How to Compare the lengths of vectors

HI, In the code below, i want to plot : "plot (f,AmpTab);" and i get this warning :

"Error using ==> plot Vectors must be the same lengths."

i want to compare the length of AmpTab to the length of f, but i don't want to loss any value or date of AmpTab.

i know that there are operations of "zeros" or " padarray " but i don't know how to use it correctly.

can you help me please ?

clear all;
close all;
Fs = 200
t= 0:1/Fs:1
y = 3*sin(2*pi*10*t) + 7*sin(2*pi*20*t) + 11*sin(2*pi*30*t); % input in time domain
L=length (y);
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1);

figure(1)
plot(f,2*abs(Y(1:NFFT/2+1))) ;

figure(2)
[B] = sort(2*abs(Y(1:NFFT/2+1))); %order the amplitudes
A1=B(end); %amplitude of the first peak
A2=B(end-1); %amplitude of second peak
AmpTab=[A1 A2];
plot (f,AmpTab);

Answer 1

[Wayne King]

Not sure why you want to pad the DFT in this case, you end up not getting accurate frequency estimates by doing that.

 Fs = 200;
 t= 0:1/Fs:1-1/200;
 y = 3*sin(2*pi*10*t) + 7*sin(2*pi*20*t) + 11*sin(2*pi*30*t); 
 L = length(y);
 Y = fft(y)/L; 
 f = 0:Fs/length(y):100;
 figure(1), plot(f,2*abs(Y(1:length(y)/2+1))) ; xlabel('Hz');
 [B,I] = sort(2*abs(Y(1:length(y)/2+1)),'descend'); %order the amplitudes 
 figure(2)
 stem(f(I(1:2)),B(1:2),'color',[0 0 1],'markerfacecolor',[0 0 1]);
 set(gca,'xlim',[0 100]); xlabel('Hz');

[engineer bsc]

HI Wayne King,

Thank you for you answer, my main aim is to calculate the amplitude and the frequency of the signal after the FFT.

i don't know how to do that, i saw that there is an operation "sort" so i tried to calculate the amplitude with that, that is the reason i asked about the padding and comparing the vectors length

can you guide me, how can i do this correctly please ?