Select / assign to data.table when variable names are stored in a character vector

Two ways to programmatically select variable(s):

1. with = FALSE:

    DT = data.table(col1 = 1:3)
    colname = "col1"
    DT[, colname, with = FALSE] 
    #    col1
    # 1:    1
    # 2:    2
    # 3:    3
   

2. ‘dot dot’ (..) prefix:

    DT[, ..colname]    
    #    col1
    # 1:    1
    # 2:    2
    # 3:    3
   

For further description of the ‘dot dot’ (..) notation, see New Features in 1.10.2 (it is currently not described in help text).

To assign to variable(s), wrap the LHS of := in parentheses:

DT[, (colname) := 4:6]    
#    col1
# 1:    4
# 2:    5
# 3:    6

The latter is known as a column plonk, because you replace the whole column vector by reference. If a subset i was present, it would subassign by reference. The parens around (colname) is a shorthand introduced in version v1.9.4 on CRAN Oct 2014. Here is the news item:

Using with = FALSE with := is now deprecated in all cases, given that wrapping
the LHS of := with parentheses has been preferred for some time.

colVar = "col1"

DT[, (colVar) := 1]                             # please change to this
DT[, c("col1", "col2") := 1]                    # no change
DT[, 2:4 := 1]                                  # no change
DT[, c("col1","col2") := list(sum(a), mean(b))]  # no change
DT[, `:=`(...), by = ...]                       # no change

See also Details section in ?`:=`:

DT[i, (colnamevector) := value]
# [...] The parens are enough to stop the LHS being a symbol

---

And to answer further question in comment, here’s one way (as usual there are many ways) :

DT[, colname := cumsum(get(colname)), with = FALSE]
#    col1
# 1:    4
# 2:    9
# 3:   15 

or, you might find it easier to read, write and debug just to eval a paste, similar to constructing a dynamic SQL statement to send to a server :

expr = paste0("DT[,",colname,":=cumsum(",colname,")]")
expr
# [1] "DT[,col1:=cumsum(col1)]"

eval(parse(text=expr))
#    col1
# 1:    4
# 2:   13
# 3:   28

If you do that a lot, you can define a helper function EVAL :

EVAL = function(...)eval(parse(text=paste0(...)),envir=parent.frame(2))

EVAL("DT[,",colname,":=cumsum(",colname,")]")
#    col1
# 1:    4
# 2:   17
# 3:   45

Now that data.table 1.8.2 automatically optimizes j for efficiency, it may be preferable to use the eval method. The get() in j prevents some optimizations, for example.

Or, there is set(). A low overhead, functional form of :=, which would be fine here. See ?set.

set(DT, j = colname, value = cumsum(DT[[colname]]))
DT
#    col1
# 1:    4
# 2:   21
# 3:   66