re-indexing slices of a matrix

Hi all,

I have a matrix A where its second column represents the index of each row. I want to replace these values so that they start from 1. I tried to do this using the code below, but clearly I'm not succeeding.

mainidx = 4; %main indices
desiredidx = 5; %waht i want the indices to be
%the matrix
A = [1 4 
    4 4 
    3 4
    6 3
    3 3
    2 1
    3 1
    7 2 
    5 2
    9 2
    5 1
    4 1];
%reindexing
for i = 1:mainidx
    for j=1:desiredidx
        idx = A(:,2)==mainidx;
        A(A(:,2)==desiredidx) = mainidx;
        A(idx)  = desiredidx;
    end
end

I want this matrix to look like the following:

A = [1 1 
    4 1 
    3 1
    6 2
    3 2
    2 3
    3 3
    7 4 
    5 4
    9 4
    5 5
    4 5];

Any help would be appreicted.

Answer 1

[Voss]

A = [1 4 
    4 4 
    3 4
    6 3
    3 3
    2 1
    3 1
    7 2 
    5 2
    9 2
    5 1
    4 1];

A(:,2) = 1+cumsum([false; logical(diff(A(:,2)))]);

disp(A)

     1     1
     4     1
     3     1
     6     2
     3     2
     2     3
     3     3
     7     4
     5     4
     9     4
     5     5
     4     5

[LH]

Many thanks! This works the way I want.

[Voss]

You're welcome!