How to reference a variable which is not in data.table header?

I have a data.table DT, which contains 2 columns.

library(data.table)

DT = data.table(a = c(1, 2), b = c(2, 3))

I have a variable, named i

i <- "A"

Now I want to use i to determine how to create a new column in data.table. If i == "A", I want to create a new column, which is a*b.

However, if I use DT[ , NEW := ifelse(i == "A", a*b, a)]

The result is in below, which is not what I want.

   a b NEW

1: 1 2   2

2: 2 3   2

The result that I am looking for is

   a b NEW

1: 1 2   2

2: 2 3   6

I know if I generate a new column named i, I can get what I want.

DT[ , i := i]

DT[ , NEW := ifelse(i == "A", a*b, a)]

   a b i NEW

1: 1 2 A   2

2: 2 3 A   6

Is there a simple way to reference variable i, without generating a new column named i?

Thanks

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

use if/else not ifelse
– MichaelChirico
Nov 19 '18 at 18:20

stackoverflow.com/questions/17252905/else-if-vs-ifelse
– MichaelChirico
Nov 19 '18 at 18:20

also may be a situation where you consider using switch; see ?switch
– MichaelChirico
Nov 19 '18 at 18:21

3

Or raise 'b' to the power of your logical test, DT[ , NEW := a * b^(i == "A")]
– Henrik
Nov 19 '18 at 18:44

@MichaelChirico Thank you! Simple if/else works.
– SQLNewUser
Nov 19 '18 at 21:00

|
show 1 more comment

I have a data.table DT, which contains 2 columns.

library(data.table)

DT = data.table(a = c(1, 2), b = c(2, 3))

I have a variable, named i

i <- "A"

Now I want to use i to determine how to create a new column in data.table. If i == "A", I want to create a new column, which is a*b.

However, if I use DT[ , NEW := ifelse(i == "A", a*b, a)]

The result is in below, which is not what I want.

   a b NEW

1: 1 2   2

2: 2 3   2

The result that I am looking for is

   a b NEW

1: 1 2   2

2: 2 3   6

I know if I generate a new column named i, I can get what I want.

DT[ , i := i]

DT[ , NEW := ifelse(i == "A", a*b, a)]

   a b i NEW

1: 1 2 A   2

2: 2 3 A   6

Is there a simple way to reference variable i, without generating a new column named i?

Thanks

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

use if/else not ifelse
– MichaelChirico
Nov 19 '18 at 18:20

stackoverflow.com/questions/17252905/else-if-vs-ifelse
– MichaelChirico
Nov 19 '18 at 18:20

also may be a situation where you consider using switch; see ?switch
– MichaelChirico
Nov 19 '18 at 18:21

3

Or raise 'b' to the power of your logical test, DT[ , NEW := a * b^(i == "A")]
– Henrik
Nov 19 '18 at 18:44

@MichaelChirico Thank you! Simple if/else works.
– SQLNewUser
Nov 19 '18 at 21:00

|
show 1 more comment

I have a data.table DT, which contains 2 columns.

library(data.table)

DT = data.table(a = c(1, 2), b = c(2, 3))

I have a variable, named i

i <- "A"

Now I want to use i to determine how to create a new column in data.table. If i == "A", I want to create a new column, which is a*b.

However, if I use DT[ , NEW := ifelse(i == "A", a*b, a)]

The result is in below, which is not what I want.

   a b NEW

1: 1 2   2

2: 2 3   2

The result that I am looking for is

   a b NEW

1: 1 2   2

2: 2 3   6

I know if I generate a new column named i, I can get what I want.

DT[ , i := i]

DT[ , NEW := ifelse(i == "A", a*b, a)]

   a b i NEW

1: 1 2 A   2

2: 2 3 A   6

Is there a simple way to reference variable i, without generating a new column named i?

Thanks

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

I have a data.table DT, which contains 2 columns.

library(data.table)

DT = data.table(a = c(1, 2), b = c(2, 3))

I have a variable, named i

i <- "A"

Now I want to use i to determine how to create a new column in data.table. If i == "A", I want to create a new column, which is a*b.

However, if I use DT[ , NEW := ifelse(i == "A", a*b, a)]

The result is in below, which is not what I want.

   a b NEW

1: 1 2   2

2: 2 3   2

The result that I am looking for is

   a b NEW

1: 1 2   2

2: 2 3   6

I know if I generate a new column named i, I can get what I want.

DT[ , i := i]

DT[ , NEW := ifelse(i == "A", a*b, a)]

   a b i NEW

1: 1 2 A   2

2: 2 3 A   6

Is there a simple way to reference variable i, without generating a new column named i?

Thanks

r data.table

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

edited Nov 20 '18 at 20:40

Henrik

40.9k992107

asked Nov 19 '18 at 18:06

SQLNewUser

103

asked Nov 19 '18 at 18:06

SQLNewUser

103

asked Nov 19 '18 at 18:06

SQLNewUser

103

use if/else not ifelse
– MichaelChirico
Nov 19 '18 at 18:20

stackoverflow.com/questions/17252905/else-if-vs-ifelse
– MichaelChirico
Nov 19 '18 at 18:20

also may be a situation where you consider using switch; see ?switch
– MichaelChirico
Nov 19 '18 at 18:21

3

Or raise 'b' to the power of your logical test, DT[ , NEW := a * b^(i == "A")]
– Henrik
Nov 19 '18 at 18:44

@MichaelChirico Thank you! Simple if/else works.
– SQLNewUser
Nov 19 '18 at 21:00

|
show 1 more comment

use if/else not ifelse
– MichaelChirico
Nov 19 '18 at 18:20

stackoverflow.com/questions/17252905/else-if-vs-ifelse
– MichaelChirico
Nov 19 '18 at 18:20

also may be a situation where you consider using switch; see ?switch
– MichaelChirico
Nov 19 '18 at 18:21

3

Or raise 'b' to the power of your logical test, DT[ , NEW := a * b^(i == "A")]
– Henrik
Nov 19 '18 at 18:44

@MichaelChirico Thank you! Simple if/else works.
– SQLNewUser
Nov 19 '18 at 21:00

use if/else not ifelse
– MichaelChirico
Nov 19 '18 at 18:20

stackoverflow.com/questions/17252905/else-if-vs-ifelse
– MichaelChirico
Nov 19 '18 at 18:20

also may be a situation where you consider using switch; see ?switch
– MichaelChirico
Nov 19 '18 at 18:21

Or raise 'b' to the power of your logical test, DT[ , NEW := a * b^(i == "A")]
– Henrik
Nov 19 '18 at 18:44

@MichaelChirico Thank you! Simple if/else works.
– SQLNewUser
Nov 19 '18 at 21:00

|
show 1 more comment

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