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Re: Conditional list indexing

  • To: mathgroup at smc.vnet.net
  • Subject: [mg95923] Re: [mg95855] Conditional list indexing
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 29 Jan 2009 06:01:30 -0500 (EST)
  • Reply-to: hanlonr at cox.net

a = Table[Prime[i], {i, 10}];

b = Select[a, # > 15 &];

If you have list b:

Flatten[Position[a, #] & /@ b]

{7,8,9,10}

Without calculating list b:

Flatten[Position[a, _?(# > 15 &)]]

{7,8,9,10}


Bob Hanlon

---- daniele <danielelinaro at gmail.com> 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.

Thank you,
Daniele



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