Finding the first instance of a value in an array

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Pelajar UM 2021-10-29

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https://ww2.mathworks.cn/matlabcentral/answers/1574953-finding-the-first-instance-of-a-value-in-an-array

编辑： Pelajar UM 2021-10-30

I have an array with many rows and many columns. I want to search the array row by row, find the first instance of a value that is above 170 (yellow cells below) and then store the colum index it was found in (green cells below) in a new array. If there are no values above 170, then return 0 as the index.

This should explain it:

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Answer 1

Walter Roberson 2021-10-29

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https://ww2.mathworks.cn/matlabcentral/answers/1574953-finding-the-first-instance-of-a-value-in-an-array#answer_819908

YourArray = randi([100 180], 10, 20)
YourArray = 10×20
   110   163   114   129   160   127   126   132   104   147   134   159   169   117   170   117   153   127   180   165
   103   137   155   141   159   129   137   162   172   109   159   160   134   156   166   110   179   166   137   136
   116   168   151   105   110   180   116   112   171   152   112   178   154   101   169   137   176   144   137   159
   163   153   100   132   104   153   174   180   127   164   162   127   153   118   113   133   112   158   108   129
   105   118   159   147   137   158   114   168   124   164   146   163   114   107   104   153   144   146   162   130
   122   138   149   162   156   122   113   105   147   147   164   107   149   160   149   117   150   122   138   152
   142   122   149   105   141   127   151   103   133   179   164   133   100   164   129   134   116   124   140   177
   127   162   180   176   175   143   110   149   160   156   171   145   106   156   174   118   159   179   131   161
   104   126   102   109   137   126   118   139   155   180   168   165   151   173   168   167   137   101   120   127
   142   107   130   136   159   126   129   152   127   164   140   129   107   178   122   177   169   169   155   100
idx = sum(cumprod(YourArray <= 170, 2),2) + 1;
idx( idx == size(YourArray,2) + 1 ) = 0;
idx
idx = 10×1
    19
     9
     6
     7
     0
     0
    10
     3
    10
    14

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Pelajar UM 2021-10-30

编辑：Pelajar UM 2021-10-30

Perfect. Now as you see YourArray is an Nx9 double. It represents a certain intensity at 9 points in space. I need to use the idx generated here to sort these 3D point array which is Nx(3x9) ( 3 for x,y,z times M points).

How can I do that? Do I need the row index too?

And my idea was those point that correlate with the idx of 0 will be removed in the sorted array but this is not how it works. I get this error:

Index in position 1 is invalid. Array indices must be positive integers or logical values.

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Answer 2

Chris 2021-10-29

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https://ww2.mathworks.cn/matlabcentral/answers/1574953-finding-the-first-instance-of-a-value-in-an-array#answer_819813

A = magic(5)
A = 5×5
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9
for idx = 1:size(A,1)
    test = find(A(idx,:)>22,1);
    if isempty(test)
        test = 0;
    end
    colidxs(idx,1) = test;
end
colidxs
colidxs = 5×1
     2
     1
     0
     0
     3

A is your array, and replace 22 with 170.

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Chris 2021-10-29

For something a bit faster, try

A = magic(5);
colidxs = rowfun(@findidx,table(A))
function idx = findidx(row)
    idx = find(row > 22,1);
    if isempty(idx)
        idx = 0;
    end
end