interchange entries in list

*To*: mathgroup at smc.vnet.net*Subject*: [mg27419] interchange entries in list*From*: Murray Eisenberg <murray at math.umass.edu>*Date*: Sun, 25 Feb 2001 00:53:46 -0500 (EST)*Organization*: University of Massachusetts, Amherst*Sender*: owner-wri-mathgroup at wolfram.com

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}]]; t ] 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?