rounding to zero small numbers (NOT round function)

atabak mostafavi さんによって質問されました 2019 年 10 月 30 日

最新アクティビティ Walter Roberson さんによってコメントされました 2019 年 10 月 31 日

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hey

so i want matlab to consider every number below a certain level (for example 0.001) as zero in the whole script. while right now it returns some computation result even in 10e-15, is there any defult value for matlab ?

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回答者: dpb 2019 年 10 月 30 日

No. ML basic numeric type is the floating point double which around zero has roughly the above magnitude of precision.

You'll have to set values less than your chosen threshold to zero explicitly.

The round function with the optional second option could be useful tool in this, but simply

x(x<threshold)=0;

will avoid the rounding that occurs in it. Of course, you'll want to use abs to get both positive and negative sides of the rounding that will likely occur.

You might also find ismembertol of use depending upon your actual code.

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Walter Roberson 2019 年 10 月 31 日

What result are you getting, exactly, for which there is "no explicit solution" ? Do you mean the result of null() on the symbolic numeric array is the empty array? If so then as far as the routine can tell, the array is full rank.

atabak mostafavi 2019 年 10 月 31 日

ok when I excute this:

det(B)

coeff_sol=null(sym(B))

I get this:

ans =

-8.7503252381931774667359844484583e-38

coeff_sol =

Empty sym: 1-by-0

Walter Roberson 2019 年 10 月 31 日

I would be interested to see what shows up for

dets = det(sym(B))
detn = double(dets)

However for large B this can take a fair while.

You might want to look at https://cseweb.ucsd.edu/classes/wi15/cse252B-a/nullspace.pdf at how null space can be determined by SVD. That form would give you the flexibility to permit columns that you consider "close enough" to zero.

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