Re: Logical comparisons of items in a two lists

*To*: mathgroup at smc.vnet.net*Subject*: [mg75478] Re: Logical comparisons of items in a two lists*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Wed, 2 May 2007 03:53:54 -0400 (EDT)*Organization*: The Open University, Milton Keynes, UK*References*: <f16q2u$7er$1@smc.vnet.net>

actuary at mchsi.com wrote: > Hello: > > I have two lists of real numbers, a & b. I want two compare > individual items in one list to the corresponding items in the other > list. For example Is a[[1]] > b[[1]]. At the end of the comparisons, > I want to count the "Trues". I know how to do this use a "Table" > statement and a "Count" statement. Is there a quicker, more efficient > way of counting the number of "Trues". > > Thanks > > Larry > > Hi Larry, Since you show neither your code (or a similar example) nor any timing, it is hard to tell where the bottleneck is located, although I would bet on the *Table* part. In the code below, we compare two lists of 10,000,000 real numbers (machine precision) each. Comparing the pairs of elements with *Table* took nearly twice the time needed by *MapThread*. (Note that the difference is less dramatic for smaller values of n.) Moreover, you can see that the built-in function *Count* is highly efficient: it took less than 0.8 second to count all the *True* values. In[1]:= $HistoryLength=0; n=7; a=Table[Random[],{10^n}]; b=Table[Random[],{10^n}]; Timing[comp=Table[a[[i]]>b[[i]],{i,10^n}];] Timing[comp=MapThread[#1>#2&,{a,b}];] Timing[Count[comp,True]] Out[5]= {30.563 Second,Null} Out[6]= {18.766 Second,Null} Out[7]= {0.797 Second,4998689} Of course, you may have done something completely different, by I could not read your mind! Best regards, Jean-Marc