Re: Conditional list indexing

*To*: mathgroup at smc.vnet.net*Subject*: [mg95868] Re: Conditional list indexing*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Thu, 29 Jan 2009 05:51:24 -0500 (EST)*Organization*: The Open University, Milton Keynes, UK*References*: <glpgd4$lha$1@smc.vnet.net>

In article <glpgd4$lha$1 at smc.vnet.net>, daniele <danielelinaro at gmail.com> wrote: > I'd like to know how I can get the indices of the elements in a list > that satisfy a certain condition. Here's an example: suppose I have > the list > > a = Table[Prime[i],{i,10}]; > > that contains {2,3,5,7,11,13,17,19,23,29}. > > If I execute the command > > b = Select[a,#>15&]; > > b will contain {17,19,23,29}. I'd like to find a way to obtain a third > list, say c, that will contain {7,8,9,10}, i.e. the indices of the > elements in list a that are greater than 15. Use *Position* to get the "indices". In[1]:= a = Table[Prime[i], {i, 10}] Out[1]= {2, 3, 5, 7, 11, 13, 17, 19, 23, 29} In[2]:= b = Select[a, # > 15 &] Out[2]= {17, 19, 23, 29} In[3]:= c = Position[a, p_?(# > 15 &)] Out[3]= {{7}, {8}, {9}, {10}} In[4]:= c = Position[a, p_ /; p > 15] Out[4]= {{7}, {8}, {9}, {10}} In[5]:= c = Flatten[Position[a, p_ /; p > 15]] Out[5]= {7, 8, 9, 10} Regards, --Jean-Marc