How to get tally (rolling sum) by group in R?

KC Ray Published at Dev

KC Ray

I would like to create a column called "tally" in my dataset that takes sums the count of each type and rank.

type <- c("A","A","A","B","B","C")
rank <- c("low", "med", "high","med", "high", "low")
count <- c(9,20,31,2,4,14)

df <- data.frame(type, rank, count)

My desired output would be:

type rank count tally
1    A  low     9     9
2    A  med    20    29
3    A high    31    60
4    B  med     2     2
5    B high     4     6
6    C  low    14    14

I guess another way to describe it would be a rolling sum (where it takes into account the low to high order)? I have looked around but I can't find any good functions to do this. Ideally, I could have a for loop that would allow me to get this "rolling sum" by type.

akrun

We can use cumsum after grouping by 'type'

library(dplyr)
df <- df %>%
   group_by(type) %>%
    mutate(tally = cumsum(count)) %>%
    ungroup

-output

# A tibble: 6 x 4
  type  rank  count tally
  <chr> <chr> <dbl> <dbl>
1 A     low       9     9
2 A     med      20    29
3 A     high     31    60
4 B     med       2     2
5 B     high      4     6
6 C     low      14    14

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