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$Post vs. $PrePrint

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  • Subject: [mg36423] $Post vs. $PrePrint
  • From: "Ersek, Ted R" <ErsekTR at>
  • Date: Fri, 6 Sep 2002 03:16:54 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

I am trying to find an example that will demonstrate the difference between
$PrePrint  and   $Post.   I found an old thread in this news group where a
user wanted to display all matrices using MatrixForm.  Some users suggested
the following:

     In[1]:=   $Post=(#/.mtrx_?MatrixQ:>MatrixForm[mtrx]&);

Then Dave Withoff said it's better to assign this to  $PrePrint  since the
objective here is to adjust the display rather than the result of the
calculation.  With the assignment to  $Post  you could, for example, get
unexpected results from calculations using %, since matrices will be wrapped
in MatrixForm.

However, if we use  $Post  above, the next input will compute the inverse
the matrix.  I did verify that Inverse can't take a matrix wrapped in
MatrixForm.  Can somebody give an example where doing this with  $PrePrint
instead of  $Post  gives a different result. 

    In[2]:=   m={{2,3},{0,1}};

   Out[3]=   (*  Inverse of (m) in MatrixForm, not shown. *)

   Ted Ersek
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