Rounding selected columns of data.table in R

Question

I have following data and code to round selected columns of this data.table:

> dput(mydf)
structure(list(vnum1 = c(0.590165705411504, -1.39939534199836, 
0.720226053660755, -0.253198380120377, -0.783366825121657), vnum2 = c(0.706508400384337, 
0.526770398486406, 0.863136084517464, 0.838245498016477, 0.556775856064633
), vch1 = structure(c(2L, 4L, 1L, 3L, 3L), .Label = c("A", "B", 
"C", "E"), class = "factor")), .Names = c("vnum1", "vnum2", "vch1"
), row.names = c(NA, -5L), class = c("data.table", "data.frame"
))

> mydf[,round(.SD,1),]
Error in Math.data.frame(list(vnum1 = c(0.590165705411504, -1.39939534199836,  : 
  non-numeric variable in data frame: vch1

> cbind(mydf[,3,with=F], mydf[,1:2,with=F][,round(.SD,1),])
   vch1 vnum1 vnum2
1:    B   0.6   0.7
2:    E  -1.4   0.5
3:    A   0.7   0.9
4:    C  -0.3   0.8
5:    C  -0.8   0.6

Is there a better method (shorter code)? Thanks for your help.

Answer 1

Using dplyr

If you want to round multiple columns at once:

mydf %>% mutate_at(vars(vnum1, vnum2), funs(round(., 1))) 

Or, if you want to change all columns except "vch1":

mydf %>% mutate_at(vars(-vch1), funs(round(., 1))) 

Or, if you want to change all columns starting with "vnum":

mydf %>% mutate_at(vars(starts_with("vnum")), funs(round(., 1))) 

Or, if you want to change only numeric columns:

mydf %>% mutate_if(is.numeric, ~round(., 1)) 

You get:

  vnum1 vnum2 vch1 1   0.6   0.7    B 2  -1.4   0.5    E 3   0.7   0.9    A 4  -0.3   0.8    C 5  -0.8   0.6    C 

Answer 2

If you don't mind overwriting your original mydf:

cols <- names(mydf)[1:2]
mydf[,(cols) := round(.SD,1), .SDcols=cols]
mydf

#   vnum1 vnum2 vch1
#1:   0.6   0.7    B
#2:  -1.4   0.5    E
#3:   0.7   0.9    A
#4:  -0.3   0.8    C
#5:  -0.8   0.6    C