Error using plot Vectors must be the same length.

good evening family I was calculating the autocorrelation of the function cos (wt) but every time I compile I get an error like Error using plot Vectors must be the same length.

Help me please.

this is my code :

function autocorrx
f0=20; N=1000  ;
k=5; Tmax=k/f0;
t=0/2:Tmax/N:Tmax;
x=cos(2*pi*f0*t); 
subplot(1,1,1);
plot(t,x);
axis([0 0.1 -1 1]);grid;
xlabel('t(s)');
ylabel('cos(wt)');

C = xcorr(x,x)*Tmax;
subplot(2,2,2) ; plot(t,C) ;
axis([-0 0.1 -1 1]);
grid
xlabel ('t') ;
ylabel ('Cxx(t)') ;
title ('autocorrelation ')
end 

[Adam Danz]

That error means that "Vectors must be the same length". It's pretty clear.

K>> size(t)
ans =
           1        1001
K>> size(C)
ans =
           1        2001

Clearly t and C and not the same length. So Matlab doesn't know how to pair those coordinates which is required for plotting.

[sabayke bremso]

thank you for your answer first the code has no more problem but I can not see autocorrelation of cos (wt) do you know why ???

[Adam Danz]

Because you're crushing the axis limits.

axis([-0 0.1 -1 1]);

Remove that line.

[sabayke bremso]

thank you sir sooooooooooooooooooooooooo much :0

[Adam Danz]

Glad I could help!

Answer 1

[Adam Danz]

The cross correlation function xcorr() will produce an output with 2*M-1 elements where M is the number of elements in the input. Perhaps you want to plot the results as a function of the lag.

[C,lags]= xcorr(x,x);
C = C*Tmax;  % <--- this is from your code; 
plot(lags,C); 

Note that your subplots are not defined correctly. Assuming you want two vertically stacked subplot, the subplot definitions should be

subplot(2,1,1)
subplot(2,1,2)