starting indexing of an array at values greater than 1

Hi,

I would like to reindex specific arrays to start at given user input values

for example: A = [1,2,3;4,5,6;7,8,9];

start_values = input('Please enter start values in brackets'); >> [3,3]

%some function for index conversion of A to start at 3,3

%then i can reference the matrix this way

display(A(3,3));

ans =

1

%and therefore i could also call

display(A(5,5));

ans =

9

%%%%%%%%%%%%%%%%%%%%%%%%%%%

P.S. this is for correlating local stiffness coefficients to a global stiffness coefficient in the direct stiffness method.

thanks!

Answer 1

[Stephen23]

Just save the origin-indices in a variable. Then simply subtract them from all the indices after that. You could even make a function to help you do this:

>> A = [1,2,3;4,5,6;7,8,9];
>> X = [3,3]; % origin
>> fun = @(x,y)sub2ind(size(A),1+x-X(1),1+y-X(2));
>> A(fun(3,3))
ans = 1

>> A(fun(5,5))
ans = 9

[Matt N]

I would actually like to make the re-indexing permanent for each index, so that I can call the values at anytime without the need of the function. Is that possible?

[Stephen23]

Short answer: No.

MATLAB indices start from 1 (linear indexing), which is standard in mathematics (and matrix manipulation in particular).

Long answer: You could overload the functions SUBSREF and SUBSASGN, but this would be a massive undertaking, likely be quite buggy, and make your code completely non-portable. Another option is to create some special data object that allows other indexing: you can find some examples on MATLAB File Exchange:

http://www.mathworks.com/matlabcentral/fileexchange/1168-varbase

My advice: stick to one-based indexing.

[Jan]

I agree with Stephen: Instead of reinventing the indexing methods of the programming language, simply add the required offset to the indices.

[David Young]

Matt N asked: "I would actually like to make the re-indexing permanent for each index, so that I can call the values at anytime without the need of the function. Is that possible?"

I think it is possible in one sense - see my answer.

Answer 2

[David Young]

All MATLAB arrays are indexed from 1, but an alternative to Stephen Cobeldick's answer is to make an array Ashifted such that the value of A(1,1) is at Ashifted(3,3) and so on. Like this:

% test data
A = [1,2,3;4,5,6;7,8,9];
start_values = [3 3];

% construct shifted matrix
rowStart = start_values(1);
colStart = start_values(2);
Ashifted = NaN(size(A,1)+rowStart-1, size(A,2)+colStart-1);
Ashifted(rowStart:end, colStart:end) = A;

% check if it has worked
disp(Ashifted(3,3));   % same as A(1,1)
disp(Ashifted(4,5));   % same as A(2,3)

If you are happy to lose the original indexing, you can assign Ashifted to A.

Note that this uses extra memory, because Ashifted has extra rows and columns. I've filled them with NaN values to show they're not meaningful data.

Whether it's preferable to use this approach or to add an offset to the indices each time you access A depends on a number of factors, with tradeoffs in memory, time and code complexity.

[David Young]

Incidentally, I agree with the comments above that overriding subsref is not something worth undertaking.

[Stephen23]

Accidentally setting the origin to [1e7,1e7] could be very interesting... this method is good for low indices, but is not a general solution.

It also completely removes the ability to use several of MATLAB's rather useful indexing methods :

• A(1,:) would not return the first data row
• A(1) would not return the first data value
• A(:) is not a vector of all data values
• etc

In fact it completely removes the ability to use the colon to select a whole row/column (x,:), because this would always include the padding values.