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Dan
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Subscript indices must either be real positive integers or logicals.

Asked by Dan
on 29 Sep 2013
Latest activity Commented on by azure roy on 3 Jun 2016
%Number of components
K = zeros(1,111);
%sampling frequency/timing interval
fs = 8000;
Fs = 10*fs;
T = 2;
%frequency components
fk =100:30:3400;
%amplitude components
Ak = rand(1,length(K));
%phase components
phik = 2*pi*rand(1,length(K));
for t = 0:(1/Fs):T %time interval
K = Ak.*cos(2*pi.*fk*t + phik);
y = t/(1/Fs)+1;
xt(1,y) = sum(K,2);                        
end                        

I'm trying to run this code but for some reason there is an error at y = 14 which is no where near the end of the loop. I'm pretty sure that xt(1,14) is a valid index. Any reason why this might be happening?

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3 Answers

Answer by Matt J
on 29 Sep 2013
Edited by Matt J
on 29 Sep 2013
 Accepted Answer

y will not be 14 exactly because you use non-integer arithmetic to generate it. Using round() might be a possible fix.

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Answer by dpb
on 29 Sep 2013
Edited by dpb
on 30 Sep 2013

The answer is in the following--

>> fs = 8000;
>> Fs = 10*fs;
>> t=0:1/Fs:2;
>> y = t/(1/Fs)+1;
>> all(fix(y)==y)
ans =
   0
>> neq=find(fix(y)~=y);
>> neq(1)
ans =
  14
>> y(14)-fix(y(14))
ans =
 1.7764e-15
>> length(neq)
ans =
     39209
>> length(y)
ans =
    160001
>> 

Moral: Don't use FP calculations as indices...

  2 Comments

Hmm where does that discrepancy in y values come from?

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Answer by Wayne King
on 29 Sep 2013
Edited by Wayne King
on 29 Sep 2013

I'm not sure what you are trying to do, create a superposition of sine waves with different frequencies and a random phase and amplitudes? If so, I'll show you how to do that.

But your immediate issue here is that y=0 when t = 0 and then you try to access the zero-th column of xt(), but that can't exist.

You can do this.

Fs = 8000;
t = (0:1/Fs:2-1/Fs)';
fk = 100:30:3400;
X = zeros(length(t),length(fk));
phi = -pi+2*pi*rand(1,length(fk));
Ak = rand(1,length(fk));
tmat = repmat(t,1,length(fk));
omega = 2*pi*fk;
omega = repmat(omega,size(tmat,1),1);
phi = repmat(phi,size(tmat,1),1);
freq_phi = omega.*tmat+phi;
Ak = repmat(Ak,size(tmat,1),1);
sigz = Ak.*cos(freq_phi);
sig = sum(sigz,2);

Now plot the resulting signal

plot(t,sig)

  1 Comment

theta_i(i,j)=2*pi*rand(1) this is showing subscript indices should be integer or logical value. please help with the solution.

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