I know a certain y1 and I want to find the respective x1.

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roberta perna 2019-6-14

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评论： James Browne 2019-6-15

I have a graph where x and y are two vectors.

plot(x,y)

I know a certain y1 and I want to find the respective x1.

thanks

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Answer 1

Torsten 2019-6-14

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x1 = fzero(@(z)interp1(x,y,z)-y1,1)

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darova 2019-6-14

sorry, it works

darova 2019-6-14

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It's difficult to choose intial condition:

x = linspace(0,3,10);
y = x.^2;
y1 = 4;
x1 = fzero(@(z)interp1(x,y,z)-y1,1)
plot(x,y,x1,y1,'or')

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Answer 2

darova 2019-6-14

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I'd use polyxpoly or intersections

x = linspace(0,10,20);
y = sin(x);
y1 = 0.6;                           % find x at y=0.6
xc = [min(x) max(x)];               % boundaries of horizontal line
yc = [y1 y1];
[x1,y1] = polyxpoly(x,y,xc,yc);
plot(x,y,'.-b')
hold on
plot(xc,yc,'.-g')
plot(x1,y1,'or')
hold off

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Answer 3

roberta perna 2019-6-14

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Sorry not work.

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darova 2019-6-14

What doesn't work? What did you try?

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Answer 4

James Browne 2019-6-15

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编辑：James Browne 2019-6-15

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Greetings,

I believe I have a solution to your problem, I wrote a scrip to demonstrate the concept, let me know if this is not what you are looking for and I will try to find a better solution. Here you go:

close all
clear
clc
%set arbitrary values of x, x can be any set of inputs to y(x)
x = [ 1 2 3 4 5 6 7 ];
%generate an arbitrary  set of values y(x), y(x) can be any function
y = 2*x;
%Select a known, arbitrary value fromthe y-vector, y1
y1 = y(randi(length(y)))
%Now we first find the index of the value y1
idx = find( y == y1)
%Since every entry in the y-vector is dependant on a corrosponding entry in
%the x-vector, we can use the index value of y1 to find the corrosponding
%value in the x-vector ( x1 ) as follows:
x1 = x(idx)
plot(x,y,x1,y1,'kd')
title('y = f(x) and (x1, y1)')
ylabel('y(x)')
xlabel('x')
legend('y = f(x)','(x1, y1)','location','southeast')