# copying matrix values based on a specific value in another column

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okoth ochola il 20 Gen 2023
Commentato: Jiri Hajek il 20 Gen 2023
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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Jiri Hajek il 20 Gen 2023
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 il 20 Gen 2023
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 il 20 Gen 2023
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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