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MathGroup Archive 2012

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Re: Transforming/expanding a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg127990] Re: Transforming/expanding a list
  • From: awnl <awnl at gmx-topmail.de>
  • Date: Fri, 7 Sep 2012 04:58:30 -0400 (EDT)
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  • References: <k29m0t$a8r$1@smc.vnet.net>

Hi,

> 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}
>

I think this will do what you need:

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

FoldList[Replace[#2, NULL :> #1] &, First[row], Rest[row]]

hth,

albert



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