find a value between two points
显示 更早的评论
Hey everyone,
I have the following case : (picture linked to the tocpic).
Let's suppose that i have two points with their value on y-axis equals to -20.5 and -19.5. I also know their value on x-axis (let's suppose 10^9 and 2*10^9). I don't have more points between them.
I would like to know : How can I find the value on X-axis of the point that have the value on y-axis equal to -20.
If it's not understandable, I hope the picture linked to it will be helpful !
Thanks you very much !
2 个评论
Mathieu NOE
2021-9-15
hi
seems you want to have this intermediate point from a curve that passes through the 2 points. have you tried to create this curve or a approximation (fit) of it ? then you can pick anypoint from this curve.
otherwise you could do simple interpolation, but with only two input points this would be limited to a simple linear interpolation , which is not what your picture says.
采纳的回答
Example —
x = linspace(0, 50);
y = 30*(1-exp(-0.1*x));
f = @(x) 30*(1-exp(-0.1*x));
x_20 = interp1(y, x, 20) % From Data
f_20 = fzero(@(x) f(x)-20, rand) % From Function
figure
plot(x, y)
grid
hold on
plot(x_20, 20, '+r')
plot(f_20, 20, 'xg')
hold off
.
4 个评论
I am not certain what your function (or data) look like, so I can’t address it precisely. The best way to approach something like that is to subtract 20 from it, and then use interp1 at the zero-crossings:
x = linspace(0, 50, 250);
y = 20*(1-exp(-0.1*x)) + 5*sin(2*pi*x*15);
yfind = 20;
yz = find(diff(sign(y-yfind)));
for k = 1:numel(yz)
idxrng = max(1,yz(k)-2) : min(numel(x),yz(k)+2); % Index Range
xi(k) = interp1(y(idxrng),x(idxrng),yfind);
end
xi
figure
plot(x, y)
yline(yfind, '--r')
hold on
plot(xi, ones(size(xi))*yfind, 'sg', 'MarkerSize',10)
hold off
grid
legend('Data','Target Value','Interceptions', 'Location','best')
Experiment with my code and your (x,y) data.
.
dsq dq
2021-9-15
Star Strider
2021-9-15
As always, my pleasure!
.
更多回答（1 个）
KSSV
2021-9-15
Read about interp1. If you have curve data as (x,y), at the point xi you can seek the value using:
yi = interp1(x,y,xi)
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