在foreach包的mannual中，高效并行的例子：将行分块后，发送到各个workers：applyKernel <- function(newX, FUN, d2, d.call, dn.call = NULL, ...) { foreach(x = iblkcol(newX, 3), .combine = "c", .packages = "foreach")%dopar% { foreach(i = 1:ncol(x)) %do% FUN(array(x[, i], d.call, dn.call), ...) }}applyKernel(matrix(1:16, 4), mean, 4, 4)会报错：Error in eval(expr, envir, enclos) : could not find function "iblkcol"解决方法一：将以下iblkcol函数加入foreach/iterators包的source中iblkcol <- function(a, chunks) { n <- ncol(a) i <- 1 nextElem <- function() { if (chunks <= 0 || n <= 0) stop('StopIteration') m <- ceiling(n / chunks) r <- seq(i, length=m) i <<- i + m n <<- n - m chunks <<- chunks - 1 a[,r, drop=FALSE] } structure(list(nextElem=nextElem), class=c('iblkcol', 'iter'))}nextElem.iblkcol <- function(obj) obj$nextElem()
解决方法二：改用通用的iter函数参数iterator
applyKernel <- function(newX, FUN, d2, d.call, dn.call = NULL, ...) { foreach(x = iter(newX,chunks=3), .combine = "c", .packages = "foreach")%dopar% { foreach(i = 1:ncol(x)) %do% FUN(array(x[, i], d.call, dn.call), ...) }}applyKernel(matrix(1:16, 4), mean, 4, 4)
