R Filter and subset a dataframe

Please give me some 开发者_JAVA百科advice on the followings.

There is a data frame with three columns, the first one is a datetime(precise to seconds), the second one is a person's name, and the third one is a message.

I want to retrieve the following information and plot them.

1. Messages per person during a certain minute.
2. A certain person's messages during a certain minute.

Thanks in advance.

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There are lots of different date formats in R. If you haven't got a preference for one of them, then use the lubridate package.

library(lubridate)

Some sample data:

the_present <- now()
dfr <- data.frame(
  person  = rep(c("Richie", "user900168"), each = 3),
  time    = seq(the_present, by = "-1 min", length.out = 6),
  message = letters[1:6]
)

Pick an interesting minute:

start_time <- floor_date(the_present, unit = "min")
end_time <- ceiling_date(the_present, unit = "min")

Use subset and table to solve your problems.

table(subset(dfr, time > start_time & time <= end_time, person))
subset(dfr, person == "Richie" & time > start_time & time <= end_time)