# How to convert values in alternate rows to negative in data.table object in R?

To convert values in alternate rows to negative in data.table object in R, we can follow the below steps −

• First of all, create a data.table object.

• Then, use vector multiplication with 1 and minus 1 to convert values in alternate rows to negative.

## Example

#### Create the data.table object

Let’s create a data.table object as shown below −

library(data.table)
x<-sample(1:5,30,replace=TRUE)
DT<-data.table(x)
DT

## Output

On executing, the above script generates the below output(this output will vary on your system due to randomization) −

x
1:  2
2:  1
3:  3
4:  2
5:  3
6:  3
7:  1
8:  4
9:  3
10: 5
11: 1
12: 5
13: 5
14: 4
15: 2
16: 5
17: 4
18: 5
19: 1
20: 4
21: 3
22: 3
23: 3
24: 1
25: 1
26: 4
27: 3
28: 3
29: 1
30: 3
x

Convert values in alternate rows to negative

Using vector multiplication with 1 and minus 1 to convert values in alternate rows of column x of data.table object DT to negative −

library(data.table)
x<-sample(1:5,30,replace=TRUE)
DT<-data.table(x)
DT$x<-DT$x*c(1,-1)
DT

## Output

x
1:   2
2:  -1
3:   3
4:  -2
5:   3
6:  -3
7:   1
8:  -4
9:   3
10: -5
11:  1
12: -5
13:  5
14: -4
15:  2
16: -5
17:  4
18: -5
19:  1
20: -4
21:  3
22: -3
23:  3
24: -1
25:  1
26: -4
27:  3
28: -3
29:  1
30: -3
x

Updated on: 15-Nov-2021

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