Re: Types in Mathematica, a practical example

*To*: mathgroup at smc.vnet.net*Subject*: [mg62818] Re: [mg62800] Types in Mathematica, a practical example*From*: Kristen W Carlson <carlsonkw at Gmail.com>*Date*: Tue, 6 Dec 2005 00:03:14 -0500 (EST)*References*: <200512051841.NAA21133@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Ingolf, How about, on an initialized one per your spec: In[11]:= Table[ Table[0, {2}], {2}] Out[11]= {{0,0},{0,0}} In[12]:= MatrixQ[Out[11]] Out[12]= True on yours, or an arbitrary one, for which you have to adjust the Depth argument: In[19]:= idlist = {{{{0, 1}, {2, 3}}, {{-1, 0}, {1, 2}}}, {{{-2, -1}, {0, 1}}, {{-3, -2}, {-1, 0}}}}; ArrayQ[idlist, 4] Out[20]= True Note you can also initialize via indexed arrays (See Array in Help), and then replace by index (eg f[1,1] = 2) or by MapAt. You can also query some of the qualities of each array element; qv Help 2.3.5, middle section. Kris On 12/5/05, Ingolf Dahl <ingolf.dahl at telia.com> wrote: > To MathGroup, > > I am not an advocate for strong typing in Mathematica, but consider the > following simple example: I want to see if two matrices are equal. One of > them was the result from some equation, and is given inside a rule. Then I > write some code similar to this: > > > > a = {{1, 2}, {3, 4}}; > > x - a /. {x -> a} > > > > I of course hope to get a matrix filled by zeroes, but if x is undefined, > the following is returned: > > > > {{{{0, 1}, {2, 3}}, {{-1, 0}, {1, 2}}}, {{{-2, -1}, {0, 1}}, {{-3, -2}, {-1, > 0}}}} > > > > First x was assumed to be a number, and (x - a) was evaluated. Then x was > substituted by the matrix a. No bug in Mathematica, but it was not what I > wanted as user. It is easy to make such a mistake in the programming. Of > course there are many ways to get around this problem, but is there any > reasonably simple way to "type" x to be a list of lists without specifying > the elements, in such a way that the above example works? > > > > I could do > > > > ReleaseHold[Hold[x - a] /. {x -> a}] > > > > but then we are not in the "typing business" any longer. > > > > I think this question illuminates one aspect of the typing issue in > Mathematica. I remember that I as a newbie looked for ways to declare > matrices, in such a way that I later could specify matrix elements > one-by-one, without initializing them first. I soon learned that there are > other ways to achieve similar results, but still I do not see any good > reason why I cannot force Mathematica to give the following response from > x-a, if x in some way is declared to be a 2x2 list of lists: > > > > {{x[[1,1]] - 1, x[[1,2]] - 2},{x[[2,1]] - 3, x[[2,2]] - 4}} > > > > I am not allowed to Unset or Clear any part of a list either. Why not? > > > > Ingolf Dahl > > Sweden > > >

**References**:**Types in Mathematica, a practical example***From:*"Ingolf Dahl" <ingolf.dahl@telia.com>

**Re: Types in Mathematica**

**Re: Types in Mathematica thread**

**Types in Mathematica, a practical example**

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