# How to "automatically" show an equation, written in the matlab editor, as a text on a plot

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Sim on 24 Aug 2022
Commented: Sim on 28 Aug 2022
%Input
x = 1 : 10;
a = 0.5;
y = exp(-a*x); % equation to show inside the plot
plot(x,y)
% Desired Output
Note 1: my goal would be to have a command that, if I change my equation here
y = exp(-a*x);
the text in the plot changes automatically!
Note 2: Currently, I use a "manual" command that I sould update if I change my original equation (y = exp(-a*x)), i.e.
text(2,0.5,sprintf('$$y = e^{-ax}$$'),'Interpreter', 'latex', 'fontsize', 20);
##### 2 CommentsShowHide 1 older comment
Sim on 24 Aug 2022
So... Does it mean that what I asked is currently impossible with Matlab? :-)

Star Strider on 26 Aug 2022
I apologise for the delay.
Re-posting —
syms x
% x = 1 : 10 ;
a = 0.5 ;
y = exp(-a*x); % equation to show inside the plot
figure
fplot(y, [0 10 ])
text(2,0.5, ['$y = ' latex(y) '$'], 'Interpreter','latex', 'FontSize',16)
I am not certain what you mean by ‘Note: I do not know if it is possible, but the best for me would be to use your solution (or something very similar), but showing the variable name "a", instead of its value "0.5", in the plot's text’ however it is possible to put the text object in a location determined by the data:
figure
hfp = fplot(y, [0 10]);
Xv = hfp.XData; % Get Data
Yv = hfp.YData; % Get Data
YL = ylim;
Ytxt = median(YL); % This Can Be Any Appropriate 'Y' Value
Xtxt = interp1(Yv, Xv, Ytxt); % Corresponding 'X' Value For Curve
text(Xtxt,Ytxt, ['$y = ' latex(y) '$'], 'Interpreter','latex', 'FontSize',16, 'Horiz','left', 'Vert','bottom')
To use an ‘x’ value (instead of a ‘y’ value) to initially determine the position, reverse the ‘Xtxt’ and ‘Ytxt’ code.
It is also possible to put it in the 'best' location as determined by the legend function (providing the legend itself is not also desired, since if it is, that approach will obviously fail). See Best Text/number location on the plot for that approach.
.
Sim on 28 Aug 2022
thanks a lot @Star Strider, very kind! :-)

Sim on 26 Aug 2022
@Star Strider why did you delete your solution ?
syms x
% x = 1 : 10 ;
a = 0.5 ;
y = exp(-a*x); % equation to show inside the plot
figure
fplot(y, [0 10 ])
text(2,0.5, ['$y = ' latex(y) '$'], 'Interpreter','latex', 'FontSize',16)
##### 2 CommentsShowHide 1 older comment
Sim on 26 Aug 2022
Edited: Sim on 26 Aug 2022
Thanks a lot @Star Strider! I try to reply with bullet points....
1. Your code did what I needed. Indeed, by writing just one time a certain equation (in this case: "y = exp(-a*x); "), that certain equation was "automatically" written in the plot's text, as desired, just through a combination of the latex and text functions....
2. Yes please, if you re-post your solution I will accept it..
3. Note: I do not know if it is possible, but the best for me would be to use your solution (or something very similar), but showing the variable name "a", instead of its value "0.5", in the plot's text...

Matt J on 24 Aug 2022
Does the text have to be situated inside the plot? ezplot() will automatically create a plot title displaying the mathematical form of the function.
syms x
a=0.5;
ezplot(exp(-a*x))
Sim on 26 Aug 2022
Edited: Sim on 26 Aug 2022
thanks for your reply @Matt J ... well, I would like to have the text near the curve (maybe exporting the string and use it with the text function)...Other way around... Is there a way to extract the text "exp(-x/2)" from ezplot (not recommended) or fplot ?

vamshi sai yele on 26 Aug 2022
Hi there,
My understanding is that you wish to display a matlab-written equation as text on a graphic and as per my knowledge, MATLAB currently does not support this but the work around would be
You can accomplish this by utilising the "text" function. The code for the same is displayed below:
x = linspace(0,10,50);
y = sin(x);
plot(x,y)
u = '\leftarrow sin(\pi) = 0';
text(pi,sin(pi),u)
In the above code, first and second arguments of ‘text’ function are (x,y) coordinates respectively on the plot where the equation starts.
To address specifically about the equation you mentioned, last line should be modified as:
text(2.5,0.5,'\leftarrow y=e^-^a^x')
or
text(2.5,0.5,'\leftarrow y=e^-^a^x')
Make changes in the arguments accordingly to achieve the desired results…!!
Sim on 26 Aug 2022
Thanks @vamshi sai yele, but what you wrote is what I did already to show my desired output (please read my "Note 2")..

Walter Roberson on 26 Aug 2022
Write your "master" equation as a character vector or string instead of as a symbolic expression.
Then have the code use str2sym() to convert the character vector or string into something symbolic that you could then calculate with and plot. You can then text() the character (or string) form of it.
In this way you only have to change the character (or string) form of the equation and re-run in order to get the description updated.
Sim on 28 Aug 2022
oh cool! I did not think about it !!! I will try it :-)
Many thanks @Walter Roberson :-)