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Re: MatrixForm affects values instead of only printing ?????

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100327] Re: [mg100298] MatrixForm affects values instead of only printing ?????
  • From: "David Park" <djmpark at comcast.net>
  • Date: Mon, 1 Jun 2009 07:07:28 -0400 (EDT)
  • References: <13516177.1243766422228.JavaMail.root@n11>

The documentation is either incorrect or misleading on the effect of
MatrixForm. It certainly does affect how subsequent expressions are
evaluated. But this is not all bad and one can learn how to use it.

test2 = {-3, 7, 5} // MatrixForm
test2 // FullForm

Now, what about:

test2.test2
% /. MatrixForm -> Identity

Would you like the first dot product to evaluate, or would you prefer to see
it in that form?

Often, when I only want temporary MatrixForm display I write definitions
with an extra pair of brackets:

(test2 = {-3, 7, 5}) // MatrixForm
test2[[3]]

Part does work on the FullForm of the expression and considers any form
wrappers as part of the expression.

test3 = NumberForm[a + b + N[\[Pi]], 12]
test3 // FullForm
test3[[3]] 
test3[[1, 3]]

Part[FullForm[a + b], 1, 2]

I admit, this is all somewhat confusing, and the documentation is
misleading. But it is manageable. Maybe someone else will give a clearer
explanation.


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  



From: summer [mailto:summertan at hotmail.com] 

Here is the example:

test1={-3, 7, 5}
test1[[3]]

returns the list and 5 as expected;

test2={-3, 7, 5}//MatrixForm
test2[[3]]

returns a column vector and error: "part 3 of blah blah doesn't exist". The
actual list itself is test2[[1]]. To get the right 3rd element I have to do:

test2[[1,3]]

which returns 5 as expected.


The problem is MatrixForm is NOT supposed to change the expression it is
applied on according to the manual. Is that a bug or there's logical
explanation?




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