Re: Using implicit information about row indices

*To*: mathgroup at smc.vnet.net*Subject*: [mg68337] Re: [mg68254] Using implicit information about row indices*From*: Daniel Lichtblau <danl at wolfram.com>*Date*: Tue, 1 Aug 2006 07:00:12 -0400 (EDT)*References*: <200607310745.DAA26729@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

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 Not pretty, but... info = { {{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[19]:= Normal[Apply[SparseArray,{Flatten[ MapIndexed[Map[Function[{x},{#2[[1]],x[[1]]}->x[[2]]],#]&, info]]}]] Out[19]= {{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}} There is probably a cleaner way to do the MapIndexed[Map[...]] part. Daniel Lichtblau Wolfram Research