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Subscript indices must either be real positive integers or logicals.

Asked by Dan on 29 Sep 2013
Latest activity Edited by dpb on 30 Sep 2013
%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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Dan

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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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Matt J
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

Dan on 29 Sep 2013

Hmm where does that discrepancy in y values come from?

dpb on 30 Sep 2013

FP rounding...

dpb
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)

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Wayne King

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