(x==17).subs(x=19).plot() plots a line at height 2

[pelegm]

(x==17).subs(x=19).plot()

plots a line at height 2. In general, replacing 17 with a and 19 with b results in a line at height b-a.

This is of course silly, as if one wishes to plot the function a == b, it should probably be at height 1 if indeed a=b, or at height 0 otherwise.

Component: symbolics

Keywords: bool, expression, plot

Issue created by migration from https://trac.sagemath.org/ticket/22829

[pelegm]

comment:1

Perhaps related:

(x==17).plot()

plots the same line as

(x-17).plot()

[kcrisman]

comment:2

Perhaps related:

(x==17).plot()

plots the same line as

(x-17).plot()

This at least is not a bug. Sage should plot the same thing for x^2+1 and x^2+1==0, if it plots anything at all, as in other contexts they are considered the same. However, I'm not sure if anything should be plotted here.

See https://github.com/sagemath/sage-prod/blob/master/src/sage/symbolic/expression.pyx#L11388 for where this is defined. This is very old code, so it may take some work to get right. Note that the subs you point out gives 19 == 17 which then https://github.com/sagemath/sage-prod/blob/master/src/sage/ext/fast_callable.pyx interprets as 2.

But I don't know that I'd interpret 19==17 as either 1 or 0, as we don't plot truth values, we plot numbers.