copying matrix values based on a specific value in another column

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okoth ochola 2023-1-20

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评论： Jiri Hajek 2023-1-20

Hello, suppose we have A=[1 1 1 1 2 2 2 2 3 3 3 3]' and B=[3 4 6 7 8 9 0 12 4 5 6 8]', so that C=[A B]. Is there a way I can copy all element values in column 2 corresponding to say all 1 in column 1 then put them in another matrix? The actual matrices am dealing are much much larger than what have given above there. Basically I want to extraxt values from column 2 corresponding to specific values in column 1 for larger matrix.

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Answer 1

Jiri Hajek 2023-1-20

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Hi, for this, se use logical indices in MATLAB:

A=[1 1 1 1 2 2 2 2 3 3 3 3]'; B=[3 4 6 7 8 9 0 12 4 5 6 8]'; C=[A B];
selectedRowsLogicalIndices = A == 1;
subsetBdata = B(selectedRowsLogicalIndices)
subsetBdata = 4×1
     3
     4
     6
     7

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okoth ochola 2023-1-20

Thank you, one more question kindly. Suppose matrix A was consisting of date strings from jauary to Dc say A=[01/01/2020 02/01/2020 03/01/2020 ... december], now i want to pick values that corresponds to only specific month say january. I was thinking I can convert all the dates to months only and then use the criterion above. Do you have any ideas kindly.

Jiri Hajek 2023-1-20

You can use all logical operators in the expression, where you define logical indices. Also, note that dates (datetime class) can be compared using <, > operators, so it should be straightforward. So there is no need to convert dates to a different class (e.g. double).

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