# How to get x value for a given y value in a interp1 figure?

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wuwei han el 28 de Mzo. de 2024
Comentada: Star Strider el 28 de Mzo. de 2024
If I want to find x value for y==0.5, how can I find that? We only have 9 points.
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Dyuman Joshi el 28 de Mzo. de 2024
Use the interp1 function as you have mentioned.
What seems to be the problem?
Refer to its documentation for information regarding accepted syntaxes.
wuwei han el 28 de Mzo. de 2024
Sorry, I described wrong situation.
vv=[0.190934428093434,0.277000826121106,0.477820361464529,0.703789686451856,1,0.703789686451856,0.477820361464529,0.277000826121106,0.190934428093434]
plot(-40/3/8:10/3/8:40/3/8,vv, 'r*-')
like this, how can I find x for y==0.5?

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John D'Errico el 28 de Mzo. de 2024
Editada: John D'Errico el 28 de Mzo. de 2024
You need to understand there is no unique solution. So asking for THE value of x has no answer, since there are two possible solutions.
vv=[0.190934428093434,0.277000826121106,0.477820361464529,0.703789686451856,1,0.703789686451856,0.477820361464529,0.277000826121106,0.190934428093434]
vv = 1×9
0.1909 0.2770 0.4778 0.7038 1.0000 0.7038 0.4778 0.2770 0.1909
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plot(-40/3/8:10/3/8:40/3/8,vv, 'r*-')
Can you solve the problem? Well, yes. It not not that difficult, as long as you accept the idea of multiple non-unique solutions, and you use a tool that can do the job correctly. One such tool is intersections, as found on the file exchange (written by Doug Schwarz.)
You asked for the x-value of those points where the curve crosses y==0.5.
[xint,yint] = intersections(-40/3/8:10/3/8:40/3/8,vv,[-2 2],[.5 .5])
xint =
-0.79244
0.79244
yint =
0.5
0.5
intersections is fast and robust. It can handle cases where the curve has an infinite slope for example, where interp1 will fail miserably.
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wuwei han el 28 de Mzo. de 2024
Thank you very much! I have already known that.

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### Más respuestas (2)

Star Strider el 28 de Mzo. de 2024
Editada: Star Strider el 28 de Mzo. de 2024
To get both of them —
vv=[0.190934428093434,0.277000826121106,0.477820361464529,0.703789686451856,1,0.703789686451856,0.477820361464529,0.277000826121106,0.190934428093434];
L = numel(vv)
L = 9
xv = -40/3/8:10/3/8:40/3/8;
dv = 0.5;
zxi = find(diff(sign(vv - dv)));
for k = 1:numel(zxi)
idxrng = max(1, zxi(k)-1) : min(L,zxi(k)+1);
xp(k) = interp1(vv(idxrng), xv(idxrng), dv);
end
format long
xp
xp = 1×2
-0.792436118381859 0.792436118381859
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figure
plot(xv, vv, '*-r')
hold on
plot(xp, ones(size(xp))*dv, 'sg', 'MarkerSize', 10)%, 'MarkerFaceColor','g')
plot(xp, ones(size(xp))*dv, '+k', 'MarkerSize',10)
hold off
yline(0.5, '--k')
.
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wuwei han el 28 de Mzo. de 2024
Star Strider el 28 de Mzo. de 2024
My pleasure!

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Yash el 28 de Mzo. de 2024
Hi Wuwei,
You can use the "interp1" function to find the values from the interpolation.
Given below is an example:
x_ref = [-1.7 -1.2 -0.7 -0.4 0 0.4 0.7 1.2 1.7];
y_ref = [0.2 0.3 0.5 0.7 1 0.7 0.5 0.3 0.2];
x_test = [0.5];
y_test = interp1(x_ref, y_ref, x_test)
y_test = 0.6333
plot(x_ref, y_ref, 'r', x_test, y_test, 'ko')
Since "x" is not a function of "y" here (one value of "y" leads to multiple values of "x"), you cannot directly use "interp1" for "x" vs "y". Either use half of the points in that case, or use some other techniques to make "x" a function of "y".
You can refer to the following documentation of the interp1 function for more details: https://www.mathworks.com/help/matlab/ref/interp1.html
Hope this helps!
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wuwei han el 28 de Mzo. de 2024
Thank you very much. I have known there may be no way to find 'x' by 'y' directly.

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