Assign new values to matrix using indices

*To*: mathgroup at smc.vnet.net*Subject*: [mg99751] Assign new values to matrix using indices*From*: Mac <mwjdavidson at googlemail.com>*Date*: Wed, 13 May 2009 05:07:34 -0400 (EDT)

This simple problem has got me confounded (despite 2 years of Mathematica experience). Say I would like to divide all lower diagonal elements of a matrix by 2. It is fairly easy to generate a list of indices, here an example for 3 by 3 matrix In[120]:= Table[{j, i}, {j, 2, cdim = 3}, {i, 1, j - 1}] // Flatten[#, 1] & Out[120]= {{2, 1}, {3, 1}, {3, 2}} Now if I would like to divide the elements of a 3 x 3 matrix by 2, I don't know how to do this. The problem is the complex interplay of Part [], Set[] and Apply[]. To set the elements of hte matrix I can do this In[219]:= t2 = {{0, 0, 0}, {2, 0, 0}, {2, 2, 0}}; f = Function[{x}, Apply[Set[Part[t2, ##], 1] &, x]]; Scan[f, {{2, 1}, {3, 1}, {3, 2}}] t2 Out[222]= {{0, 0, 0}, {1, 0, 0}, {1, 1, 0}} which seems complicated for a simple operation. However, to divide all elements with indices given in the form above i.e. {{2, 1}, {3, 1}, {3, 2}} by 2 has got me stumped. Any help would be appreciated Mac