How to get x value for a given y value in a interp1 figure?
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If I want to find x value for y==0.5, how can I find that? We only have 9 points.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1654131/image.png)
2 件のコメント
Dyuman Joshi
2024 年 3 月 28 日
Use the interp1 function as you have mentioned.
What seems to be the problem?
Refer to its documentation for information regarding accepted syntaxes.
採用された回答
John D'Errico
2024 年 3 月 28 日
編集済み: John D'Errico
2024 年 3 月 28 日
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]
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
You can find intersections for free download here:
intersections is fast and robust. It can handle cases where the curve has an infinite slope for example, where interp1 will fail miserably.
その他の回答 (2 件)
Star Strider
2024 年 3 月 28 日
編集済み: Star Strider
2024 年 3 月 28 日
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)
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
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')
.
9 件のコメント
Yash
2024 年 3 月 28 日
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)
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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