Re: Using implicit information about row indices

*To*: mathgroup at smc.vnet.net*Subject*: [mg68321] Re: [mg68254] Using implicit information about row indices*From*: János <janos.lobb at yale.edu>*Date*: Tue, 1 Aug 2006 06:59:50 -0400 (EDT)*References*: <200607310745.DAA26729@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

On Jul 31, 2006, at 3:45 AM, Diamond, Mark wrote: > I am wanting to fill a square matrix M (say, r by r) using > information from > a list, L. > The list L is also of length r, but is not a square matrix. It > contains > lists of ordered pairs {col, num} > It typically looks something like this; I have arranged it into > rows so that > it is more obvious > { > {{1, a}, {2, b}}, > > {{1, c}, {2, d}, {3, e}, {4, f}}, > > {{2, g}, {3, h}, {4, i}}, > > {{3, k}, {4, l}, {5,m}}, > > {{4, n}, {5, p}} > > } > > Now what I am trying to do is, for example, set M[[1,1]]=a, M[[1,2]] > =b, > M[[2,1]]=c, M[[2.2]]=d, M[[2,3]]=e and so forth, and leave (or set) > all the > other entries in M to zero. The row information for M is implicit > in the > structure of the list L; additionally, the column indices as they > appear > within sublist of L are guraranteed to be sequential. It seems with > all this > info, there should be an easy way to fill M correctly, but I have > struggled > without success to do anything other than an iterative process with > Do[]. I > would appreciate any guidance you might have. > > Thank you. > > Mark Diamond > > > Here is a newbie approach: In[9]:= lst = {{{1, a}, {2, b}}, {{1, c}, {2, d}, {3, e}, {4, f}}, {{2, g}, {3, h}, {4, i}}, {{3, k}, {4, l}, {5, m}}, {{4, n}, {5, p}}} In[23]:= Normal[SparseArray[ Flatten[ (Table[Flatten[ {Flatten[Position[ lst, #1]], #1[[ii, 1]]}] -> #1[[ii, 2]], {ii, 1, Length[#1]}] & ) /@ lst]]] Out[23]= {{a, b, 0, 0, 0}, {c, d, e, f, 0}, {0, g, h, i, 0}, {0, 0, k, l, m}, {0, 0, 0, n, p}} János ---------------------------------------------- Trying to argue with a politician is like lifting up the head of a corpse. (S. Lem: His Master Voice)

**Follow-Ups**:**Re: Re: Using implicit information about row indices***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>