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How do I add only the last element of one vector to the last element of another vector?

July 22, 2022

I have a dataframe with hundreds of different investments (represented by the "id" column), their cashflows, and market value. The following example demonstrates the data that I’m working with:

df <- data.frame(date = c("2020-01-31", "2020-02-29", "2020-03-31", "2020-02-29", "2020-03-31", "2020-04-30", "2020-05-31"),
                 id = c("alpha", "alpha", "alpha", "bravo", "bravo", "bravo", "bravo"),
                 cashflow = c(-100,20,4,-50,8,12,8),
                 market_value = c(100,90,80,50,110,120,115))

I ultimately want to calculate the IRR per investment. However, before I can do that, I need to add only the last market value number to the corresponding cashflow. I don’t care about any market values before that. In this case, the last cashflow for "alpha" investment must be 84 (i.e., 80 market value + 4 cashflow) and the last cashflow for "bravo" investment must be 123 (i.e., 115 market value + 8 cashflow).

Desired output:

id	cashflow
alpha	-100
alpha	20
alpha	84
bravo	-50
bravo	8
bravo	12
bravo	123

Thanks!

>Solution :

I’m not too sure on what final output you want but here’s how you’d just take the last.

df %>%
mutate(total = cashflow + market_value) %>%
group_by(id) %>%
slice_max(order_by = date) %>%
ungroup()

#> # A tibble: 2 × 5
#>   date       id    cashflow market_value total
#>   <chr>      <chr>    <dbl>        <dbl> <dbl>
#> 1 2020-03-31 alpha        4           80    84
#> 2 2020-05-31 bravo        8          115   123

^{Created on 2022-07-22 by the reprex package (v2.0.1)}

EDIT – just seen what I think is your desired output, how’s this?

df %>%
  group_by(id) %>%
  mutate(
    cashflow = if_else(row_number() == n(), cashflow + market_value, cashflow)
  )
#> # A tibble: 7 × 4
#> # Groups:   id [2]
#>   date       id    cashflow market_value
#>   <chr>      <chr>    <dbl>        <dbl>
#> 1 2020-01-31 alpha     -100          100
#> 2 2020-02-29 alpha       20           90
#> 3 2020-03-31 alpha       84           80
#> 4 2020-02-29 bravo      -50           50
#> 5 2020-03-31 bravo        8          110
#> 6 2020-04-30 bravo       12          120
#> 7 2020-05-31 bravo      123          115

^{Created on 2022-07-22 by the reprex package (v2.0.1)}