R help - Mar 2014 - data.table - How do I transform a set of column?

I started to use the data.table to subset, reshape large data.
But how do I transform a set of columns?

# for a data.frame I would do:

df = data.frame(a = c("a","b","c","d"),
b = 1:4,c = 1:4)
df[,2:3] = df[,2:3]^2


# but with data.table this somehow similar code produces an error.

dt = data.table(a = c("a","b","c","d"),
b = 1:4,c = 1:4)
dt[, 2:3 , with=FALSE ] = x[,2:3,with=FALSE]^2


By the way.

I need the columns selection 2:3 because column 1 in data.table
contains the keys.
With data.frame, matrix I would store the keys as rownames.
and just write:
df = df^2
(easier to read)


Is there syntactic sugar in data.table to be able to say something like:
transform all nonkey columns, replace all nonkey columns?


best regards
Witold









-- 
Witold Eryk Wolski

Hi,
May be this helps:
dt1 <- copy(dt)

?dt[,c("b","c"):=lapply(.SD,function(x) x^2),.SDcols=2:3]

#or

?for(.col in 2:3) set(dt1,j=.col,value=dt1[[.col]]^2)


identical(dt,dt1)
#[1] TRUE
A.K.




On Monday, March 17, 2014 10:27 AM, Witold E Wolski <wewolski at
gmail.com> wrote:
I started to use the data.table to subset, reshape large data.
But how do I transform a set of columns?

# for a data.frame I would do:

df = data.frame(a = c("a","b","c","d"),
b = 1:4,c = 1:4)
df[,2:3] = df[,2:3]^2


# but with data.table this somehow similar code produces an error.

dt = data.table(a = c("a","b","c","d"),
b = 1:4,c = 1:4)
dt[, 2:3 , with=FALSE ] = x[,2:3,with=FALSE]^2


By the way.

I need the columns selection 2:3 because column 1 in data.table
contains the keys.
With data.frame, matrix I would store the keys as rownames.
and just write:
df = df^2
(easier to read)


Is there syntactic sugar in data.table to be able to say something like:
transform all nonkey columns, replace all nonkey columns?


best regards
Witold









-- 
Witold Eryk Wolski

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