R - Mutate column based on another column

Question

Using R: For the dataframe:

A<-c(3,3,3,3,1,1,2,2,2,2,2)
df<-data.frame(A)

How do you add a column such that the output is the same as:

A<-c(3,3,3,3,1,1,2,2,2,2,2)
df<-data.frame(A)

B<-c(1,1,1,0,1,0,1,1,0,0,0)
mutate(df,B)

In other words, is there a formula for column 'B' - such that it looks at column 'A'....and lists '1', 3 times the puts a '0' .....etc etc.

So - the desired output (given column 'A') is:

Thankyou.

Answer 1

Here I assign a new group each time A changes, then within each group put a 1 in B in the first #A rows.

(If the values of A are distinct for each group, you could replace the first two lines with group_by(A), but unclear if that's a fair assumption.)

library(dplyr)
df %>%
  mutate(group = cumsum(A != lag(A, default = 0))) %>%
  group_by(group) %>%
  mutate(B = 1 * (row_number() <= A)) %>%
  ungroup()

result

# A tibble: 11 x 3
       A group     B
   <dbl> <int> <dbl>
 1     3     1     1
 2     3     1     1
 3     3     1     1
 4     3     1     0
 5     1     2     1
 6     1     2     0
 7     2     3     1
 8     2     3     1
 9     2     3     0
10     2     3     0
11     2     3     0