Plotting a Table requires explicit Evaluate

[sn6uv]

fs = Table[x^y, {y, 1, 5}];
Plot[fs, {x, -2, 2}]

Does not work unless used with Evaluate. e.g.

Plot[Evaluate[fs], {x, -2, 2}]

Reported here.

[GitAnt]

This problem is caused by the definition on line 140 in mathics/builtin/plot.py, changing it to
attributes = ()
solves it, dunno about the other consequences though. For sure the HoldAll attribute keeps the expression from being evaluated and it's never seen as a list at any point in the plot generation unless an evaluation of it is explicitly requested.

[bnjones]

Is this actually a bug? https://reference.wolfram.com/mathematica/ref/Table.html gives an example of Plot with Table, saying "Evaluate is needed to force evaluation of the table before it is fed to Plot". It sounds like we're doing the right thing here.

[GitAnt]

The problem is there even if fs is defined as a List, for example fs = {x^2,x^3}. So Plot[fs,{x,xmin,xmax}] gives an empty plot while Plot[{x^2,x^3},{x,xmin,xmax}] gives the expected result. In Mathematica both commands give the same plot. I think I've done some progress on this (without touching the attributes = (HoldAll,) line, I realized that that line must stay like that), I'm waiting for the other pull request to be closed before pushing the changes related to this.

[bnjones]

I did a bit of research on this - this might be obvious to people who are more familiar with Mathematica than me, but this question on Mathematica Stack Exchange explains that Plot[{f1, f2}, ...] will plot f1 and f2 as separate functions (in different colours), whereas f = {f1, f2}; Plot[f, ...] will plot f as one function that happens to return multiple values. A MathGroup thread linked from there has some more detail.

If I understand correctly, what this means for Mathics's implementation is that Plot should be able to plot functions that return a list of points.

edit - also, if we're asked to plot something we can't handle, it feels like we should issue a message rather than just return an empty plot!

[GitAnt]

From what I've see at the moment Plot in mathics does not handle function arguments, that is

f[x_]:=x^2
Plot[f[x],{x,1,2}]

does not give the expected result but I think it's a different issue respect to the one at hand. For this issue, mathics is able to plot lists of functions of the plot variable (say x) but cannot do this if such a list is "hidden" in a variable. Using Evaluate on the list before plotting is not a good solution because you can get the "wrong" result (i.e. different from mathematica) if you do something like

fs = {x^2,x^3}
x = 5
Plot[Evaluate[fs],{x,xmin,xmax}]

This would plot 2 constant functions at 25 and 125 while in mathematica

Plot[fs,{x,xmin,xmax}]

gives the expected plots since the "x" in fs is treated like a dummy variable. So Plot[Evaluate[fs],{x,xmin,xmax}] (in mathics) and Plot[fs,{x,xmin,xmax}] (in mathematica) are not equivalent.

[bnjones]

From what I've see at the moment Plot in mathics does not handle function arguments, that is

f[x_]:=x^2
Plot[f[x],{x,1,2}]

This sounds like a pretty serious problem. Could you file a separate issue for this? I'm not seeing this on current master, so it'd be good to know which git SHA you're running on and the version of Python/SymPy/etc you're using. Thanks!

[GitAnt]

Sorry, there was a conflict of definitions in the notebook I was using, stopping the server and restarting it does not reset the definitions (dunno if this is normal) and sometimes I forget to give a Quit[] command in order to start from scratch. The commands

f[x_]:=x^2
Plot[f[x],{x,1,2}]

work, sorry for the false alarm, the rest is confirmed though.