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MathGroup Archive 1995

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Re: Re: A simple swap function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg2199] Re: [mg2151] Re: [mg2132] A simple swap function
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Fri, 13 Oct 1995 02:26:05 -0400

In [mg2151] Re: [mg2132] A simple swap function
Richard Mercer sugggested a  function for swapping the values of  
two symbols.

Richard:
There are problems with

swap[v1_Symbol,v2_Symbol]:=
  If[ValueQ[v1] && ValueQ[v2], {v1,v2} = {v2,v1}];

since swap evaluates its elemenents on the left hand side.

Examples:
{a = 1; b = 2; swap[a,b],  {a,b}}
	{swap[1, 2], {1, 2}}
{a = va; b = vb; swap[a,b], {a,b}}
	{Null, {va, vb}}

Solution: give swap the attribute HoldAll

SetAttributes[swap, HoldAll]

{a = 1; b = 2; swap[a,b],  {a,b}}
	{{2, 1}, {2, 1}}
{a = va; b = vb; swap[a,b], {a,b}}
	{{vb, va}, {vb, va}}

Allan Hayes
hay at haystack.demon.co.uk


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