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Re: Re: Show doesn't work inside Do loop ?

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  • Subject: [mg101975] Re: [mg101973] Re: Show doesn't work inside Do loop ?
  • From: "David Park" <djmpark at>
  • Date: Sat, 25 Jul 2009 04:15:39 -0400 (EDT)
  • References: <32390795.1248259308283.JavaMail.root@n11> <h4951e$q2e$> <21621663.1248432817472.JavaMail.root@n11>


I have to admit that this is a source of confusion and there have been a
number of questions on MathGroup about it. I had to look more carefully at
the documentation to better understand it and not see it just as a

>From the Print Function page:

"In a notebook, Print generates a cell with style "Print"."

I believe CellPrint could also be used to generate notebook cells. And
normal evaluation of most Input statements will generate a cell, but not
from all Input statements. And not if you use ";" to suppress Output cell

On the other hand, Show does not generally generate a cell. It just returns
a Mathematica expression. In that respect it is not any different than 1+1.
One can, however, change the behavior of a plot, or of Show by adding a
DisplayFunction option. The following generates no output.

Do[Show[Plot[Sin[i x], {x, 0, 3}]], {i, 1, 3}]

The following does generate output.

Do[Show[Plot[Sin[i x], {x, 0, 3}], 
  DisplayFunction -> (Print[#]; &)], {i, 1, 3}]

Of course, it is easier to just apply Print to the Show or Plot statements.
So it is possible to make Show generate output cells, but it normally
doesn't do so. It normally only generates expressions, which because of its
special behavior a Do statement does not display.

David Park
djmpark at  

From: AES [mailto:siegman at] 

In article <h4951e$q2e$1 at>,
 "David Park" <djmpark at> wrote:

> A Do loop never generates any output on its own. Print generates the
> so add a Print header to the Show statement.


Can't quarrel with your overall response here -- but might you also 
agree that your two statements above are in a real sense contradictory.

To put it another way:

*  Show, executed in a single cell, sure generates output.

*  Print, executed in a single cell, sure generates output.

So, I think my off-cited even if mythical "ordinary user" would very 
reasonably conclude that wrapping Show *or* Print in a Do should produce 
multiple versions of the single-cell output.  Producing multiple 
versions of what some single-cell expression does is what Do loops are 
*supposed* to "do" -- nicht wahr?  And "Show" is as an imperative a verb 
as is "Print".

I'm not really interested in why Print can Do, and Show can't.  I'm just 
noting that this is an easily and reasonably misunderstood situation, 
and that's unfortunate.

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