       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)

```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

```

• Prev by Date: Re: Conditional list indexing
• Next by Date: Re: Sampling from a simplex: part 2
• Previous by thread: Re: Conditional list indexing
• Next by thread: Re: Conditional list indexing