Re: Q: efficient list operation wanted

*To*: mathgroup at smc.vnet.net*Subject*: [mg14002] Re: Q: efficient list operation wanted*From*: buttgereit at netcologne.de (Peter Buttgereit)*Date*: Sat, 12 Sep 1998 16:59:24 -0400*Organization*: Dipl.-Sportl. Peter Buttgereit*References*: <6tbvmg$mv1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

[This followup was posted to comp.soft-sys.math.mathematica and a copy was sent to the cited author.] Hi Klaus, how about ReplaceByStack[in_List,crit_,stack_List]:=Module[ {tmp,inrep=in,st=stack}, tmp=Sequence@@#&/@Position[in,crit];(*get positions of elements to be repaced;apply Sequence[] to have format compatible with Part[]*) If[ Length[tmp]>Length[stack], Message[ ReplaceByStack::stack,Length[tmp],Length[stack] ] ]; (*Error message if replacements are incomplete*) (inrep[[ #[[1]] ]]= Replace[ (*make list of replacements and*) inrep[[ #[[1]] ]], (*do replace element by element*) #[[2]] ])&/@Table[ {tmp[[i]], crit->stack[[i]]}, {i,1,Min[Length[stack],Length[tmp]]} ]; inrep (*return the modified list*) ]; In[9]:= ReplaceByStack::stack= "Stack given has Length `2`, replacements needed: `1`.\n Only the first `2` hits were replaced."; In[1]:= L1 = {1,2,3,4,5,3,7,2,3,9} Out[1]= {1,2,3,4,5,3,7,2,3,9} In[2]:= L2 = {a, b, c} Out[2]= {a,b,c} In[7]:= ReplaceByStack[L1,3,L2] Out[7]= {1,2,a,4,5,b,7,2,c,9} In[8]:= ReplaceByStack[L1,3,{a,b}] ReplaceByStack::"stack": "Stack given has Length 2, replacements needed: 3. Only the first 2 hits were replaced." Out[8]= {1,2,a,4,5,b,7,2,3,9} Well ? not really without a loop since I still need the Table[] command. However, the lists of elements to be replaced and the replacement stack somehow have to be "aligned", coping with unequal length of the lists. So, I think, in this problem we won't find an efficient way doing without a loop. Cheers, Peter In article <6tbvmg$mv1 at smc.vnet.net>, k.duellmann at uni-mannheim.deXXX says... > I am looking for an efficient solution of the following problem: > > Given two lists L1 = {1,2,3,4,5,3,7,2,3,9} and L2 = {a, b, c} > > I want to replace all numbers 3 in L1 by the elements of L2 > successively, > that is the first 3 by a, the second 3 by b and the third 3 by c. That > is I want to get the following modified L1-list: {1,2,a,4,5,b,7,2,c,9}. > > Replace[] unfortunately replaces elements with a single element only. Is > there a chance to avoid hideous Do-Loops? > > Any hints are appreciated! > > PS. Remove XXX from my e-mail-address > > K. Duellmann > >