How can I extract rows using column values?

Question

Jacob Ebilane 2022-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/1724670-how-can-i-extract-rows-using-column-values

评论： Jacob Ebilane 2022-5-22

emnist.png

I'm trying to use the emnist byclass data set which includes digits and upper/lower case letters. I don't need all of the letters so I wanted to remove the unnecessary ones. The emnist set uses the first column as a label 0-61 and only need for example 0-9 and <49, 55, 11, 32, 13, 29, 30, 20, 33, 43, 24> these ones.

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Answer 1

Walter Roberson 2022-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/1724670-how-can-i-extract-rows-using-column-values#answer_969100

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classes_needed = [49, 55, 11, 32, 13, 29, 30, 20, 33, 43, 24];
mask = ismember(emnist(:,1), classes_needed);
subset = emnist(mask,:);

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Jacob Ebilane 2022-5-22

Worked like a charm, thank you kind sir.

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Answer 2

MJFcoNaN 2022-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/1724670-how-can-i-extract-rows-using-column-values#answer_969105

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% random matrix
A=randi(50,10)
A = 10×10
    17    33    20    13     5    20     1    20    34    20
    26    13    43     3    14    46    22    22    24    41
    29    49    17     9    50    41    48    20    17    35
    11    26    20    15     4    30    50    18    35    32
    22    12     1    12    40    27    25    35     4    21
    12    17    44    47     7    26    46     4    50    47
    21    41    40    38     6    31     4     4    26    14
    23    22    42    18    20     1    23     3     8    41
    28    27    29    28     4    32     5    31    49    29
     6     3    27    14    26    45    15    39    14    32
% needed lines index
ind=ismember(A(:,1), [0:9, 49, 55, 11, 32, 13, 29, 30, 20, 33, 43, 24]);
% needed rows
B=A(ind, :)
B = 3×10
    29    49    17     9    50    41    48    20    17    35
    11    26    20    15     4    30    50    18    35    32
     6     3    27    14    26    45    15    39    14    32