intersction graph with line

Hi everyone, can u help me out im still pretty green. This is a simple one but I cant figure it out. I tried to look for similar problem but did not find satisfy answer.

I'm having some data set which I plot with basic plot command. My question is, can matlab give me right away answer for the y value at my distance of say 4.5 like in the simplify example having in mind that my graph is just a line connecting y values

x=1:10;
y=-5*rand(1,10);
plot(x,y)
hold on
plot([4.5 4.5],[0 -5],'r')

thanks in advance.

Answer 1

[Star Strider]

I am not exactly certain what you want, but my guess would be:

p = polyfit(x,y,1);

y1 = polyval(p, 4.5);

The value returned for y1 would then be the value that might be expected to be at an x value of 4.5 on that particular iteration based on a linear fit. Since your data are changing between iterations (they are random), y1 will change with them.

If you want to know what the value is on a line between connecting points, to calculate the value for 4.5, the easiest way is probably:

p = polyfit([4 5], y([4 5]), 1);

y1 = polyval(p, 4.5);

That is how I would do it.

(And thank you Walter for your editing.)

[Imo]

I'm sorry if I confused you. Your solution is what I need, thank you very much. Rand function is just to give me any set of data just for quick example here. I actually have points that represent my river profile and I need matlab to give me depths in particular places of that profile based on that interpolation.

Only problem would be here that my profile consist of a large amount of dots and i don't know the values of y bewven my specified point (in example 4.5) that i need for interpolation. I thought matlab will figure it out on its own:)

[Star Strider]

It is my pleasure to help!

I do not know what your data look like so I cannot suggest a specific solution that might work for you in all situations. However, I suggest you also look at MATLAB's various interpolation functions, for example interp1, and more generally, the functions in Interpolation and Computational Geometry.

Thank you for accepting my answer.