用最新的非NA值替换NA

Question

用最新的非NA值替换NA在data.frame（或data.table）中，我想用最近的非NA值“填充”NA。一个简单的例子，使用向量（而不是a data.frame）如下：> y <- c(NA, 2, 2, NA, NA, 3, NA, 4, NA, NA)我想要一个fill.NAs()允许我构造的函数yy：> yy[1] NA NA NA  2  2  2  2  3  3  3  4  4我需要对许多（总计~1 Tb）小尺寸data.frames（~30-50 Mb）重复此操作，其中一行是NA，其所有条目都是。解决问题的好方法是什么？我做的丑陋的解决方案使用这个功能：last <- function (x){    x[length(x)]}    fill.NAs <- function(isNA){if (isNA[1] == 1) {    isNA[1:max({which(isNA==0)[1]-1},1)] <- 0 # first is NAs                                               # can't be forward filled}isNA.neg <- isNA.pos <- isNA.diff <- diff(isNA)isNA.pos[isNA.diff < 0] <- 0isNA.neg[isNA.diff > 0] <- 0which.isNA.neg <- which(as.logical(isNA.neg))if (length(which.isNA.neg)==0) return(NULL) # generates warnings later, but workswhich.isNA.pos <- which(as.logical(isNA.pos))which.isNA <- which(as.logical(isNA))if (length(which.isNA.neg)==length(which.isNA.pos)){    replacement <- rep(which.isNA.pos[2:length(which.isNA.neg)],                                 which.isNA.neg[2:max(length(which.isNA.neg)-1,2)] -                                 which.isNA.pos[1:max(length(which.isNA.neg)-1,1)])          replacement <- c(replacement, rep(last(which.isNA.pos), last(which.isNA) - last(which.isNA.pos)))} else {    replacement <- rep(which.isNA.pos[1:length(which.isNA.neg)], which.isNA.neg - which.isNA.pos[1:length(which.isNA.neg)])         replacement <- c(replacement, rep(last(which.isNA.pos), last(which.isNA) - last(which.isNA.pos)))}replacement}该功能fill.NAs使用如下：y <- c(NA, 2, 2, NA, NA, 3, NA, 4, NA, NA)isNA <- as.numeric(is.na(y))replacement <- fill.NAs(isNA)if (length(replacement)){which.isNA <- which(as.logical(isNA))to.replace <- which.isNA[which(isNA==0)[1]:length(which.isNA)]y[to.replace] <- y[replacement]} 产量> y[1] NA  2  2  2  2  3  3  3  4  4  4......似乎有效。但是，伙计，这太丑了！有什么建议？