Help with m file

I am having a problem on the creation of my m file.

When i call my m file, i pass the parameters T and N, and then i am trying to put my signal, which is a variable of time(t). Matlab says that variable (t) is Undefined even though in my m file i calculated the time :

t= (0:Ts:(N-1)*Ts);

What is wrong with that?

My m file is here :

function [x,X_mag] = frequencyfft(T,N,signal)
Ts=T/N;
Fs=1/Ts;
df=Fs/N;
t= (0:Ts:(N-1)*Ts);
x=signal
subplot(2,1,1)
plot(t,x);
X=fft(x,N);
fourier_X=X/Fs;
X_mag=abs(fourier_X);
f=(0:df:(N-1)*df)-N/2*df;
subplot(2,1,2);
stem(f,fftshift(X_mag));

Thanks in advance

[Image Analyst]

Please give input parameters (attach your test script), and give us the complete error message. Don't snip or paraphrase the error message give us everything - ALL the red text.

[John D'Errico]

While image is right, it is hard to guess what you did. But if I had to guess from your comment, I'll bet you did something like this:

[x,X_mag] = frequencyfft(T,N,signal(t))

I'm not sure why you might have done that anyway, since your function never uses the variable signal.

It is all a complete and wild guess of course.

[Image Analyst]

A good guess - I'd put that as an "Answer" John. By the way, t should be

t= linspace(0:(N-1)*Ts, length(signal));

so that t and signal (which is also called x) will definitely have the same number of elements.

[Star Strider]

      . . .
  t= (0:Ts:(N-1)*Ts);
  x=signal                  <——«
  subplot(2,1,1)
      . . .

[Klimis]

>> [x,X_mag] = frequencyfft(0.1,128,sin(2*pi*t/T))
Undefined function or variable 't'.

Thats the error i get, even though i determined my t variable at my m file

[Klimis]

John you are right, i didnt have to use the variable signal.I could have named it x from the beggining. I named it signal for me to remember what is what :D

[Star Strider]

Does the error message also refer to the specific line in your frequencyfft function that throws the error?

Please include the entire series of error messages that a single error creates. If a function throws an error, there are usually other references in the error messages to lines within the function that can help define the problem.

[Klimis]

Image Analyst the thing is that the length of the signal is determined from its time. Thats why i have to determine time(t) first and then the signal will have the same length as time(t).

Star Strider i dont get are you trying to show

[Image Analyst]

In short, run your program and when it errors out, copy everything you see in red, and paste it back here . Make sense?

Answer 1

[Star Strider]

The error implies that you apparently did not define ‘t’ in your script that calls frequencyfft, becasue it is attempting to evaluate sin(2*pi*t/T) before it passes it as an argument to frequencyfft.

The error does not appear to be in frequencyfft itself.

[Klimis]

Yea i know thats the problem, the think is how can i make it use variable t as the one i have on my script. Is there a way to do that?

[Star Strider]

If you have defined ‘Ts’ and ‘N’ in your calling script (the one that calls frequencyfft), you can define it exactly as in your frequencyfft function by doing the same in your calling script:

t= (0:Ts:(N-1)*Ts);

Now it exists in your calling script, and the call you posted earlier should execute without error. (If all goes well, frequencyfft will execute without error as well.)

[Klimis]

but that means that the frequencyfft script is doing almost nothing.. i am doing it all again for it to work :D

[Star Strider]

Not really. You are creating the signal you want frequencyfft to analyse.

Do you want to pass a function handle to frequencyfft in your ‘signal’ argument? You can do that by passing it as for instance @sin for the sine function. Then this line:

x=signal

becomes:

x = signal(t);

Run this little function to see how it works:

function y = trigeval(fun)
      t = linspace(0,2*pi);
      y = fun(t);
end

y = trigeval(@tan)
figure(1)
plot(y)

[Star Strider]

One question I should have asked hours ago:

What do you want frequencyfft to do?

What input arguments do you want to pass to it, and what do you want it to return?

Answer 2

[Image Analyst]

You did exactly what John suspected. You did this:

>> [x,X_mag] = frequencyfft(0.1,128,sin(2*pi*t/T))
Undefined function or variable 't'.

Now, when you call that, what do you think it's supposed to use for t? You haven't defined it. You need to define t first, and then create your signal and pass it in

t= (0:Ts:(N-1)*Ts);  % This is what you were missing when you tried to call it.
signal = sin(2*pi*t/T);
[x,X_mag] = frequencyfft(0.1,128,signal)

[Klimis]

Ts is undefined also. That means i have to define T and N before i use my m file so i can define my Ts and then my t, if thats the case i am doing all the work without my m file. Or maybe i am missing something from what you answered me?