]], Timing[(#*2)& /@ Table[i,{i,10000}]][[1,1]]}, {20} ]; Check Take[data, 3] {{0.66,0.6},{0.66,0.55},{0.55,0.66}} Compare Count[data, {x_,y_}/;#[x,y]]&/@{Less,Equal,Greater} {9,2,9} Allan --------------------- Allan Hayes Mathematica Training and Consulting www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 tobias at physics.odu.edu wrote in message <79e8qt$9ea at smc.vnet.net>... >Hi all, continuing on the speed of evaluation started in another thread >about the difference between Do and Nest, I was wondering what you >thought about the difference in evaluation time between pure functions >written as > >(something depending on #i) & and >Function[{vars},samething depending on vars] > >In[1]:= Timing[x=0.3;Print[x];Nest[# 1.003456 &,x,1000000]] 0.3 > >Out[1]= >{56.8977 Second, 6.47467520 10*^1497} > >In[2]:= Timing[x=0.3;Print[x];Nest[Function[{y},y 1.003456],x,1000000]] >0.3 > >Out[2]= >{79.2468 Second, 6.47467520 10*^1497} > >Using the Function notation increases the evaluation time by 40 %. I >kind of understand this since in the Function case there has to be a >replacement between the formal parameters and the arguments to be made >which is not as streight forward as a replacement of #i by the >arguments. On the other hand, I do not get at all the difference in >evaluation time between > >Map[f,expr] and f /@ expr > >In[3]:= Timing[Map[(#*2)&,Table[i,{i,10000}]]][[1]] > >Out[3]= 1.63 Second > >In[4]:= Timing[(#*2)& /@ Table[i,{i,10000}]][[1]] > >Out[4]= 1.55 Second > >10 times more > >In[5]:= Timing[Map[(#*2)&,Table[i,{i,100000}]]][[1]] > >Out[5]= 16.31 Second > >In[6]:= Timing[(#*2)& /@ Table[i,{i,100000}]][[1]] > >Out[7]= 15.44 Second > >I expected that Map[] would be slower by a constant amount of time used >for the translation to the other form but no it's more or less >linear... > >I'd like to know if anyone knew other tricks that reduce computation >time just by changing the notation ? > >Tobias > >-----------== Posted via Deja News, The Discussion Network ==---------- >http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own > >