Re:Q: efficient list operation wanted

*To*: mathgroup at smc.vnet.net*Subject*: [mg13985] Re:[mg13973] Q: efficient list operation wanted*From*: Lou Talman <me at talmanl.mscd.edu>*Date*: Sat, 12 Sep 1998 16:59:10 -0400*Sender*: owner-wri-mathgroup at wolfram.com

K. Duellmann wrote: > 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? How about: In[1]:= CyclicReplace[L_List, x_, R_List]:= Block[{n = Length[R], pointer = Length[R]}, L /. (x :> Q[[(pointer = 1 + Mod[pointer, n])]])] In[2]:= stuff = {1, 3, 2, 4, 5, 3, 6, 3, 7, 8, 9, 3} Out[2]= {1, 3, 2, 4, 5, 3, 6, 3, 7, 8, 9, 3} In[3]:= Q = {a, b, c} Out[3]= {a, b, c} In[4]:= CyclicReplace[stuff, 3 ,Q] Out[4]= {1, a, 2, 4, 5, b, 6, c, 7, 8, 9, a} --Lou Talman