# Getting error as array indices must be positive integers or logical values in slope calculation.

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Ramesh Bala on 15 Sep 2021
Commented: Walter Roberson on 17 Sep 2021
My R matrix is 512x43 and I would like to calculate slope by central diff from row wise values.
It shows logical values error ? What's the wrong in this slope calculation.
%testing slope by central difference
dy = zeros(43,1);
ddy = zeros(43,1);
delx = 1e-4;
for ij=1:1:512
for ji = 1:1:43
dy(ij,ji) = (R(ij,ji+1)-R(ij,ji-1))/(2/delx);
ddy(ij,ji) = (R(ij,ji+1)-2*R(ij,ji)+R(ij,ji-1))/delx^2;
end
end
Index in position 2 is invalid. Array indices must be positive integers or logical values.
##### 2 CommentsShowHide 1 older comment
Ramesh Bala on 15 Sep 2021
R(1,0) doesn't is nothing- doesnt exist

Cris LaPierre on 15 Sep 2021
Edited: Cris LaPierre on 15 Sep 2021
I suspect the error is in your column index for R:
• ji-1
When ji=1, the result is 0, which is not a valid index in MATLAB.
Note that you will have another problem when hi=43, as you also use ji+1. The result will be 44, which exceeds the size of R.
Walter Roberson on 17 Sep 2021
You should be assigning to dy2(ij,1) and to dy2(ij,43)

Image Analyst on 15 Sep 2021