[datatable-help] Update data.table columns by multiplication of another column

[Arunkumar Srinivasan]

How about this?
for (j in v) set(dt, i=NULL, j=j, dt[[j]]*dt[[coeff]]) 

Arun


On Sunday, October 27, 2013 at 9:37 AM, cduprez wrote:

> Hi all,
> 
> I am trying to update columns of a data.table, whose names are in a vector,
> by multiplicating their values with the values of another column (whose name
> is in another vector).
> Example : 
> dt <- data.table(a=c(1, 1, 1, 1, 1), b=c(2, 2, 2, 2, 2), c=c(3, 3, 3, 3, 3),
> d=c(4, 4, 4, 4, 4), e=c(5, 5, 5, 5, 5), coef = c(1, 2, 3, 4, 5))
> v <- c("b", "c")
> coef <- c("coef")
> 
> dt
> a b c d e coef
> 1: 1 2 3 4 5 1
> 2: 1 2 3 4 5 2
> 3: 1 2 3 4 5 3
> 4: 1 2 3 4 5 4
> 5: 1 2 3 4 5 5
> 
> And what I am looking for, as result, is : b = b*coef, c = c*coef
> a b c d e coef
> 1: 1 2 3 4 5 1
> 2: 1 4 6 4 5 2
> 3: 1 6 9 4 5 3
> 4: 1 8 12 4 5 4
> 5: 1 10 15 4 5 5
> 
> How can I compute that result by keeping the columns to update and the coef
> column in character vectors containing the columns names.
> I precise that the coef vector still contains only one column name.
> 
> Thanks in advance for you help.
> 
> Regards,
> 
> Cedric
> 
> 
> 
> --
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