Interpolating on a semilog scale

I have a set of data, A = [25 20 10 5 2 0.5 0.07] and B = [100 90 80 72 67 56 44]. I've graphed it with A on the semilogx axis and B on the linear y axis. I would like to find the ordered pair (x1, 50), where I know the y-value but not the x-value, but using interp1 has just resulted in linear interpolation that doesn't seem to take into account the semilog scale. How can I find x1?

Answer 1

[Star Strider]

The interp1 function needs to know about the log transformation. Then, it will give you the correct result.

Try this:

A = [25 20 10 5 2 0.5 0.07];
B = [100 90 80 72 67 56 44];

y1 = 50;
x1 = exp(interp1(B, log(A), y1));

figure
semilogx(A, B)
hold on
plot(x1, y1, 'p')
hold off
grid

The plot is just to demonstrate the result graphically.

[Jenna Scott]

Exactly what I was trying to do, thank you!!

[Star Strider]

As always, my pleasure!

[Görkem ÜLKÜTAS]

Hello Star Strider,

Actually, I have the reverse version of this problem but I couldn't modified your answer correctly. In my case,

x = [0.6, 0.8, 1, 1.5, 2, 3, 4, 6, 8]; (log scale)

y = [0.0087, 0.0070, 0.0060, 0.0047, 0.0043, 0.0041, 0.0038, 0.0035, 0.0034]; (linear scale)

I want to interpolate for example x = 1.25 to get corresponding y value. Can you help me please.

Thank you!

[Ander de Marcos Arocena]

Did you find the solution to your problem? I want to do the same