Re: Automatic Display in MatrixForm

*To*: mathgroup at smc.vnet.net*Subject*: [mg19259] Re: [mg19206] Automatic Display in MatrixForm*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Thu, 12 Aug 1999 01:24:18 -0400*References*: <7or4k5$13j@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Ted, Two comments: A) and B) I show the outputs in InputForm and indented (of course we can always use TraditionalForm: Cell>Default Output Format>TraditionalForm) A) There are still problems: $Post = (# /. x_?MatrixQ :> MatrixForm[x]) &; mtrx = {{2, 0, 0}, {0, -1, 0}, {0, 0, 3}} MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}] mtrx.a MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}] . a % /. a -> mtrx MatrixForm[MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}]] . MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}] The MatrixForm wrappers prevent the Dot product. B) Perhaps a more general solution is as follows (where MatrixForm is first replaces by Identity to allow evaluation, and is then replaced appropriately). I await a list of problems with this also! $Post = (# /. MatrixForm -> Identity /. x_?MatrixQ :> MatrixForm[x]) &; mtrx = {{2, 0, 0}, {0, -1, 0}, {0, 0, 3}} MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}] mtrx.a MatrixForm[{{2, 0, 0}, {0, -1, 0}, {0, 0, 3}}] . a % /. a -> mtrx MatrixForm[{{4, 0, 0}, {0, 1, 0}, {0, 0, 9}}] Ersek, Ted R <ErsekTR at navair.navy.mil> wrote in message news:7or4k5$13j at smc.vnet.net... > Bruno Daniel wrote: > ---------------------- > I found a way to display matrices automatically in MatrixForm > without having to place the annoying "// MatrixForm" in the end > of each line. Just put the following line into your init.m: > > $Post=(#/.x_?MatrixQ->MatrixForm[x])& > > This also works for output that doesn't *consist* of a matrix, but > *contains* matrices. > ---------------------------- > > That's nice, but it sometimes spells disaster. > > > In[1]:= > $Post=(#/.x_?MatrixQ->MatrixForm[x])&; > > In[2]:= > x=3+y; > > In[3]:= > mtrx={{2,0,0},{0,-1,0},{0,0,3}} > > Out[3]//MatrixForm= > 3+y > > > What *%&^ !! > ----------------- > > You used (lhs->rhs) in $Post, and Rule uses the global value of (x) in > (rhs). Hence the result in Out[3] is very wrong! However, (mtrx) actually > has the right value. Below the first row of (mtrx) is displayed. > > > In[4]:= > First[mtrx] > > Out[4]= > {2,0,0} > > ----------------------------- > What you need to do is use (lhs:>rhs) as below. > > In[5]:= > $Post=(#/.x_?MatrixQ:>MatrixForm[x])&; > > In[6]:= > mtrx > > Out[6]//MatrixForm= > (* The desired output is displayed. *) > > ---------------------------- > > It's no wonder people keep making this mistake in spite of me pointing it > out over and over. The Mathematica Book (all editions) are full of examples > that will give the same type of error when the variable used has a global > value. The book does nothing to warn users about this problem, or how to > avoid it. Has Stephen Wolfram not been aware of this for the past 10 years, > or does he think it's in his best interest to avoid bringing it to our > attention? > > As far as I can tell WRI tech support doesn't have a web page addressing > this problem. > > Besides that there are dozens of books written about how to use Mathematica. > I have yet to find one that mentions this problem. It's even more puzzling > why few if any of the other books discuss this point. > > ------------- > Regards, > Ted Ersek >