Re: EUREKA Re: Types in Mathematica, a practical example

*To*: mathgroup at smc.vnet.net*Subject*: [mg63220] Re: EUREKA Re: Types in Mathematica, a practical example*From*: David Bailey <dave at Remove_Thisdbailey.co.uk>*Date*: Mon, 19 Dec 2005 07:01:21 -0500 (EST)*Organization*: Customer of PlusNet plc (http://www.plus.net)*References*: <do0le4$41j$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hello, If you want to compute with undefined array elements, why not use subscript elements such as Subscript[a,1,2]. More generally, the concept of a matrix can mean a number of different things: 1) A fully filled rectangular array of numbers or other expressions. (SparseArray is simply a variant of this). 2) An array in which some or all of the elements are undefined. 3) An array in which even the dimensions are undefined, but in which you can refer to individual elements. 4) In some work a single symbol is used to represent a matrix together with a non-commutative multiplication. Clearly a system cannot be expected to cater for all of these and all possible transitions between one and another. In practice, of course, Mathematica caters for case 1, and you have to supply your own notation to work with the others. This seems reasonable to me. In practice, I find that error messages generated by Part are quite common and extremely useful, and I think it would be most unhelpful if Part expressions that could not be evaluated silently returned unchanged. David Bailey http://www.dbaileyconsultancy.co.uk