# find a value between two points

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dsq dq on 15 Sep 2021
Commented: Star Strider on 15 Sep 2021
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 CommentsShowHide 1 older comment
dsq dq on 15 Sep 2021
Hi,
This curve is already created I have like ~ 8000 points, so I cannot create it. I looked for the interpolation function but I didn't get how this could help me because I would like to have the exact value for -20 ...

Star Strider on 15 Sep 2021
Use interp1 or fzero on a monotonically-increasing (or decreasing) curve such as that.
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
x_20 = 10.9885
f_20 = fzero(@(x) f(x)-20, rand) % From Function
f_20 = 10.9861
figure
plot(x, y)
grid
hold on
plot(x_20, 20, '+r')
plot(f_20, 20, 'xg')
hold off .
Star Strider on 15 Sep 2021
As always, my pleasure!
.

KSSV on 15 Sep 2021
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)
dsq dq on 15 Sep 2021
Hey thanks ! star strider explained it juste below ! thanks for your help