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# How to get value of X based on a specific value of Y in Graph

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ayinas on 27 Oct 2020
Commented: ayinas on 27 Oct 2020
How to get value of X based on a specific value of Y in Graph
for example
x = [ 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ];
y= [ -77.4236 -91.4051 -105.387 -119.368 -133.35 -147.331 -161.313 -175.294 -189.276 -203.258 ]
and i have to find at what value of x i m getting y value of 100 or any negative value of Y.

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### Accepted Answer

Walter Roberson on 27 Oct 2020
x100 = interp1(y, x, 100, 'linear', 'extrap')

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ayinas on 27 Oct 2020
Sir i m getting answer of x for any positive value of y, like the way (mentioned below) u told me,
x100 = interp1(y, x, 100, 'linear', 'extrap')
but when i m taking negative value of y then m getting an error that index must be a positive interger or logical .
x-100 = interp1(y, x, -100, 'linear', 'extrap')
can u please share the syntax for any negative value of y?
Walter Roberson on 27 Oct 2020
The left hand side should be a variable name that does not have to include the number. For example you could assign to xinterp or to x_at_negative_100
ayinas on 27 Oct 2020
Thanks alot sir.

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