find() function indexing seems confusing

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Zeyuan 2025-9-22

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https://ww2.mathworks.cn/matlabcentral/answers/2180098-find-function-indexing-seems-confusing

评论： Alexander 2025-10-7

I am writing the code where I want to find the min distance from each point to cluster and the cluster they are assigned to. A step of the k-means algorithm. So I used the function of find. I am confused because even though I tried to set examples, but I keep finding myself in a loophole.

[s_norm, ind] = sort(norm_x(i,:)); % keep i th rown in ascending order
vec_distance(i) = s_norm(1);       % take the smallest distance
index(i) = find(ind == 1);

Here is the example I work with

norm_x(i,:) = [2.5   1.0   0.8   3.2]
[s_norm, ind] = sort(norm_x(i,:))
s_norm = [0.8   1.0   2.5   3.2]
ind    = [3     2     1     4]

As a reuslt, find(ind==1) should return "3" but why is my code not running? Please help.

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the cyclist 2025-9-22

Ah, I only noticed the

[2.5 1.0 0.8 3.2]

and wondered about the 1.0. I had not paid sufficient attention to the sort in there.

Alexander 2025-10-7

在 MATLAB Online 中打开

Most likely an initialization problem before. Try one of the following:

%Example (also possible with double entries)
i=1;
norm_x(i,:) = [2.5   1.0   0.8   3.2];
% Find first smallest value + first index
[vec_distance(i), index(i)] = min(norm_x(i,:));
% Solution for an array
[vec_distance, index] = min(norm_x, [], 2);
% robust form if you you have double entries or want to find e.g. (ind == 2)
[~, ind] = sort(norm_x(i,:));
rank = zeros(1,numel(ind));   % double
rank(ind) = 1:numel(ind); % Inverse permutation
rank_of_col1 = rank(1) % should result in 3
rank_of_col1 = 3
rank_of_col2 = rank(2) % should result in 2
rank_of_col2 = 2
% Maybe you initialize "ind" incorrectly before? Try this for debugging:
[~, ind] = sort(norm_x(i,:));
class(ind)     % -> 'double'
ans = 'double'
find(ind==1,1) % should result in 3
ans = 3
find(ind==2,1) % should result in 2
ans = 2