Question

我正在尝试计算滚动的马哈拉诺比斯距离，而不求助于for循环和惨败。

这是一个示例数据集：

df <- data.frame(label = c(rep("A", 5), rep("B", 5)),
               date = rep(seq.Date(from = as.Date("2018-01-01"), by = "days", length.out = 5), 2),
               valx = c(rnorm(5, mean = 0, sd = 1), rnorm(5, mean = 1.5, sd = 1)),
               valy = c(rnorm(5, mean = 100, sd = 10), rnorm(5, mean = 115, sd = 10)),
               valz = c(rnorm(5, mean = 0, sd = 10), rnorm(5, mean = 0, sd = 30)))

我试图按组（label）计算valx，valy和valz的马哈拉诺比斯距离，但仅使用该日期之后的行（date）或更早的版本。我当前的解决方案是遍历每个label，遍历每个date，将数据集过滤到匹配数据，使用stats::mahalanobis计算距离，将该距离添加到列表中，然后do.call和rbind不在循环中*。显然，这并不理想。

我怀疑有某种写法：

cum.mdist <- function(df, cols) {...}
df %>%
  group_by(label) %>%
  arrange(date) %>%
  mutate(mdist = xapply(., c(valx, valy, valz), cum.mdist)) %>%
  ungroup()

以类似于计算滚动一元函数的方式，如下所示：

cumsd <- function(x) sapply(seq_along(x), function(k, z) sd(z[1:k]), z = x)

如果没有协方差，我可以计算到零件的距离（滚动方差方差很容易使用上述函数来计算），但是我认为我的变量 do 具有协方差，并且我不确定如何建立滚动协方差矩阵...

对此问题的解决方案是否存在于for循环之外？

*循环解决方案的代码如下：

library("tidyverse")
df <- data.frame(label = c(rep("A", 5), rep("B", 5)),
                 date = rep(seq.Date(from = as.Date("2018-01-01"), by = "days", length.out = 5), 2),
                 valx = c(rnorm(5, mean = 0, sd = 1), rnorm(5, mean = 1.5, sd = 1)),
                 valy = c(rnorm(5, mean = 100, sd = 10), rnorm(5, mean = 115, sd = 10)),
                 valz = c(rnorm(5, mean = 0, sd = 10), rnorm(5, mean = 0, sd = 30)))
mdist.list <- vector(length = nrow(df), mode = "list")
counter <- 1

for(l in seq_along(unique(df$label))){
  label_data <- df %>%
    filter(label == unique(df$label)[l])

  for(d in seq_along(unique(label_data$date))){
    label_date_data <- label_data %>%
      filter(date <= unique(label_data$date)[d])

    if(nrow(label_date_data) > 3){
      label_date_data$mdist <- mahalanobis(label_date_data %>% select(contains("val")),
                                           colMeans(label_date_data %>% select(contains("val"))),
                                           cov(label_date_data %>% select(contains("val"))))
    } else{
      label_date_data$mdist <- NA
    }

    mdist.list[[counter]] <- filter(label_date_data, 
                                    date == unique(label_data$date)[d])

    counter <- counter + 1
  }
}

mdist.df <- do.call(rbind, mdist.list)

Answer 1

不确定我是否正确理解您的要求或期望的输出，以下是使用data.table来帮助您入门的信息：

library(data.table)
setDT(df)
df[, mdist := 
    .SD[, transpose(lapply(1L:.N, function(n) {
        ma <- .SD[1L:n]
        ans <- tryCatch(mahalanobis(ma, colMeans(ma), var(ma)), error=function(e) NA)
        ans[length(ans)]            
    })), by=.(label), .SDcols=valx:valz]$V1]

输出：

    label       date         valx       valy        valz     mdist
 1:     A 2018-01-01  1.262954285   7.635935  -2.2426789        NA
 2:     A 2018-01-02 -0.326233361  -7.990092   3.7739565        NA
 3:     A 2018-01-03  1.329799263 -11.476570   1.3333636        NA
 4:     A 2018-01-04  1.272429321  -2.894616   8.0418951 2.2500000
 5:     A 2018-01-05  0.414641434  -2.992151  -0.5710677 0.7260652
 6:     B 2018-01-01 -1.539950042  -4.115108  15.1082392        NA
 7:     B 2018-01-02 -0.928567035   2.522234  32.5730809        NA
 8:     B 2018-01-03 -0.294720447  -8.919211 -20.7286152        NA
 9:     B 2018-01-04 -0.005767173   4.356833 -38.5379806 2.2500000
10:     B 2018-01-05  2.404653389 -12.375384   1.4017852 3.0800360

数据：

set.seed(0L)
df <- data.frame(label = c(rep("A", 5), rep("B", 5)),
    date = rep(seq.Date(from = as.Date("2018-01-01"), by = "days", length.out = 5), 2),
    valx = c(rnorm(5, mean = 0, sd = 1), rnorm(5, mean = 0, sd = 1)),
    valy = c(rnorm(5, mean = 0, sd = 10), rnorm(5, mean = 0, sd = 10)),
    valz = c(rnorm(5, mean = 0, sd = 10), rnorm(5, mean = 0, sd = 30)))

如果您仅寻找tidyverse解决方案，我将删除此帖子。

R

1 个答案: