How to group by every 7 rows, and aggregate those 7 values by median?

Question

I have a data frame for daily earning like this:

         date    earning
            1 -40.495074
            2 -88.636625
            3 134.400976
            4  66.325253
            5 -86.337511
            6  -1.266815
            7 267.944212
            8  65.247053
            9  86.177537
           10  64.540579        

I want to group every 7 days and aggregate 7 days earning by using median.

edit1: It doesn't have to be calendar week, just arbitrary 7 days as as group.

So It would be like

week  median-earning
   1  11
   2  22

and so on...

How can I achieve this in R?

Answer 1

library(dplyr)
theData <- data.frame(date = 1:10,
                      earning = c(-40, -88, -134, 66, -86, -1, 267, 65, 86, 64))

theData$seven_day_index <- c(0, rep(1:(nrow(theData)-1)%/%7))

group_by(theData, seven_day_index) %>%
  summarise(median_earnings = median(earning))

and the base R version

sapply(split(theData$earning, c(0, rep(1:(nrow(theData)-1)%/%7))), median)

edit: inspired by above

theData %>% mutate(seven_day_index = 1:nrow(theData) %/% 7) %>%
            group_by(seven_day_index) %>%
            summarise(median_earnings = median(earning)