How do I add a column indicating the years since a binary variable in R?

September 17, 2022

I thought this would be trivial, and I think it must be, but I am very tired and stuck at this problem at the moment.

Consider a df with two columns, one with a year, and the other with a binary variable indicating some event.

df <- data.frame(year = c(2000,2001,2002,2003,2004, 2005,2006,2007,2008,2010),
             flag = c(0,0,0,1,0,0,0,1,0,0))

I want to create a third column that simply counts the years since the last flag and that resets when a new flag appears, like so:

I thought this code would do the job:

First, add a 0 as "year_since" for every year with a flag, then, if there was a flag in the previous year, add 1 to the value of the previous "year_since".

df <- df %>% mutate(year_since = ifelse(flag == 1, 0, NA)) %>%
 mutate(year_since = ifelse(dplyr::lag(flag, n=1, order_by = "year") == 1 & is.na(year_since), 
dplyr::lag(year_since, n=1, order_by = "year")+1, year_since))

However, this returns NA for every row that should be 1,2,3, and so on.

>Solution :

You could do

df %>% 
  group_by(group = cumsum(flag)) %>% 
  mutate(year_since = ifelse(group == 0, NA, seq(n()) - 1)) %>%
  ungroup() %>%
  select(-group)
#> # A tibble: 10 x 3
#>     year  flag year_since
#>    <dbl> <dbl>      <dbl>
#>  1  2000     0         NA
#>  2  2001     0         NA
#>  3  2002     0         NA
#>  4  2003     1          0
#>  5  2004     0          1
#>  6  2005     0          2
#>  7  2006     0          3
#>  8  2007     1          0
#>  9  2008     0          1
#> 10  2010     0          2

^{Created on 2022-09-16 with reprex v2.0.2}