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RE: interchange entries in list

  • To: mathgroup at
  • Subject: [mg27470] RE: [mg27419] interchange entries in list
  • From: "Wolf, Hartmut" <Hartmut.Wolf at>
  • Date: Tue, 27 Feb 2001 00:37:22 -0500 (EST)
  • Sender: owner-wri-mathgroup at

for my answer see below:

-----Original Message-----
From: Murray Eisenberg [mailto:murray at]
To: mathgroup at
Subject: [mg27470] [mg27419] interchange entries in list

Is there any hidden trouble with the following function for
interchanging two entries in a list?

  swap[v_List, i_, j_] := v /. {v[[i]]->v[[j]], v[[j]]->v[[i]]}

It seems to work, yet I'm uncomfortable.

Note that I am _not_ uncomfortable with a construction such as

  vec[[{1,2}]] = v[[{2,1}]]

or the Module

  swapItems[v_List, i_, j_] := 
     Module[ {t = v},
       t[[{i, j}]] = t[[{j, i}]]; 

that expresses this idea.

With "swap", evidently my concern is with when what items get replaced
by what.  In other words, why SHOULD "swap"  work?


Well, Murray, it doesn't! Having looked at the Trace

In[20]:= Trace[swap[Range[5], 2, 5]]

you might try

In[21]:= swap[{1, 2, 3, 2, 1}, 2, 3]
Out[21]= {1, 3, 2, 3, 1}

which is not what swap should do.

-- Hartmut 

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