如果在 2 行中满足某些条件，如何在 R 数据框中添加新列，显示当前行和前一行中的值之和？

Question

Suppose you have a data frame of columns "a" and "b" with the values shown below, generated with df <- data.frame(a=c(0, 1, 2, 2, 3), b=c(1, 3, 8, 9, 4)) . Suppose you want to add a column "c", whereby if a value in "a" equals the value in the immediately preceding row in col "a", then the corresponding row values in col "b" are summed; otherwise a 0 value is shown. A column "c" is added to the below to illustrate what I'm trying to do:

   a  b   add col c
1  0  1       0
2  1  3       0
3  2  8       0
4  2  9       17 (since the values in col "a" rows 3 and 4 are equal, add the values in col b rows 3 and 4)
5  3  4       0

What is the easiest way to do this in native R?

Answer 1

We can use

 df %>% group_by(a) %>% mutate(c = if(n() > 1) sum(b) else 0)