CODEWITHSUNDEEP
octave
Archisman Panigrahi
Archisman Panigrahi

Reputation: 633

Only work with upto 8 decimal places in Octave

When the eigenvalues of matrices with integer entries (even 3x3 matrices) are calculated in Octave, sometimes it reports a floating point value like 9.1940e-17, whereas it should really be zero. For example, when I am plotting some numbers modulo 1, a value -2e-17 is becoming 1 in the plot, but it should actually be 0.

Although I can approximate the answers upto some decimal places later, it will still internally calculate upto 17 decimal places while calculating the eigenvalue and waste calculation time. Is it possible to work with only upto something like 8 decimal places for all quantities from the beginning?

If that is not possible, is it possible to tell octave (once and for all in a script) to approximate all quantities upto 8 decimal places while reporting?

Upvotes: 1

Views: 412

Answers (2)

Daniel
Daniel

Reputation: 36710

What you are asking for won't help, it makes it worse. Setting the precision to 8 decimal points (or significant digits) won't make numerical errors disappear, it will increase the numerical errors to that magnitude. Take a simple calculation:

x=1/3
y=x*6

Now you would end up with 1.99999998, a much larger error.

There are libraries doing the opposite, increasing the precision. In Octave that's called vpa.

Upvotes: 2

Cris Luengo
Cris Luengo

Reputation: 60615

To complement Daniel's answer, the additional precision doesn't cost any additional time. The computer works with the number as a whole, not with individual digits.

You might want to use 

A(abs(A)<1e-8) = 0;

after computing your final result A to set the near-zero values to zero.

Upvotes: 3

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