# InputField equations and Solve

I'm trying to get this piece of code to work:

Panel[
DynamicModule[{f = x, g = -x},
xmax = 10;
Column[{
InputField[Dynamic[f], ContinuousAction -> True],
InputField[Dynamic[g]],
Dynamic[ Show[Plot[{f, g}, {x, -xmax, xmax}, AspectRatio -> 1,
PlotRange -> {{-xmax, xmax}, {-xmax, xmax}}]
]
]
}]
]
]


This works and updates dynamically the graphs of f and g. Now, I would like to add the intersections, dynamically, if any (so with a IF testing), and I tried different things, without success:

solution = Dynamic[Solve[f == g, {x,y}]


gives me indeed the dynamically updated solution. But I can't use this information to plot points, nor even get Length[solution] update dynamically...

Any help with this?

• This is the static version of your question. Take a look there. solution definition looks strange, use solution:=... and use Dynamic at the end in the plotting procedure. p.s. isn't Show redundant?
– Kuba
Sep 4, 2013 at 18:04
• Thank you for this static version, it's very helpful! Sep 4, 2013 at 20:05

This solution is only for simple cases where the intersection is a point. And not adapted for cases with periodical functions when the output of the Solve contains a conditional expression (sometimes).

Panel[DynamicModule[{f = x, g = -x, xmax = 20},

Column[{
InputField[Dynamic[f], ContinuousAction -> True],
InputField[Dynamic[g]],
Dynamic @ Plot[{f, g}, {x, -xmax, xmax}, AspectRatio -> 1, PlotRange -> xmax,
Epilog -> {[email protected], Point[pt]}]
}]
, Initialization :> (pt := ({x, f} /. NSolve[f == g, x, Reals]) /. {x, f} -> {})
]] • I like this answer a lot, thank you very much. One question though, what do you mean by "/. {x, f} -> {}" ? Would this be equivalent to {x, f} /. NSolve[f == g, x, Reals])[] ? Thank you! Sep 4, 2013 at 20:04
• @Gabriel when there is no solution there is no replacement rules so in this case {x,f} is switching to {} because it will not cause any error in Epilog. I'm glad it is helpful.
– Kuba
Sep 4, 2013 at 20:10