How to find exact match for negative value.

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Kugen Raj 2012-2-29

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https://ww2.mathworks.cn/matlabcentral/answers/30703-how-to-find-exact-match-for-negative-value

Y=sum(y);
YY=abs(Y);
p=20*log10(YY/max(YY));%normalized power in dB.
P=round(p);
[c]=find(P==-3);
bw=c(1,2)-c(1,1);

I used this find() function to find all the vectors that has the exact value. But, then i have this error:

??? Attempted to access c(1,2); index out of bounds because size(c)=[1,0].

Error in ==> Q10 at 54
bw=c(1,2)-c(1,1);

回答（1 个）

the cyclist 2012-2-29

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https://ww2.mathworks.cn/matlabcentral/answers/30703-how-to-find-exact-match-for-negative-value#answer_39163

If no element of P is equal to -3, then c will be empty, and you will get this error.

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Kugen Raj 2012-2-29

i see.thanks for your suggestion. i will try this one out and check for the results.

the cyclist 2012-2-29

Two comments. One is that since you are checking against -3, not +3, the check should have been abs(P+3)<tol. Second, I would still say your rounding approach should have worked here, so my suspicion is that for some reason, your vector P does not actually have values near -3. You could breakpoint the code before the error occurs, to look at the values.

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