       Re: A simple swap function

• To: mathgroup at smc.vnet.net
• Subject: [mg2233] Re: [mg2132] A simple swap function
• From: mmcclure at liberty.uc.wlu.edu (Mark McClure)
• Date: Wed, 18 Oct 1995 01:53:03 -0400
• Organization: Washington & Lee University

Richard Mercer (richard at seuss.math.wright.edu) wrote:

:  >   I think I must be missing something straightforward.
: >  I want a function to swap the values of 2 lists.  I
: >  defined ab={a,b}, and ba={b,a}, and
: >  swap[v1_,v2_]:=Module[{temp=v1},v1=v2;v2=temp;], and I
: >  get an Iteration limit exceeded error.  When I look at
: >  ab, it looks like {Hold[a],Hold[b]} (or maybe
: >  {Hold[b],Hold[a], I don't remember), and ba looks
: >  opposite.  When I tried to use the same function on
: >  numbers, it didn't work either.  What's wrong with what
: >  I'm doing, and how can I do what I want to do?

: All of this is unnecessary.
: Assuming the variables a and b have been assigned values,

: the assignment

: {a,b} = {b,a}

: works. Try it! I do NOT recommend this if a and b have not been
: assigned values!

: If you want to be ultracareful, you could use

: swap[v1_Symbol,v2_Symbol]:=

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

Even if a and b have been assigned values, howabout
swap[{a_,b_}] := {b,a}
?

Mark
mmcclure at wlu.edu
http://wlu.edu/~mmcclure

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