from a column in a table, get row indices where value changes

Question

Centauri Jolene 2020-9-14

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/593170-from-a-column-in-a-table-get-row-indices-where-value-changes

评论： Centauri Jolene 2020-9-14

采纳的回答： Stephen23

data.mat

在 MATLAB Online 中打开

I have a table with a column of 0s and 1s and I need to get the row indices where the value changes.

Essentially, I need the row indicies corresponding to the sets/sequences of 1s.

For exmaple, in this data, I would need row indicies 2, 4 and 8, 10.

I atatched an example dataset (as a table) with the column called 'B' which has the 0s and 1s.

I tried the following codes:

        yIdx = find(data.Y, 1); % only returns the first instance of 1
        [~, yIdx] = ismember(1, data.Y); % only returns the first instance of 1
        
        index = any(data.Y == 1, 2),  %returns a vector of logicals
        
        test = find(any(data.Y == 1, 2)); % this gives me all the row indices for all instances of 1. I need just the indices where a sequence of 1s starts and ends.

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Answer 1

Stephen23 2020-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/593170-from-a-column-in-a-table-get-row-indices-where-value-changes#answer_494122

编辑：Stephen23 2020-9-14

在 MATLAB Online 中打开

The trick is to use diff, e.g. with the example data from your question:

>> Y = [0;1;1;1;0;0;0;1;1;1;0;0];
>> D = diff([0;Y;0]);
>> S = find(D>0) % start of each run of ones
S =
    2
    8
>> E = find(D<0)-1 % end of each run of ones
E =
    4
   10

For the uploaded table's column Y this gives:

>> D = diff([0;data.Y;0]);
>> S = find(D>0) % start of each run of ones
S =
          99
         149
        5481
        6073
>> E = find(D<0)-1 % end of each run of ones
E =
         105
         157
        5538
        6076