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

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Ramesh Bala 2021-9-15

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https://ww2.mathworks.cn/matlabcentral/answers/1453514-getting-error-as-array-indices-must-be-positive-integers-or-logical-values-in-slope-calculation

评论： Walter Roberson 2021-9-17

采纳的回答： Cris LaPierre

R.mat

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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.

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Walter Roberson 2021-9-15

R(ij,ji-1)

What's that when ji is 1?

Ramesh Bala 2021-9-15

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

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采纳的回答

Cris LaPierre 2021-9-15

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1453514-getting-error-as-array-indices-must-be-positive-integers-or-logical-values-in-slope-calculation#answer_788004

编辑：Cris LaPierre 2021-9-15

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.

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Ramesh Bala 2021-9-15

在 MATLAB Online 中打开

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 2021-9-17

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

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