用最新的非 NA 值替换 NA [英] Replacing NAs with latest non-NA value

问题描述

在data.frame(或data.table)中，我想向前填充"具有最接近的先前非 NA 值的 NA.一个简单的例子，使用向量(而不是 data.frame)如下:

In a data.frame (or data.table), I would like to "fill forward" NAs with the closest previous non-NA value. A simple example, using vectors (instead of a data.frame) is the following:

> y <- c(NA, 2, 2, NA, NA, 3, NA, 4, NA, NA)

我想要一个函数 fill.NAs() 允许我构造 yy 使得:

I would like a function fill.NAs() that allows me to construct yy such that:

> yy
[1] NA NA NA  2  2  2  2  3  3  3  4  4

我需要对许多(总共约 1 Tb)小型data.frame(约 30-50 Mb)重复此操作，其中一行是 NA 是它的所有条目.解决问题的好方法是什么?

I need to repeat this operation for many (total ~1 Tb) small sized data.frames (~30-50 Mb), where a row is NA is all its entries are. What is a good way to approach the problem?

我做的丑陋的解决方案使用了这个函数:

The ugly solution I cooked up uses this function:

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] <- 0
isNA.neg[isNA.diff > 0] <- 0
which.isNA.neg <- which(as.logical(isNA.neg))
if (length(which.isNA.neg)==0) return(NULL) # generates warnings later, but works
which.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
}

函数fill.NAs的用法如下:

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]
} 

输出

> y
[1] NA  2  2  2  2  3  3  3  4  4  4

...这似乎有效.但是，伙计，它是丑陋的！有什么建议吗?

... which seems to work. But, man, is it ugly! Any suggestions?

推荐答案

您可能想要使用 zoo 包以向前进行最后的观察以替换您的 NA 值.

You probably want to use the na.locf() function from the zoo package to carry the last observation forward to replace your NA values.

这里是帮助页面的使用示例的开头:

Here is the beginning of its usage example from the help page:

library(zoo)

az <- zoo(1:6)

bz <- zoo(c(2,NA,1,4,5,2))

na.locf(bz)
1 2 3 4 5 6 
2 2 1 4 5 2 

na.locf(bz, fromLast = TRUE)
1 2 3 4 5 6 
2 1 1 4 5 2 

cz <- zoo(c(NA,9,3,2,3,2))

na.locf(cz)
2 3 4 5 6 
9 3 2 3 2 

这篇关于用最新的非 NA 值替换 NA的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！