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MathGroup Archive 1998

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ReplacePart question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13065] ReplacePart question
  • From: walter-seaman at uiowa.edu (Walter Seaman)
  • Date: Sat, 4 Jul 1998 16:45:06 -0400
  • Organization: The University of Iowa
  • Sender: owner-wri-mathgroup at wolfram.com

hello,
Here is what m'ca ReplacePart  is supposed to do according to the online
help and the manual:

"ReplacePart[expr, new, pos, npos] replaces parts at positions pos in 
expr by parts at positions npos in new."

For example:

ReplacePart[{a, b, c,d},{x,y},{{1},{4}},2]  gives output
{y,b,c,y}

which is ok. I cannot get m'ca to replace selected parts of expr with 
selected parts of new.  For example, suppose I want to take the list 
{a,b,c,d} and replace a with y and x with b.  It seems I should be able
to use the command

ReplacePart[{a, b, c,d},{x,y},{{1},{4}},{2,1}]

but the output is
{{x,y}\[LeftDoubleBracket]2,1\[RightDoubleBracket],b,
  c,{x,y}\[LeftDoubleBracket]2,1\[RightDoubleBracket]}

i.e. m'ca is looking for {x,y}[[1,2]] (which is not defined) and trying 
to replace it in positions 1 and 4.
If I try

ReplacePart[{a, b, c,d},{x,y},{{1},{4}},{{2},{1}}]

I still get the output 

{{x,y}\[LeftDoubleBracket]{2},{1}\[RightDoubleBracket],b,
  c,{x,y}\[LeftDoubleBracket]{2},{1}\[RightDoubleBracket]}

Can anyone tell me how to accomplish this selective replacement? Thanks,
W.Seaman


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