Trouble with ez-plot

I'm writing a program to find principle stresses as assigned in a class that I am taking. I am trying to use ezplot to solve for areas to find the roots of the stresses. I keep running into trouble with this because the program keeps printing an error message about elements in I must equal B. I used it earlier to plot a quadratic and it worked fine. I tried to read some posts about this but it didn't make much sense. Any help would be greatly appreciated in advance. How do I check the sizes of the elements in I and in B? I started working exclusively with the string that was giving me the trouble. Here is the code along with the command I issue in the command window.

 function [ z ] = Untitled3(sig )
  sig=-200:200;
  z=sig.^3-.120*sig.^2-11.8*sig-(-844)

end
sig=(-200:200);ezplot(@(z)Untitled3(sig),[-20,20])

I took three zeros off the coefficients of sig trying to help but it doesn't seem to matter. The error message displayed is this: ??? In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in ==> ezplotfeval>applyfun at 79 z(i) = feval(f,x(i));

Error in ==> ezplotfeval at 66 z = applyfun(x);

Error in ==> ezplot>ezplot1 at 472 [y, f, loopflag] = ezplotfeval(f, x);

Error in ==> ezplot at 146 [hp, cax] = ezplot1(cax, f{1}, vars, labels, args{:});

Any help would be appreciated. And if you could explain in layman's terms that would be great too.

Answer 1

[Andrew Newell]

The syntax of your command is wrong. This is how you do it:

f = @(sig) sig.^3-.120*sig.^2-11.8*sig-(-844);
ezplot(f,[-200,200])

The domain you're plotting over should not appear inside the function. You could also define your function in a separate file as follows:

function y = f(x)
y = x.^3 - .120*x.^2 -11.8*x-(-844);

By the way - do you really mean to write -(-844), which is the same as +844?

[Adam Anderson]

I was actually plugging numbers into an equation. In the original code it finds those constants and puts them in itself. I cann't believe that is what it is. I have been messing with this for hours today.

[Adam Anderson]

wow. The syntax I was using is something similar to what the teacher posted in our class notes.

[Andrew Newell]

You might find this page useful: http://www.mathworks.com/help/techdoc/learn_matlab/f4-2525.html. It's part of the "Getting Started" section of the MATLAB documentation.

Answer 2

[Walter Roberson]

Do not overwrite sig inside of Untitled3 . ezplot is going to call Untitled3 with however many elements it wants in the arguments that it feels like, but because you ignore the argument passed in and use your own sig, you always return 401 outputs instead of the size of the argument that ezplot() passes in.

[Adam Anderson]

I wish I could accept both answers. They should incorporate that in Matlab forums. I appreciate all he help I receive from all the members on here. Basically when you declare inputs / outputs in the function you cannot repeat those terms elsewhere in the function. Or can I declare inputs just no outputs?

[Walter Roberson]

You *can* use the input or output arguments elsewhere in a function, and there are good uses for that sometimes.

However, when you do it, you have to be careful that the size of whatever you do return matches the size expected by the calling function. Also if you assign a value to the output parameter, you have to be careful in case you end up having an early "return" statement, or in case a section of code you are thinking will always write in a new value does not *always* get executed. It isn't that these things cannot be programmed, but you need to take care when you do the programming.

You _can_ declare a function with no outputs:

function myfun(x)

[...]

end

If, though, this gets invoked in a circumstance where the caller expects an output value, then you will get an error.

Also, if you have a function that has no outputs, then the only purpose in calling the function is for "side effects" -- e.g., values printed, global variables changed, shared variables changed, object properties set(). Functions of this nature, executed for their side effects, occur quite often when programming GUIs, but otherwise are _mostly_ used for printing values (or some Object Oriented Programming purposes.)

It does not appear to me that using a function with no outputs would be useful in your circumstances.

[Andrew Newell]

Adam, you can't accept both answers but you can vote for Walter's.