Re: how to transform a list in a sequence of arguments?

*To*: mathgroup at smc.vnet.net*Subject*: [mg77330] Re: how to transform a list in a sequence of arguments?*From*: dh <dh at metrohm.ch>*Date*: Thu, 7 Jun 2007 03:37:24 -0400 (EDT)*References*: <f43i7q$3lt$1@smc.vnet.net>

Hi Alessandro, if I understand correctly, you have a matrix m and indices given as list {i,j,k} and you want to get m[[i,j,k]]. This can be done e.g. by: m[[Sequence@@#]]& hope this helps, Daniel alexxx.magni at gmail.com wrote: > I believed it was fairly trivial, yet I was unable to find a > solution ... > > say I have a 3d matrix, > > In[64]:= s > Dimensions[s] > > Out[64]= {{{-1, 1, 1}, {-1, 1, 1}, {-1, 1, 1}}, {{1, 1, 1}, {1, 1, 1}, > {1, -1, -1}}, {{1, -1, 1}, {-1, 1, -1}, {-1, 1, -1}}} > Out[65]= {3, 3, 3} > > and I want to access&modify it in a procedure, in which the indices > i,j,k are found sequentially: {1,1,1}, {1,1,2}, .... > > How can I do it, since s[[{i,j,k}]] is not the same - of course - of > s[[i,j,k]] ??? > > I finally wrote the construct: > > Apply[s[[##]] &, {i,j,k}] > > ant this works OK in reading, but not in writing since it reports: > > s[[##]] & @@ {1,2,1} = "0" > Set::write: Tag Apply in (s[[##1]]&)@@{1,2,1} is Protected. >> > > any idea on how to make it work read/write??? > > P.S. > it <does> work, for now, because I'm using this (where i0 is the > indices list): > s = ReplacePart[s, i0 -> - Extract[s, i0]]; > > but I fear that it must be much slower than a Part[] access > > > Thank you! > > Alessandro Magni > >