#3916 if statement in function ignored while plotting

[Alfons Schmid]

if statement ignored while plotting

[Alfons Schmid]

Hi, if I use an if statement in a functions it's working properly with aa value.
While plotting the if is ignored.
Please try:
f(x):=block(x,if x>0 then x:0,x);
f(10)= 0. This is right.
Plotting it, the if statement is ignored e.g. with wxplot2d(f(x),[x,-100,100]);.
In the result you will see a straight line. Alfons

[Barton Willis]

This isn't a bug. Instead try plotting

wxplot2d('f(x),[x,-100,100]);

Why is the quote needed? Consider:

(%i15)  f(x):=block(x,if x>0 then x:0,x)$

(%i16)  f(x);
(%o16)  x

If you all have more questions, please ask on the Maxima discussion list.

[Alfons Schmid]

Thanks a lot,

Didn't really get the sense of the quote.

If I use an if else statement it will also work without quote:

f(x):=block(if x>0 then x:0 else x)$

wxplot2d(f(x),[x,-100,100])

Von: Barton Willis [mailto:willisbl@users.sourceforge.net]
Gesendet: Mittwoch, 12. Januar 2022 17:40
An: [maxima:bugs]
Betreff: [maxima:bugs] #3916 if statement in function ignored while plotting

This isn't a bug. Instead try plotting

wxplot2d('f(x),[x,-100,100]);

Why is the quote needed? Consider:

(%i15) f(x):=block(x,if x>0 then x:0,x)$

(%i16) f(x);
(%o16) x

If you all have more questions, please ask on the Maxima discussion list.

---

[bugs:#3916] https://sourceforge.net/p/maxima/bugs/3916/ if statement in
function ignored while plotting

Status: not-a-bug
Group: None
Labels: wrong plot
Created: Wed Jan 12, 2022 04:25 PM UTC by Alfons Schmid
Last Updated: Wed Jan 12, 2022 04:38 PM UTC
Owner: nobody
Attachments:

• wxmaxima bug if statement ignored while plotting.wxmx
  https://sourceforge.net/p/maxima/bugs/3916/attachment/wxmaxima%20bug%20if%2 0statement%20ignored%20while%20plotting.wxmx (12.4 kB;
  application/octet-stream)

if statement ignored while plotting

---

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[Jaime E. Villate]

To see the real command you are giving to Maxima, after simplification, try quoting the plot2d to see what is going to be done:

(%i1) f(x):=block(x,if x>0 then x:0,x)$
(%i2) 'plot2d(f(x),[x,-100,100]);
(%o2)                     plot2d(x, [x, - 100, 100])
(%i3) g(x) := if x>0 then 0 else x$
(%i4) 'plot2d(g(x),[x,-100,100]);
(%o4)           plot2d(if x > 0 then 0 else x, [x, - 100, 100])

See the difference between %o2 and %o4? The issue is not with plot2d but with the way Maxima has been designed. I hope you understand that the difference between f and g is that f has 2 commands while g has only one. The result of a function is always the result of the last command.

[Alfons Schmid]

thanks for support. I see it is quite different to Octave or to C where a function is a function whatever you implement. Hm, so I think it's a good idea to compare the plot results with another application in future.

[Jaime E. Villate]

Actually, the way you defined f , it has 3 commands.

[Jaime E. Villate]

A function is a function in Maxima too, whatever you implement! f is the function; f(x) is the result of applying the function on x. You should then use:

f(x):=(if x>0 then x:0,x);
plot2d(f,[x,-100,100]);

(No need for "block" in this case). And once again, let's not use this bug tracking system to discuss about Maxima; it is better to use the Maxima list.

[Alfons Schmid]

https://sourceforge.net/p/wxmaxima/discussion/create_topic/
https://sourceforge.net/p/wxmaxima/discussion/create_topic/ I cannot create
a topic. "You do not have permission to post in any forums."
Is this the right forum?

[Jaime E. Villate]

No! "wxmaxima" is a project separate from Maxima. The discussion list where you should register if you want to talk about Maxima is the one described in the Mailing list section of our website:
https://maxima.sourceforge.io/maximalist.html