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Re: Map and functional constructs to replace iterative statements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96882] Re: Map and functional constructs to replace iterative statements
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Thu, 26 Feb 2009 08:00:32 -0500 (EST)
  • References: <go31ba$f47$1@smc.vnet.net>

Andreas wrote:
> I have begun to replace Table, Do, While, or For, but in my thinking with Map, Inner, Outer and other functional programming constructs as I learn more about Mathematica.
> 
> But, I'm stuck.  I need to figure out how to use Map (or some related approach) when a function needs both a current value and a previous value in a time series, such as a percentage change function (or perhaps more generally stated, when I need to refer to previous values in a series or even previous calculations).  But first a simple case:
> 
> percentChange[currentValue_, previousValue_] := 100 * ( currentValue - previousValue) / previousValue
> 
> I know how to apply this iteratively, but how can I do it with Map or some other functional construct?
> 
> Say I have a list: timeSeries 
> 
> I thought to try something like this:
> 
> Map[percentChange[?,?], timeSeries] 
> 
> but I don't know how what to put in for "?".
> 
> Any help much appreciated.
> 

This is one possibility do what you want:

percentChange @@@ Partition[timeseries, 2, 1]

Partition will create a list of pairs, @@@ is a short notation for Apply
at level 1. You might want to look at the documentation for Partition
and Apply for more details...

hth,

albert


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