Rename the indices of a vector in a certain way.

Hi all,

My code below has a vector A and its index vector idx. It goes throguh the elements of A and when it finds a zero element, it removes its index from the orginal index vector.

My problem here is: suppose the updated index vector is . How can I rename this vector so it increases in an acending order? For example: the first element in the updated index vector is 1, I want this to be 1, the second element in the updated index vector is 3, I want this to become 2, and so on so forth.

%define the index vector
idx = [1 2 3 4 5];
%define the vector
A   = [5 0 6 0 3];
%initialise the vector that has all indices of zero elements
zero_indices = [];
%go through the elements of vector A
for uu = 1:numel(A)   
    %if the element is zero
    if A(uu)==0
        %assign its index to the zero-elements index vector
        zeroindex = uu;
        %collect all results from iterations
        zero_indices = [zero_indices ; zeroindex];
    end
end
%update the original index vector by removing the indicies that correspond
%to zero elements
idx(zero_indices) = [];
%rename the indices os that they beocome 1, 2, 3, ...

Any help would be appreicted.

Thanks.

Answer 1

[Garmit Pant]

Hello LH

Given that you have extracted the indices of non-zero elements in the vector ‘A’, you can rename the index vector to have ascending values using the following code-snippet:

% Given non-zero value indices are [1 3 5] 
idx = [1 3 5]; 

% Rename the indices so that they become 1, 2, 3, ... 
new_idx = 1:numel(idx); 

% Display the updated index vector 
disp('Updated index vector:'); 

Updated index vector:

disp(new_idx); 

     1     2     3

I hope you find the above explanation and suggestions useful!

[LH]

Thanks Garmit, it's indeed helpful.

What if the problem becomes a bit more complicated and the index vector is like . If we extract the unique values of this vector we will get using:

idx_unique = unique(idx);

which will give us the three main values. Renameing these indices using your line:

% Rename the indices so that they become 1, 2, 3, ... 
new_idx = 1:numel(idx_unique); 

But now, how can we apply this on the orginal idx vector so it besomes: ?

Thanks.

[Garmit Pant]

This can be solved by mapping the values of the 'idx' vector to ascending values. The following code snippet does the same:

% Using the idx vector given in the above comment
idx = [1 3 3 5 1 5 3 1]

idx = 1x8

1 3 3 5 1 5 3 1

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% Extract unique values from the updated index vector
idx_unique = unique(idx);

% Rename the indices so that they become 1, 2, 3, ...
new_idx = 1:numel(idx_unique);

% Create a mapping from unique values to new indices
mapping = containers.Map(idx_unique, new_idx);

% Apply the mapping to the original index vector to get the renamed indices
renamed_idx = arrayfun(@(x) mapping(x), idx);

% Display the updated index vector
disp('Renamed index vector:');

Renamed index vector:

disp(renamed_idx);

     1     2     2     3     1     3     2     1

[LH]

That's great! Many thanks Garmit!

[Stephen23]

The simpler MATLAB approaches:

idx = [1,3,3,5,1,5,3,1]

idx = 1x8

1 3 3 5 1 5 3 1

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[~,~,idy] = unique(idx)

idy = 8x1

1 2 2 3 1 3 2 1

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idy = discretize(idx,[unique(idx),Inf])

idy = 1x8

1 2 2 3 1 3 2 1

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