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MathGroup Archive 1997

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extracting elements from a list of lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg5811] extracting elements from a list of lists
  • From: Olivier Georg <olivier.georg at studi.epfl.ch>
  • Date: Wed, 22 Jan 1997 00:44:15 -0500
  • Organization: EPF Lausanne
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

Suppose I have lists within a list, like:

  myList = {{a,b}, {c,d}}

and I want to extract some element (let's say the d).  
I can request:

  myList[[2,2]].

Now suppose what I have the index of the element as
the list {2,2}.  Then I can do:

  Fold[Part, myList, {2,2}]

Is it correct to suppose that it will first return the 
2nd sublist, and then extract the second element from it?
How efficient is it?  This may be a crucial point
if I have long lists, I think.  Then isn't there a
better way of doing this?

Olivier
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