Getting all max, min values of a function as a set of coordinate points

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rezheen
rezheen 2025-5-6
评论： Walter Roberson 2025-5-9
Ran in:
My code only returns 1 set of points for max, min. There are more points based on the graph I provide. This is my code:
clear, clc
syms x y real
y=abs(x^3-9*x); fplot(y, [-5 5])
dy=diff(y); x_cps=solve(dy==0);
y_cps=subs(y,x_cps); y_max=(1); x_max=(1); y_min=(1); x_min=(1);
% % x_cps & y_cps are the critical points
for k=1:length(y_cps)
if y_cps(k)>y_max
y_max=y_cps(k);
x_max=x_cps(k);
elseif y_cps(k)<y_min
y_min=y_cps(k);
x_min=x_cps(k);
end
end
fprintf('(%s,%s)\n',x_max,y_max)
(3^(1/2),6*3^(1/2))
fprintf('(%s,%s)\n',x_min,y_min)
(-3,0)

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回答（1 个）

Cris LaPierre
Cris LaPierre 2025-5-6
Ran in:
I would use findpeaks
syms x y real
y=abs(x^3-9*x);
[X,Y] = fplot(y, [-5 5]);
Warning: Having two output arguments for fplot will be removed in a future release. Use the XData and YData properties instead.
[pk, loc] = findpeaks(Y,X);
[pkN, locN] = findpeaks(-Y,X);
plot(X,Y,loc,pk,'r*',locN,pkN,'m*')
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rezheen
rezheen 2025-5-9

@ Steven Lord: Can we find intervals of increasing & decreasing with the code you have? That'd be awesome.

Walter Roberson
Walter Roberson 2025-5-9
@rezheen
No, you cannot find intervals of increasing and decreasing with the code posted by @Steven Lord -- not without stripping down to all but the first 5 lines and then adding notable additional code.

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