Question: I have the following data.table:

```
name = c("Bob","Mary","Jane","Kim")
weight = c(60,65,45,55)
height = c(170,165,140,135)
dft = data.table(name,weight,height)

```

I want to change `weight` to be equal to `height + 13` . I know numerous ways can do this, for example

```
dft[, weight := height + 13

```

or

```
dft[, "weight" := height + 13

```

However, since I have a huge dataset whose column names are like `V1, V2,....,V1000`, I wish to use column names to enter modification. In the above example, however,

```
dft[, "weight" := "height" + 13

```

is not working.

So I wonder how to use  `"height"` to modify `weight`. Thanks

Answer: 
You can use :

```
library(data.table)
name = c("Bob","Mary","Jane","Kim")
weight = c(60,65,45,55)
height = c(170,165,140,135)
dft = data.table(name,weight,height)

col1 <- 'weight'
col2 <- 'height'

dft[, (col1) := get(col2) + 13]
dft

#   name weight height
#1:  Bob    183    170
#2: Mary    178    165
#3: Jane    153    140
#4:  Kim    148    135

```
I find `data.table` to be awful on so many levels.

In base R you would just need:

```
df[, 'height'] + 13 -> df[, 'weight' ]

```