# bug #53509: log1p: difference result in Octave and MATLAB.

 Submitter: Dildar Sk Submitted: Thu 29 Mar 2018 12:14:25 PM UTC Category: Octave Function Severity: 3 - Normal Priority: 5 - Normal Item Group: Matlab Compatibility Status: Wont Fix Assigned to: None Originator Name: Dildar Sk Open/Closed:  * Closed Release:  * dev Operating System:  * GNU/Linux Fixed Release: None Planned Release: None
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## Discussion

Thu 29 Mar 2018 03:37:52 PM UTC, comment #2:

Both Octave and Matlab are numerical computation languages, rather than pure math.  It is important to get a sense of the size of the error, and if it is on the order of a few eps then that is all that can be expected.

In Octave,

log1p(2) - log(3)
ans =   -2.2204e-16

so the calculation is as accurate as it can be expected given that

eps (2)
ans =    4.4409e-16

Rik <rik5>
Thu 29 Mar 2018 02:42:40 PM UTC, comment #1:

I think this shouldn't be reported as a bug: the function log1p is used to compute in a more stable way when x is small in module. In fact in Octave log1p(x) is computed as log (1+x) when x < - 0.5 or x > 1:

>> log1p(2.1) == log(3.1)

ans = 1

>> log1p(2) == log(3)

ans = 0

Probably Matlab simply uses a different threshold to decide when use the definition log1p(x)==log(1+x) and when an alternative expansion:

>> log1p(0.3) == log(1.3)

ans =

logical

1

>> log1p(0.2) == log(1.2)

ans =

logical

0

Michele Ginesi <m_ginesi>
Thu 29 Mar 2018 12:14:25 PM UTC, original submission:

In MATLAB,

>> log1p (2) == log (3)

ans =

logical

1

>>

In Octave,

>> log1p (2) == log(3)
ans = 0
>>

Dildar Sk <hodor123456>

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