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What I want to ask is, algorithmically, what do the rowMeans() and colMeans() functions do to optimize speed?

Paul Hiemstra
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user1482678
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    I don't think it's algorithmic, I think it's a matter of what can be coded directly in C and what has to go through the R interpreter. – Ben Bolker Oct 06 '12 at 13:05

2 Answers2

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In addition, consider what lapply() does. It sets up repeated calls to the function mean(). So as well as the overhead of actually computing a mean (which is done in fast C code), the lapply() version repeatedly incurs the overhead of the sanity checking code and method dispatch associated with mean().

rowMeans() and colMeans() incur only a single set of sanity checks as internally, their C code is optimised to loop over the rows/columns there rather than via separate R calls.

Gavin Simpson
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rowMeans and colMeans are faster than because they call C code directly, rather than being interpreted by the R interpreter.

Andrie
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    Is there a difference in speed/efficiency between ``rowMeans`` and ``colMeans``? Thanks. (looked at various questions on so, but couldn't find it mentioned, did I miss a related post on this?) – PatrickT Oct 25 '17 at 19:00