问题
What I want to ask is, algorithmically, what do the rowMeans() and colMeans() functions do to optimize speed?
回答1:
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.
回答2:
rowMeans and colMeans are faster than because they call C code directly, rather than being interpreted by the R interpreter.
来源：https://stackoverflow.com/questions/12759937/why-are-colmeans-and-rowmeans-functions-faster-than-using-the-mean-funct