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MathGroup Archive 1996

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Re: Re; Reordering lists?!

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4541] Re: Re; Reordering lists?!
  • From: earhart at athena.mit.edu (Elizabeth J Earhart)
  • Date: Wed, 7 Aug 1996 04:17:47 -0400
  • Organization: Massachvsetts Institvte of Technology
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4thaug$j6e at dragonfly.wolfram.com>,
Andre and Alain <abindon at gpu.srv.ualberta.ca> wrote:
>        Hey! I have a list of Numbers, 1 to 4, and wish to rescale them so
>that 1 replkaces 4 and 3 replaces 2 and 2 replaces 3 etc. I made a little
>code to do this on an element per element basis and it doesn't seem to do
>it! Could someone let me know what I'm doing wrong or whether it's a Mma
>problem? Thanks
>
[nonworking code snipped]

The problem is that when you encounter 1's or 2's, you're changing them
into 4's or 3's respectively, and then you hit the next If[] statement
and change them back again!

The easiest way to accomplish what you want is

newgsub = (5-#)& /@ gsub;

('/@' is shorthand for Map[], '(5-#)&' is a pure function equivalent
to f[x_] := (5-x); ).

If you're not always going to be doing your substitution in such a
way that the numbers add up to a constant, you can define an encoding
function as follows:

encode[x_] := Switch[x,
		1, 4,
		2, 3,
		3, 2,
		4, 1,
		_, x];

(That last line isn't absolutely necessary - it leaves unchanged anything
that doesn't match any of the defined cases).

Then you will get the correct result if you do

newgsub = encode /@ gsub;

(Equivalently, newgsub = Map[encode, gsub, {1}];)

-Elizabeth

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