# Create filling between two trig functions

Working with trig functions, period, amplitude, etc.

I found a worksheet at a calculus website, with some nice questions related to interpreting trig functions, at least, they provide some "fun" context.

It has to do with a couple, different phases in their relationship, etc.

Anyway, my Mathematica question, is related to filling between two curves.

In the worksheet, consider that "happy" means positive. A couple is “happy” when they both like each other. A couple is unhappy when they both dislike each other.

Here are the two curves as given in the worksheet.

Plot[{Sin[9 \[Pi] t/10], 2 Sin[7 \[Pi] t/10]}, {t, 0, 20},
AspectRatio -> 0.2, AxesLabel -> {"weeks", "affection"}]


Is there a clever Mathematica way to fill in just those portions of the curve where the couple are happy, I.E. both are positive? Is there a nice way to visualize this?

• I'm not sure of etiquette here, didn't want to change my question, and I'm delighted with the graphs, it's helped a lot... the specific question in the worksheet, which I should have asked, is what is the PERCENTAGE of time the couple are happy....? Nov 15 '12 at 23:05
• THANKS to all the responders, everything was helpful and I learned a lot. hard to chose an answer but went with Andy's, though Chris' was the simplest to follow. Andy's (I think) illustrated the actual band of time better. Thanks again to everyone who responded. Nov 16 '12 at 20:34

Seems incredibly wasteful to repeat the plots but this does at least work. Notice the use of ColorFunctionScaling->False. This is necessary here.

Show[
Plot[{2 Sin[7 \[Pi] t/10],Sin[9 \[Pi] t/10]}, {t, 0, 20}, Filling -> Axis,
ColorFunctionScaling -> False,
ColorFunction ->
Function[{x, y},
If[2 Sin[7 \[Pi] x/10] > 0 && Sin[9 \[Pi] x/10] > 0, Green,
Red]]]
,
Plot[{2 Sin[7 \[Pi] t/10], Sin[9 \[Pi] t/10]}, {t, 0, 20},
PlotStyle -> Black],
AxesLabel -> {"weeks", "affection"}, AspectRatio -> .2
]


Edit:

You could alternatively use RegionPlot to do the shading. The PlotPoints need to be set quite high to pick up the smaller regions.

Show[Plot[{2 Sin[7 \[Pi] t/10], Sin[9 \[Pi] t/10]}, {t, 0, 20},
AxesLabel -> {"weeks", "affection"}, AspectRatio -> .2],
RegionPlot[
2 Sin[7 \[Pi] t/10] > 0 && Sin[9 \[Pi] t/10] > 0, {t, 0,
20}, {y, -4, 4}, PlotStyle -> Directive[Opacity[.25], Green],
PlotPoints -> 250]]


Edit 2:

I seem to be unable to stop having fun with this one. It is much faster to use Plot with Boole than to use RegionPlot in this case since the regions are simple rectangles.

Show[Plot[{2 Sin[7 \[Pi] t/10], Sin[9 \[Pi] t/10]}, {t, 0, 20},
AxesLabel -> {"weeks", "affection"}, AspectRatio -> .2],
Plot[{-2, 2} Boole[
2 Sin[7 \[Pi] t/10] > 0 && Sin[9 \[Pi] t/10] > 0], {t, 0, 20},
Filling -> Axis, PlotStyle -> None,
FillingStyle -> Directive[Opacity[.5], Pink], PlotPoints -> 150]]


• Ok, I managed to bring it down to effectively one plot call (although, I'd go with Chris' answer): Show[{#, DeleteCases[#, _Polygon | HoldPattern[VertexColors -> v___], ∞] /. h_Hue :> Black} &@ Plot[{2 Sin[7 π t/10], Sin[9 π t/10]}, {t, 0, 20}, Filling -> Axis, ColorFunctionScaling -> False, ColorFunction -> Function[{x, y}, If[2 Sin[7 π x/10] > 0 && Sin[9 π x/10] > 0, Green, Red]]]]
– rm -rf
Nov 15 '12 at 22:27
• @rm-rf definitely cool but also sufficiently scary that I think I'll leave what I have :). Nov 15 '12 at 22:43
• This is more complicated, but in some ways seems to more accurately reflect a "band of time". I really like the elegance of Chris's answer, but which one would be a more helpful "mathematical" answer? Nov 15 '12 at 23:34
• Incidentally, I think my latest edit makes it clear how one might compute the percentage of time they are both happy. NIntegrate[ Boole[Sin[7 \[Pi] t/10] > 0 && Sin[9 \[Pi] t/10] > 0], {t, 0, 20}]/20 Nov 16 '12 at 0:26
• Hmm.. not sure. I can't find a way to arrive at 33 1/3%. Nov 16 '12 at 2:06

Here's happy:

Plot[{Sin[9 \[Pi] t/10], 2 Sin[7 \[Pi] t/10]}, {t, 0, 20},
AspectRatio -> 0.2, AxesLabel -> {"weeks", "affection"},
Filling -> {1 -> {Axis, {None, Green}}, 1 -> {{2}, White}}]


• Nice! This is the first time I've fully understood the more exotic uses of Filling. Nov 15 '12 at 22:19
• I knew there had to be some kind of filling option that would work, but wouldn't have managed that, nice! I think we need a better color for "affection though" 8^) Nov 15 '12 at 22:43
• They don't seem to get along too well on the whole... coming soon: Method -> {"EventLocator", "Event" -> "Divorce"}! Nov 16 '12 at 0:34
• @Tom, Filling -> {1 -> {Axis, {None, Pink}}, 1 -> {{2}, Transparent}}... Nov 16 '12 at 2:13

may be another option

f1[t_] := Sin[9 Pi t/10];
f2[t_] := 2 Sin[7 Pi t/10];
data=Table[{t,If[f1[t]>0 && f2[t] > 0, Max[{f1[t],f2[t]}], 0]},{t,0,20,0.01}];
p1 = ListPlot[data, Joined -> True, Filling -> Axis,FillingStyle -> Green];
p2 = Plot[{f1[t], f2[t]}, {t, 0, 20},AxesLabel->{"weeks","affection"},Evaluated -> True];
Show[p2, p1]


 {f1, f2} = {Sin[9 \[Pi] t/10], 2 Sin[7 \[Pi] t/10]};
Plot[{f1, f2, ConditionalExpression[#, # >= 0] &[Min[f1, f2]]}, {t, 0, 20},
Filling -> {3 -> {Axis, Green}}, PlotStyle -> Thick,
AspectRatio -> 0.2, Frame -> False, Axes -> False]


(Axes removed to show that the added function does not produce a line at zero.)

To answer the second, rather different question concerning the percentage of time the couple are happy, here is one way of calculating it:-

(* Find roots up to t = 20 (period of sin(ax) is 2 Pi/a) *)
r1 = Table[t, {t, 0, 20, 10/9}];
r2 = Table[t, {t, 0, 20, 10/7}];

(* Establish when going positive *)
d1 = Sign[D[Sin[9 \[Pi] t/10], t] /. t -> r1];
d2 = Sign[D[2 Sin[7 \[Pi] t/10], t] /. t -> r2];

(* Join in a list for processing *)
m1 = Transpose[{r1, d1, ConstantArray[0, Length[r1]]}];
m2 = Transpose[{r2, ConstantArray[0, Length[r2]], d2}];
m = Sort[Join[m1, m2]];

process[{a_, b_, c_}] := Module[{},
Which[b == 1, y = 1, b == -1, y = 0];
Which[c == 1, z = 1, c == -1, z = 0];
AppendTo[list, If[y + z == 2, {"happy", a}, {"not so happy", a}]]]

y = z = 0;
list = {};

process /@ m;

ToString[First[Total[
{Last[#2] - Last[#1]} & @@@ Cases[Partition[list, 2, 1],
{{"happy", _}, _}]]/20*100.]] <> "%"


25.3968%