Interpolate between specific values in a data set

I am having trouble trying to figure out how to interpolate between specific y values for 2 given x values from excel.

For example in excel:

X            Y
1            0.2
2.2            0.35
3            0.4
4            0.5

then given an X value of 3.5 from a matlab output I would like to find the corresponding Y value, in this case it would be 0.45. Then store that and use that in the future. I am not sure how to search for where 3.5 would sit within the X column and then find the surrounding values to do the interpolation.

Answer 1

[the cyclist]

You can use the interp1 function to do interpolation.

x = [1, 2.2, 3, 4];
y = [0.2, 0.35, 0.4, 0.5];

x0 = 3.5;

y0 = interp1(x,y,x0)

y0 = 0.4500

[Reece Smith]

oh okay thanks, didn't realise it was as simple as that. I implemented it into the code but the values seem to be slightly off incomparison to my hand calculated values which I have double checked are right, I am guessing this is just since interp1 does it a little different?

[Torsten]

0.45 = 0.4 + (0.5-0.4)/(4-3) * (4-3.5)

Linear interpolation between (3/0.4) and (4/0.5).

What's your method ?

[Reece Smith]

Yea its the one you just put. I am comparing my answers to someone elses interpolations so maybe they just rounded earlier and made it slightly different.

[the cyclist]

@Reece Smith, if you can upload an example of the data and code that give a different result from what you expect, we could try to help you understand the difference.