get indices for part of a matrix

I have the matrix T, I want to get the indices of a part this matrix. I would imagine using a function of the form:

 ind1  = getindexes(T,(1:35,:));

which would get me the indexes for the (1:35,:) part of matrix T. What is the fastest way to do this?

[Azzi Abdelmalek]

Your question is not clear, you are asking for getting indices that you already have

Answer 1

[Star Strider]

I’m not certain what you want to do. Here are two possibilities:

If you want to find the indices of particular elements in a vector or matrix, use the find function. You will probably have to experiment a bit with it to get the result you want.

If you want to get all the elements in the specific rows from 1:35 for all columns of ‘T’, this will work:

Tx = T(1:35,:);

[Christopher]

I want the former, but this doesn't work:

 ind1=find(T(2:35,:))

because it doesn't find the indices of T, it finds the indices of T(2:35,:). How do I find the indices corresponding to the original matrix?

Edit: changed T(1:35,:) to T(2:35,:) to reinforce my point.

[Star Strider]

If I define ‘T’ as:

T = randi(10,50,50);

This command:

[IndR,IndC] = find(T(2:35,:));

will return two 1700 columns with the first offset by 1, so that ‘IndR’ would result a vector of [1:34] repeating 50 times. (You would have to provide the offset.)

If you want the indices of particular elements om your matrix that meet specific conditions, for instance >10 and <20, the find function for that becomes:

[Ridx1020,Cidx1020] = find(T>=10 & T<=20);

where ‘Ridx1020’ are the row indices and ‘Cidx1020’ are the corresponding column indices of those value of ‘T’ that meet those criteria. To verify that they meet the criteria (always a good check), this will tell you:

for k1 = 1:length(Ridx1020)
    Telm(k1,k1) = T(Ridx1020(k1), Cidx1020(k1));
end

We’ll keep iterating this in the morning.