# Help with matrix indexing.

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Lucy on 26 Jan 2011
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?

the cyclist on 26 Jan 2011
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

Siddharth Shankar on 26 Jan 2011
if "i" is 1, then you can not index into c with the row index "i-1". MATLAB uses 1-based indexing. You need to "special case" your code to avoid indexing into row 0, or a row value that is negative, as the error message suggests.

Lucy on 26 Jan 2011
Hi, I solved the problem by using
if i==1
cellindex=16;
elseif i~=1
cellindex=i-1;
c(cellindex,:)
end
Thank you both anyway.
##### 2 CommentsShowHide 1 older comment
Lucy on 26 Jan 2011
I can see your point now. Thanks

Paulo Silva on 26 Jan 2011
Lucy there's no need for any if statements or other fancy functions
nrows=16 %choose the number of rows (horizontal lines)
indrow=2:nrows+1 %create the index of the rows, the trick is here (+1)
C(indrow-1,:) %no problem now, indrow-1 is never zero
##### 2 CommentsShowHide 1 older comment
Paulo Silva on 26 Jan 2011
Now I understand, thanks for the good explanation :) forget about my code, it just rotates the rows 180º