Very, very simple MATLAB question (drawing a line) [SOLVED]

urahonky

I know this is going to be dreadfully easy but I'm not sure how to do it. Basically I want to draw a continuous line on a plot that I have already drawn. I know the m, and the b. How do I draw this line? The values are:

c = [1.0018,2.1111]

Just not sure how I use this to draw a line... Any answers? Thanks!

Rook

I don't actually have matlab in front of me but it's something like:

y = polyval(c,x)

where x is a vector containing the start and end points of your line. then you can just plot(x,y,'-')

urahonky

So if I've got a dataset with x's and y's... I just use the beginning and ending values as the x and y's in that?

Daenris

urahonky wrote: »

So if I've got a dataset with x's and y's... I just use the beginning and ending values as the x and y's in that?

Assuming you are drawing a straight line (and since you mention you have the slope and intercept, I assume you are) then you just need 2 x values for the line (no y values, they will be calculated based on the coefficients you provide).

Just figure out where you want the lowest and highest x-value to be to make a good looking graph, and put those into the polyval function along with your c variable containing slope and intercept.

For example if you have c = [1.0018,2.1111], and you want to plot it along values of x from -2 to 2, you would do

y = polyval(c,[-2,2]);
plot([-2,2],y);

urahonky

You guys are the best. Thank you, it worked perfectly!

e: Does this work for a quadratic function? Or is there something like "quadval"?

inlemur

urahonky wrote: »

You guys are the best. Thank you, it worked perfectly!

e: Does this work for a quadratic function? Or is there something like "quadval"?

Polyval works for any polynomial. You give it a vector P, and based on the number of values in P it will interpret it as the corresponding polynomial. That is, if P has 3 values, they are read as [A B C] in Ax^2+Bx+C.

Also MATLAB's built in help is outstanding. Try "help polyval"

urahonky

Yeah I figured it out. I think their website is better than their help, since it has some examples. I don't think their help function gives those!

inlemur

urahonky wrote: »

Yeah I figured it out. I think their website is better than their help, since it has some examples. I don't think their help function gives those!

Actually, it does. And the command window help generally links to the full help files, with further details and examples. I really can't think of a program with better built-in help than MATLAB.

urahonky

inlemur wrote: »

urahonky wrote: »

Yeah I figured it out. I think their website is better than their help, since it has some examples. I don't think their help function gives those!

Actually, it does. And the command window help generally links to the full help files, with further details and examples. I really can't think of a program with better built-in help than MATLAB.

Wow yeah I'm stupid. I don't know why I thought it didn't have them....

thanks