MATLAB: Help with matrix indexing.

indexingMATLAB

C is a matrix with i rows and 5 columns. i=1:16
I am interested in finding the value of C(i-1,:).
Using the following matlab code, I try to direct matrix c row i-1 any column to c row 16 any column when i equals 1.
if i==1
  c(i-1,:)=c(16,:);
end
 Error: Subscript indices must either be real positive integers or logicals.
Problem: It gives me above error message as it seems always recognize the c(i-1,:)=c(0,:) first and therefore ignore the statement after that which is equalling to the c(16,:).
Any idea how to solve it?

Best Answer

You seem to be treating this in a sorta symbolic way trying to tell MATLAB that the c(i-1) element is really element c(16,0) when i==1. However, what you have done in practice is an assignment statement,
>>  c(0,:) = c(16,:)
which is not valid because you can't assign to element c(0,:).
I think the easiest way to accomplish what you want is to use the CIRCSHIFT command:
>> index = (1:16)'
>> down_one_index = circshift(index,1)
The guts of CIRCSHIFT, in your case, is the following use of MOD:
>> mod((0:15)-1, 16)+1
This creates the periodic indexing that you want. Hope that helps.
the cyclist
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