MATLAB: Subscript indices must either be real positive integers or logicals

coding errorfaq

Hello, I'm trying to define the Sherman-Morrison formula:

using the code

invA = A.^(-1)

n=size(A,1)

u=ones(n,1)

Z = 1 + u'*invA*u

inv(A + u*u')=invA – 1/Z * invA * u*u' * invA.

I'm using Z here instead of alpha in the problem. It always returns to an error "Subscript indices must either be real positive integers or logicals". What does it mean? Can somebody help with it??

Matlab new starter here, all help would be appreciated. Thanks heaps!!

Best Answer

There is a more fundamental issue here:

inv(A + u*u')=invA - 1/Z * invA * u*u' * invA

The above line indicates that you need to go over some basic MATLAB tutorials. You can't just type and equation with variables on both sides and solve it like you would in Maple or Mathematica. While:

New = inv(A + u*u')

is a perfectly valid line:

inv(A + u*u') = New

is not.

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MATLAB: How to fix Index exceeds matrix dimensions

First, you should predefine the size of u instead of changing the size throughout your script.

Second, in the first for loop you have written

 u(i,1) = it0(x(i));

But x is not defined, did you mean to use the variable a as input? Why do you even need a loop, the following should be fine (assuming u is predefined)

 u(:,1) = it0(a);

When I run your code I do not get the "Index exceeds matrix dimensions" error but instead I get an error because the number of rows in u is not equal to the number of values in a. Because you did not predefine the size of u the size will be 81*81 and the vector a will have length 120.

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Your boundary condition function includes the line

pl = ul-TT(i);

TT(i) is not defined within view of the function, so that is what is causing the error. To make that work you need to pass TT in as an extra parameter. But, still, the index i is undefined. So I would recommend just doing something like:

function [pl,ql,pr,qr] = pdex1bc(xl,ul,xr,ur,t)
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If you really want to pass in TT as a parameter, change the header of the boundary function to

function [pl,ql,pr,qr] = pdex1bc(xl,ul,xr,ur,t,TT)

and then define the variable

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Also, although these don't cause errors some other possible issues are:

The constant k appears in your flux term f = ..., but not in your original equation.
Your source term should just be s = 0 from the original equation, but you have lines to define y, F, and s.

Best Answer

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MATLAB: How to fix Index exceeds matrix dimensions

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