Question

我正在尝试使用dplyr包计算两个相邻行中两个时间戳之间的时间差。这是代码：

    tidy_ex <- function () {

    library(dplyr)

    #construct example data
    data <- data.frame(code = c(10888, 10888, 10888, 10888, 10888, 10888, 
                                    10889, 10889, 10889, 10889, 10889, 10889,
                                    10890, 10890, 10890),
                           station = c("F1", "F3", "F4", "F5", "L5", "L7", "F1",
                                       "F3", "F4", "L5", "L6", "L7", "F1", "F3", "F5"),
                           timestamp = c(1365895151, 1365969188, 1366105495,
                                           1367433149, 1368005216, 1368011698,
                                           1366244224, 1366414926, 1367513240,
                                           1367790556, 1367946420, 1367923973,
                                           1365896546, 1365907968, 1366144207))

    # reformat timestamp as POSIXct
    data$timestamp <- as.POSIXct(data$timestamp,origin = "1970-01-01")

    #create tbl_df
    data2 <- tbl_df(data)

    #group by code and calculate time differences between two rows in timestamp column 
    data2 <- data2 %>%
            group_by(code) %>%
            mutate(diff = c(difftime(tail(timestamp, -1), head(timestamp, -1))))

    data2

    }

代码会产生错误消息：

 Error: incompatible size (5), expecting 6 (the group size) or 1

我想那是因为最后一行的差异会产生一个NA（因为没有更多的相邻行）。然而，difftime / head-tails方法适用于plyr包而不是dplyr (see this StackOverflow post)

如何使用dplyr让它工作？

Answer 1

感谢Victorp提出的建议。我将mutate行更改为：

mutate(diff = c(difftime(tail(timestamp, -1), head(timestamp, -1)),0))

（我放在最后的0，所以差异计算将从第一行开始）。

Answer 2

或者，您可以尝试：

... %>%
mutate(diff = c(0,diff(timestamp)))

或者，如果要显式分配单位并将列转换为数字以进行其他计算：

... %>%
mutate(diff = c(0,as.numeric(diff(timestamp), units="mins")))

使用dplyr在行之间进行difftime

2 个答案: