Re: reading different locations of a matrix specified

*To*: mathgroup at smc.vnet.net*Subject*: [mg113868] Re: reading different locations of a matrix specified*From*: Oliver Ruebenkoenig <ruebenko at wolfram.com>*Date*: Tue, 16 Nov 2010 06:13:30 -0500 (EST)

On Tue, 16 Nov 2010, Nasser M. Abbasi wrote: > On 11/15/2010 3:28 PM, Nasser M. Abbasi wrote: >> Given a matrix A, and list of locations withing the matrix, >> where each location is given by {i,j} entry, I wanted to >> find the most efficient and clear way to obtain the entries >> in the matrix by these locations. >> >> I wanted to ask the experts here what they think of this >> method, and if they can suggest a better way. >> >> Example, given >> >> A={{1,2,3}, >> {4,5,6}, >> {7,8,9}} >> >> and wanted to obtain the entries at say {1,1} and {3,3}, >> which will be 1 and 9 in this example. So I first made a >> list of the locations : >> >> pos={ {1,1},{3,3}}; >> >> Then typed >> >> Map[ A[[Sequence@@ # ]]& , pos ] >> >> and got back >> >> {1,9} >> >> So, this seems to work. >> >> Any suggestion of may be of a more 'obvious' or better way? >> > > While looking around, I Found one can use ##& in place of Sequence > > ------------------------------------ > A={{1,2,3},{4,5,6},{7,8,9}}; > pos={{1,1},{3,3}}; > > A[[ ##& @@ #]]& /@ pos > --------------------------- > > {1,9} > > > --Nasser > > Extract[A, pos]