Interpolation on tables using matlab

Hello, I am solving several probems in Thermodynamics and one of my most time consuming things solving problems is the interpolation.

That not exactly the same as an interpolation on a plot, so thats why Im writing a new post.

I had my exam and succeed in MatLab last week but was a short tour of 15 hours so Im still new at this! :)

So the thing is I have a table giving several temperatures and for the given temperatures I have columns next to them giving the related energy, enthalpy and entropy.

So if I have temperature that is not given on the table I have to do an interpolation for the temperature I need, so I was thinking about a function that could read the 3 temperatures as x1 x2 x3 then read the 2 energys lets say as y1 y3 and I will need to calculate the y2 which is what Im looking for.

The equation Im using on my calculator looks like y2={[(x2-x1)/(x3-x1)]*(y3-y1)}+y1

I know is simple but Im just too short on time and cant get it to work :(

Answer 1

[Cris LaPierre]

If you are using MATLAB, just use the interp1 function. Just be sure to follow the expected syntax, described in the linked documentation page.

[ORESTIS KOKKINOS]

Thank you I wasnt sure if it will work as I coundnt understand fully the descreption I will check it thanks!

[Cris LaPierre]

Syntax splits the knowns and unknowns.

% known x,y values
x = [1 3];
y = [2 6];

% Interporate to find y when x=2.
xq = 2;
yq = interp1(x,y,xq)

yq = 4

% Just to check, plot results
plot(x,y)
hold on
plot(xq,yq,'ro')

[ORESTIS KOKKINOS]

unfortunatly this is not faster than calculating by hand.

Iv seen a function in Fortran that you could just type in 5 values and it calculates the 6th value for you.

Like your example will be 1 (2)3 2 6 and the function knows the order and has the equation in the memory solving for 2, each time you give a number you just press enter , the program was like: read x1 read x2 ... etc, can something similar be done on matlab?

[Cris LaPierre]

You could write your own function to do whatever you want.

Not sure what is making this so slow for you. I would argue that interp1 also takes 5 values and calculates a 6th.

yq = interp1([1 3],[2 6],2)

yq = 4

[ORESTIS KOKKINOS]

yes it takes the value and calculates the 6th, but is just way more time consuming than in fortran, I just cant instal fortran on my laptop or I would so I tought of trying to do it in matlab.

fortran program looks like

program interpolation

implicit none

real x1,x2,x3,y1,y3,y2

print*, ''First the 3 known values then the 2 values than contain the unknown value in the middle

read*,x1

read*,x2

read*,x3

read*,y1

read*,y3

y2=(((x2-x1)/(x3-x1))*(y3-y1))+y1

print*, y2

end program interpolation

hope this helps,

I tried writing something simillar but it wont work thats why I thought maybe someone can help on this one.

I tried this on a friends laptop and this takes just a few secs you just type in the values in order and it gives the y2 value in no time

[Cris LaPierre]

Like I said, you can write a similar function in MATLAB if you are particular about the calling syntax. You could write that same function in 3 lines in MATLAB. If that's what you prefer, see this documentation page on how to create a function.

[ORESTIS KOKKINOS]

Thanks I will try it!