Find minimum value in matrix and test for symmetry

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David du Preez 2017-3-8

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https://ww2.mathworks.cn/matlabcentral/answers/328717-find-minimum-value-in-matrix-and-test-for-symmetry

评论： David du Preez 2017-3-13

采纳的回答： Jan

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Hi. I have a 2160x6 matrix (attached). The columns represent: Year(1), month(2), day(3), hour(4), UVI(5),SZA(6).

The data is hourly and I need to find the minimum SZA value for every 24 (hours) for rows. Can I use something like this to find the minimum value?

a=CP_06_07DJF(1:24,6);
  min1=min(a(:))

Then I need to test if the UVI values are symmetric about the point where the SZA is lowest and repeat this for everyday.

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kowshik Thopalli 2017-3-8

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Looking at your data for the first few hours in every 24 hrs the value of SZA is zero. Could you tell us how would you define symmetry now?.

Yes the code that you have will give min value . using the below line will also give you the index at which the minimum value appears

[min1,index]= min(a)

David du Preez 2017-3-8

Once I have the minimum SZA (solar zenith angle)for each day. I need to determine if the UVI data is symmetric. The UVI data represents solar ultraviolet radiation. Therefore when the SZA and UVI the is no data.

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Answer 1

Jan 2017-3-8

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https://ww2.mathworks.cn/matlabcentral/answers/328717-find-minimum-value-in-matrix-and-test-for-symmetry#answer_257813

编辑：Jan 2017-3-8

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Date  = datenum(CP_06_07DJF(1:3), 6);  % Year, Month, Day
Value = CP_06_07DJF(:, 6);
G     = findgroups(Date);
Min1  = splitapply(@min, Value, G);

Or if the data are dense (you get 24 values for each day and no additional hours:

Value = CP_06_07DJF(:, 6);
Min1  = min(reshape(Value, 24, []), [], 1);

This reshape the value to a [24 x N] matrix and finds the minimum along the first dimension.

The "symmetric" part is not clear to me. Can you post a small example?

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Jan 2017-3-13

编辑：Jan 2017-3-13

Please give a short example. How do you define "symmetry" here? Please note that I'm still not sure what "UVI" and "SZA" is. Simply post some values, because the names and meanings do not matter for Matlab also. ;-)

David du Preez 2017-3-13