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

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.

Thanks in advance.

%testing slope by central difference
load('R.mat');

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.

[Walter Roberson]

R(ij,ji-1)

What's that when ji is 1?

[Ramesh Bala]

R(1,0) doesn't is nothing- doesnt exist

Answer 1

[Cris LaPierre]

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.

[Ramesh Bala]

Thank you for your valubale reply Cris

But its as per the Central diff scheme (pic) ,even if I reduce ji as 1:1:42 (ji+1 =43) I get similar error.

[Cris LaPierre]

You haven't solved the first issue yet (ji-1 = 0 when ji=1)

Consider setting your loop counter to "for ji = 2:42"

[Ramesh Bala]

Yes that's right R(1,0) doesn't exist.

[Ramesh Bala]

Let's say if I stop it till 1 and then use another loop for rest? then join all together

for ji = 1
    dy(ij,ji)  = (R(ij,ji+1))/(2/delx);
    
    for kj =2:1:42
        
    dy2(ij,kj)  = (R(ij,kj+1)-R(ij,kj-1))/(2/delx); 
    
    end
end

[Walter Roberson]

You should be assigning to dy2(ij,1) and to dy2(ij,43)

Answer 2

[Image Analyst]

A thorough discussion of the error is in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22