在 R 中，如何自动检测首次出现非标准时差的位置？

Question

I have a data frame where each row is a different timestamp. The older data in the data frame is collected at 30-minute intervals while the more recent data is collected at 15-minute intervals. I would like to run a for loop (or maybe an ifelse statement) that calulates the time difference between each row, if the difference is equal to 30 minutes (below example uses 1800 seconds) then the loop continues, but if the loop encounters a 15 minute time difference (below example uses 900 seconds) it stops and tells me which row this first occured on.

x <- as.POSIXct("2000-01-01 01:00", tz = "", "%Y-%m-%d %H:%M")
y <- as.POSIXct("2000-01-10 12:30", tz = "", "%Y-%m-%d %H:%M")
xx <- as.POSIXct("2000-01-10 12:45", tz = "", "%Y-%m-%d %H:%M")
yy <- as.POSIXct("2000-01-20 23:45", tz = "", "%Y-%m-%d %H:%M")

a.30 <- as.data.frame(seq(from = x, to = y, by = 1800))
names(a.30)[1] <- "TimeStamp"
a.15 <- as.data.frame(seq(from = xx, to = yy, by = 900))
names(a.15)[1] <- "TimeStamp"

dat <- rbind(a.30,a.15)

In the example dat data frame, the time difference switches from 30 minute to 15 minute intervals at row 457. I would like to automate the process of identifing the row where this change in time difference first occurs.

Answer 1

We can use difftime to calculate the difference in time in mins and create a logical vector based on the difference

library(dplyr)
dat %>%
   summarise(ind = which.max(abs(as.numeric(difftime(TimeStamp, 
       lag(TimeStamp, default = TimeStamp[2]), unit = 'min'))) < 30))
#  ind
#1 457

Answer 2

Here's another way that uses slightly different logic. Calculate the difference, and create a column with the row number. Then filter to where the difference is 15, and take the first row.

library(tidyverse)

dat %>% mutate(Diff = TimeStamp - lag(TimeStamp), rownum = row_number()) %>% 
  filter(Diff == 15) %>% 
  slice(1)

            TimeStamp    Diff rownum
1 2000-01-10 12:45:00 15 mins    457