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How to create a string that depends on user input variables?

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Joseph Wales
Joseph Wales on 28 Apr 2018
Commented: Rik on 1 Mar 2021
Hi,
I am currently writing a script in which data collected from a separate file is used to plot curve fits.
I also need to include the equation used for the curve fit on the plot.
I encountered an issue when attempting to do this with a general polynomial, as the degree of the polynomial is decided by the user.
A shortened version of my code is this:
data = load('data_1.txt');
x = data(:,1);
y = data(:,2);
d = input('To what degree would you like your polynomial?');
p = polyfit(x,y,d);
yfit = polyval(p,x);
plot(x,y,'bo',x,yfit,'r--')
I need a way to create a string that is the correct form of the equation of the polynomial so that it can be properly displayed on the plot.
For example, if the user chooses degree 2, I need to display the equation "p(1)*x^2 + p(2)*x + p(3)" on the plot.
Is there a way to do this for any value of degree?
Any help is appreciated, thanks.

Answers (2)

Rik
Rik on 28 Feb 2021
You can create this text with sprintf and some minor tweaks:
str=sprintf('p(%d)*x^%d +', [1:d;(d-1):-1:0]);
str=strrep(str,'^1','');
str=strrep(str,'x^0 + ','');

Siyu Guo
Siyu Guo on 28 Apr 2018
Try poly2str function. For example, s = poly2str(p, 'x').
  1 Comment
培林 王
培林 王 on 28 Feb 2021
Well, I can't use this function now and I didn't find it in the toolbx. what's the problem?

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