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MathGroup Archive 1995

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Re: Re: interchanging elements in a list

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg2049] Re: [mg2005] Re: [mg1993] interchanging elements in a list
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Sat, 16 Sep 1995 01:43:38 -0400

Richard Mercer <richard at seuss.math.wright.edu>
in [mg2005] Re: [mg1993] interchanging elements in a list
gives four ways of interchanging elements (see code below - I  add  
indices to distinguish the four functions).

Richard,

Interchange1 and Interchange4 are fine but the other two replace by  
value and can give incorrect results:

Interchange2[ {1,1,2},2,3]
	{2, 2, 1}

Interchange3[ {1,1,2},2,3]
	{2, 2, 1}

Allan Hayes
hay at haystack.demon.co.uk


******************
Interchange1[list_List,n_,m_]:=
   ReplacePart[ReplacePart[list,list[[n]],m],list[[m]],n];

Interchange2[list_List,n_,m_]:=
   list /. {list[[m]]->list[[n]],list[[n]]->list[[m]]};

Interchange3[list_List,n_,m_]:=
   list /. {#1->#2,#2->#1}& @@ Part[list,{m,n}];

Interchange4[list_List,n_,m_]:=
   Part[list,Range[1,Length[list]] /. {m->n,n->m}];


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