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

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

Answer 1

[Torsten]

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

[darova]

sorry, it works

[darova]

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')

Answer 2

[darova]

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

Answer 3

[roberta perna]

Sorry not work.

[darova]

What doesn't work? What did you try?

Answer 4

[James Browne]

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')

[darova]

Im afraid it doesn't work

[James Browne]

I somehow messed up on the upload, I corrected the previous answer, it should work now.