# Error using for loop

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i'm currently working on the discrete Wigner ville transform, wich require to use a concatenated for loops. But when i run the code i get an error message : * Subscript indices must either be real positive integers or logicals.*. a simple version of my code:
for i = 1 :length(A)
for k = 1:length(B)
a = A(i+k)*A(i-k)
end
end

#### 댓글 수: 1

Jos (10584) 12 Feb 2018
It would be helpful if you give
1. the actual code
2. short examples of the inputs A and B,
3. the expected output
4. the complete error message

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### 답변(3개)

Jan 11 Feb 2018
Posting a simplified version of the real failing code has the disadvantage, that the actual problem might be concealed.
Use the debugger to examine the problem:
dbstop if error
Now your code will stop at:
a= A(i+k) * A(i-k)
when i-k is not a positive integer. Neither 0 nor negative indices are allowed in Matlab.

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Mohamed Sebnat 12 Feb 2018
exactly my code stop working at
a = A(i+k)*A(i-k)
i understand that 0 and negative indices are not allowed in Matlab, so i will reformulate my question: how can I work around this issue? in other word how can I write the same formulae with respect to Matlab Syntax?

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possibility 11 Feb 2018
There are two main fails in your code. Elaborating Jan's post;
(1) when i=1 and k=1, i.e. first loop, A(i-k) = A(0) fails. The first element is starting with index 1.
(2) Even if you correct (1), you will see the same error when i=length(A), k=1. Because you attempt to an index A(i+k) which is larger than length(A).

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Mohamed Sebnat 12 Feb 2018
how can I work around this tow issues, any ideas ?

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In first iteration , value of i and k is 1 so A(i-k) means A(0) and in matlab "0" can not be index of any array so this cause error in your case. So, i-k should not be equal to zero otherwise it will generate error.

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