Thanh Quang - 1 year ago 53

R Question

I have a question regarding data.table in

`R`

i have a dataset like this

`data <- data.table(a=c(1:7,12,32,13),b=c(1,5,6,7,8,3,2,5,1,4))`

a b

1: 1 1

2: 2 5

3: 3 6

4: 4 7

5: 5 8

6: 6 3

7: 7 2

8: 12 5

9: 32 1

10: 13 4

Now i want to generate a third column c, which gonna compare the value of each row of a, to all previous values of b and check if there is any value of b is bigger than a. For e.g, at row 5, a=5, and previous value of b is 1,5,6,7. so 6 and 7 is bigger than 5, therefore value of c should be 1, otherwise it would be 0.

The result should be like this

`a b c`

1: 1 1 NA

2: 2 5 0

3: 3 6 1

4: 4 7 1

5: 5 8 1

6: 6 3 1

7: 7 2 1

8: 12 5 0

9: 32 1 0

10: 13 4 0

I tried with a for loop but it takes a very long time. I also tried shift but i can not refer to multiple previous rows with shift. Anyone has any recommendation?

Answer Source

```
library(data.table)
data <- data.table(a=c(1:7,12,32,13),b=c(1,5,6,7,8,3,2,5,1,4))
data[,c:= a <= shift(cummax(b))]
```