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Re: Why don't my plots appear? My 5.2 code no longer works


You'll probably get zillions of responses to your question.

The treatment of Plot and related functions has changed with version 6: 
the graphic created is no longer a side-effect of evaluating the 
function, but rather the actual, direct RESULT of the function.  (In 
Mathematica 5 and before, the display was a side-effect whereas the 
direct result was a -Graphics- object.)

A consequence is that following a Plot with a semi-colon now has EXACTLY 
the same effect of following any other command with a semi-colon: it 
suppresses the result.  More precisely, it produces a Null result, which 
by default is not shown in any way.

In general, a semi-colon is used to separate the parts of a compound 
expression, for example:

   a = 2; Plot[Sin[a x], {x, 0, Pi}]; "Hello, world!"  (* NO! *)

has result the string 'Hello, world!.  And indeed in Mathematica 6 you 
the semi-colon after the Plot expression appears in red to warn you.  To 
make the plot appear, use Print:

   a = 2; Print[Plot[Sin[a x], {x, 0, Pi}]]; "Hello, world!"  (* Yes *)

Note this has nothing to do, per se, with Modules -- just compound 
expressions in general.


Charlie Brummitt wrote:
> Hello,
> I am running into difficulties with Plot. Some code of mine that worked in
> 5.2 no longer works correctly in 6.0.2.0. Specifically, no plots appear
> whenever Plot (or Plot3D or DensityPlot or ListPlot) is called within a
> function that I have defined.
> 
> Here is a closely related question: What does the red semicolon ";" mean at
> the end of "Plot[ ... ];"? Why is it red? In 5.2, placing a semicolon at the
> end of Plot[] allowed me to output a plot in the middle of a computation,
> without halting the computation. Now I can no longer seem to output a plot
> in the middle of a long computation. (I only see the output of Print[]
> commands.) I must be able to see these plots during long computations, like
> I was able to in 5.2.
> 
> What's wrong? Can anyone please help?
> 
> Thanks,
> 
> Charlie
> 
> 

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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