CODEWITHSUNDEEP
matlabfloating-point
FalafelPita
FalafelPita

Reputation: 293

Matlab: Find next lesser floating point number

The Matlab function eps(x) returns "the positive distance from abs(x) to the next larger floating-point number of the same precision as x." I use this to calculate the smallest floating-point number greater than x, via x + eps(x). I would also like to obtain the largest floating point number less than x, but I'm unaware of a function similar to eps that would facilitate this. How can I find the largest floating point number less than x?

Upvotes: 4

Views: 589

Answers (2)

Simon Byrne
Simon Byrne

Reputation: 7874

For all numbers except subnormals (i.e. those smaller than 2.225073.8585072014e-308) you can do:

v = 1 - eps()/2;
b = x / v; # next larger in magnitude
c = x * v; # next smaller in magnitude

(based on this answer).

Upvotes: 0

Ander Biguri
Ander Biguri

Reputation: 35525

You can subtract eps in almost all cases.

However as you probably have realized, this does not apply when the mantisa changes, or in other words, when you want to subtract from a power of two.

A negative-side eps then is easy to implement, knowing that the current eps is smaller than the distance to the next power of two that will trigger a step change. Therefore, the eps of our number minus its eps should do the trick.

function out=neps(in)

out=eps(in-eps(in));

This seem to work fine

eps(2)

     4.440892098500626e-16

neps(2)

     2.220446049250313e-16

Upvotes: 6

Related Questions