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MathGroup Archive 2012

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Re: Transforming/expanding a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg127978] Re: Transforming/expanding a list
  • From: Ray Koopman <koopman at sfu.ca>
  • Date: Fri, 7 Sep 2012 04:54:29 -0400 (EDT)
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On Sep 6, 1:15 am, Mark Coleman <markspcole... at gmail.com> wrote:
> I'm working on some Mathematica code that will help transform a large set of lists drawn from my company's legacy database systems. A typical row of the raw data looks like
>
> {x1,NULL,NULL,x2,NULL,NULL,NULL,NULL,x3,NULL,NULL,x4,x5,NULL} ...
>
> The lists are sequences of real numbers, separating some some number of Strings which for convenicence sake we call "NULL" (as opposed to a proper Mathematica Null). The goal is to create a new list where each NULL is replaced by the real number that most recently proceeds it (note that any number of NULLs may occur before or after any real values). I would like the code to return
>
> {x1,x1,x1,x2,x2,x2,x2,x2,x3,x3,x3,x4,x5,x5}
>
> Thanks
>
> Mark

data = {x1,NULL,NULL,x2,NULL,NULL,NULL,NULL,x3,NULL,NULL,x4,x5,NULL};

x = x0 (* just in case the first element in data is NULL *);
If[# === NULL, x, x = #]& /@ data

{x1, x1, x1, x2, x2, x2, x2, x2, x3, x3, x3, x4, x5, x5}



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