[Rd] Non identical numerical results from R code vs C/C++ code?

Renaud Gaujoux renaud at mancala.cbio.uct.ac.za
Fri Sep 10 12:46:30 CEST 2010


suppose you have two versions of the same algorithm: one in pure R, the 
other one in C/C++ called via .Call().
Assuming there is no bug in the implementations (i.e. they both do the 
same thing), is there any well known reason why the C/C++ implementation 
could return numerical results non identical to the one obtained from 
the pure R code? (e.g. could it be rounding errors? please explain.)
Has anybody had a similar experience?

By not identical, I mean very small differences (< 2.4 e-14), but enough 
to have identical() returning FALSE. Maybe I should not bother, but I 
want to be sure where the differences come from, at least by mere curiosity.

Briefly the R code perform multiple matrix product; the C code is an 
optimization of those specific products via custom for loops, where 
entries are not computed in the same order, etc... which improves both 
memory usage and speed. The result is theoretically the same.

Thank you,

Renaud Gaujoux
Computational Biology - University of Cape Town
South Africa



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