Finding multiple x values in plot for given y

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Geroge Papas
Geroge Papas 2020-12-17
回答： Geroge Papas 2020-12-17
I have to find the values for x when y=0 but i cant seem to make it work. I have tried with interp1 but it seems that it gives me an answer of x=2.3490 which is clearly wrong,since there are 2 points in the plot where y=0. Here's the code:
x = [0:0.001:pi]
f = exp(x/4).*sin(x)-1;
plot(x,f)
xlabel('Axis x');
ylabel('Axis y');
newLim = get(gca,'XLim');
newx = linspace(newLim(1), newLim(2), 30);
set(gca,'XTick', newx);
% what I tried:
yi = 0 ;
xi = interp1(f,x,yi) ;
hold on
semilogy(xi,yi,'*r')

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采纳的回答

Geroge Papas
Geroge Papas 2020-12-17
Welp I solved it! I used zero-crossings estimation code and got the index vectors i needed to find the values of x when y aproximates 0. Here is the finished code and graph:
x = [0:0.001:pi]
f = exp(x/4).*sin(x)-1;
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);
zx = zci(f);
figure(1)
plot(x, f, '-r')
hold on
plot(x(zx), f(zx), 'bp')
hold off
grid
legend('f(x)', 'Approximate Zero-Crossings')
%x1=0.9180
%x2=2.5900

更多回答（1 个）

Daniel Pollard
Daniel Pollard 2020-12-17
I think you want fsolve. Give it a function handle and an initial starting point, and it'll tell you (approximately) what x gives f(x)=0.

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