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

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.

Answer 1

[Walter Roberson]

x100 = interp1(y, x, 100, 'linear', 'extrap')

[ayinas]

Thanks alot sir, nd what if i have to check at what value of x i m getting let suppose negative value of y forexample -176

[Walter Roberson]

same technique, just replace the 100 with the query value.

[ayinas]

Sir m getting an error

while giving this command

x(-21) = interp1(y, x, -21, 'linear', 'extrap')

m geting below error

[Walter Roberson]

don't assign to x(-21). You are not defining a function, you are storing into a variable. For example x_21 if you have reason to name it specifically after -21. otherwise you should be talking about positions

xinterp(idx) = interp1(y, x, yquery(idx))

The output should not be the same variable as you are using to do the interpolation from.

[ayinas]

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]

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]

Thanks alot sir.