# How to find out standard deviation of a matrix when there is a lot of NaN values?

3 views (last 30 days)
Ashfaq Ahmed on 14 Dec 2021
Commented: Ashfaq Ahmed on 14 Dec 2021
Hi altruists,
I have a 71*294 matrix that contains a lot of NaN values.
In fact, the first 7 rows, the last 7 rows, and row 30 to 41 are just NaNs! But the rest of the matrix has values. I want to calculate the row-wise std (standard deviation). How can I write down the code that automatically neglects the NaNs and calculate the std of the non NaN values?

Kevin Holly on 14 Dec 2021
Edited: Kevin Holly on 14 Dec 2021
You can ignore NaN values with the std function by adding 'omitnan' as an input.
A = rand(9,9);
A(3,4) = NaN;
A(7,8) = NaN;
A(8:9,:) = NaN
A = 9×9
0.8525 0.7498 0.3202 0.4618 0.2857 0.0152 0.0025 0.7856 0.8587 0.9196 0.9043 0.7895 0.8567 0.1709 0.5375 0.7785 0.3022 0.3562 0.5906 0.8111 0.0398 NaN 0.4670 0.0575 0.9813 0.1220 0.8259 0.1217 0.8268 0.8372 0.2509 0.8764 0.6745 0.6064 0.4309 0.4537 0.0781 0.7542 0.5271 0.0940 0.1519 0.8002 0.9233 0.3918 0.2708 0.5682 0.1108 0.3855 0.1143 0.4872 0.0658 0.5214 0.7553 0.8456 0.2884 0.0712 0.7995 0.0562 0.5466 0.5103 0.9874 NaN 0.8934 NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN
B = std(A,0,2,'omitnan')
B = 9×1
0.3457 0.2871 0.3766 0.2690 0.3229 0.2840 0.3600 NaN NaN
Kevin Holly on 14 Dec 2021
From the documentation on the function provided:
S = std(A,w,dim) returns the standard deviation along dimension dim for any of the previous syntaxes. To maintain the default normalization while specifying the dimension of operation, set w = 0 in the second argument.
The first input to the function, A, is your matrix. The second input, w, is the weight (default is 0 which I had chosen). The third is the dimension to operate along. In this case, 2 was chosen so that the standard deviation of the elements would be calculated in each row. A value of 1 (default value) would calculate the standard deviation for each column.

_ on 14 Dec 2021
std(x,[],2,'omitnan')
where x is your matrix, 2 tells std to go by row, and 'omitnan' tells std to ignore NaNs in x.
Ashfaq Ahmed on 14 Dec 2021
Thank you so much :)