Re: Conditional list indexing

*To*: mathgroup at smc.vnet.net*Subject*: [mg95919] Re: Conditional list indexing*From*: Bill Rowe <readnews at sbcglobal.net>*Date*: Thu, 29 Jan 2009 06:00:45 -0500 (EST)

On 1/28/09 at 6:43 AM, danielelinaro at gmail.com (daniele) wrote: >Hello everybody, 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. Here is one way to do what you want using Pick In[4]:= Pick[Range@Length@a, Sign[a - 15], 1] Out[4]= {7,8,9,10}