Indexing error in function code.

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Jim Oste
Jim Oste 2015-2-23
评论： dpb 2015-2-23
I have the following function:
function [ r ] = ost_approx_rate_conv( h_1,B )
n = 1:length(B);
k = 2:1:length(B);
h = zeros(size(n));
h(k+1) = h(k-1)/2;
h(1) = h_1;
h = logical(h);
r(h) = log((B(2*h)-B(2*h))/(B(h)-B(2*h)))/log(2);
and when I try to run it I get "Subscript indices must either be real positive integers or logicals." error for my r(h) term. How exactly would I remedy this?
  1 个评论
dpb
dpb 2015-2-23
Need to first define what it is you're trying to do. This is peculiar at best and undoubtedly not what you intend but I can't tell what is wanted from the code alone.
For example, k is vector of integers from 2:length(B) and h is a zero-valued vector of length(B) but then you assign the 3rd thru (N+1)th positions to the values from the 1:(N-1)th (dividing by 2, but zero/2 is still zero, so all you've really done in the end is
h=zeroes(1,length(B)+1);
h(1)=h_1;
after all that.
The explicit cause of the error, however, is that when you write
B(2*h)
despite have previously converted h to logical array, that you've subsequently multiplied by 2 means its numeric and most (if not all) the entries are zero. Matlab arrays are one-based so 0 is not either a positive integer nor a logical.
BTW, the expression
log((B(2*h)-B(2*h))
is log(0) as the two terms are identical so this also is clearly not what you're intending but I've no kluw what that might be...

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回答（2 个）

Andrew Newell
Andrew Newell 2015-2-23
编辑：Andrew Newell 2015-2-23
You'll get the same error with this code snippet:
h = logical([1 0]);
B(2*h)
or, for that matter, just
B(0)
The problem is that by multiplying h by 2, you're converting it back to a double precision number. So you have indices
2*h
ans =
2 0
and the second index is zero. As to how you might fix it, I can't suggest anything because I don't understand what you are trying to do.
Michael Haderlein
Michael Haderlein 2015-2-23
You initialize h as zeros. k goes from 2 to length(B). The line
h(k+1) = h(k-1)/2;
remains all h as zeros (and I'm questioning if this line is doing what you want - extending h by one zero).
The line
h = logical(h);
sets all these zeros (only the first element of h might have been set nonzero) to false. However, the line
r(h) = log((B(2*h)-B(2*h))/(B(h)-B(2*h)))/log(2);
wants to check B(2*h). The multiplication of a double with a logical returns a double. In case of the logical being false, the double will be zero. So, finally, you ask for B(0) and that does not exist in Matlab.
I guess there are quite some operations here which are not intended. Maybe you might want read about logical indexing as a keyword. Also, you can explain the purpose of your code and we might be able to hint you to working code.

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