add a column in data table using for loop

Question

I have a data.table called md that looks like this

group_1 group_2
a       a
b       a
b       b
c       c

I use the following code: groups <- c("group_1","group_2")

    for (group in groups)
    {
     md[!get(group)=="a",get(group):="b"]
     md[,get(group):=factor(x = get(group),levels = c("a","b"),ordered = T)]
    }

So I want for both columns in the data.table if the value for every row is not "a" then replace the value with "b" and then add ordered levels to it.

But I get an error Error in get(group): object 'group_1' not found

Any ideas ?

but then it adds a column called group, whereas i want to replace the values at the existing columns — quant
– quant, Commented Mar 28, 2017 at 10:04

akrun · Accepted Answer · 2017-03-28 12:50:32Z

1

Another option is using set

 for(group in groups){
   set(md, i = which(md[[group]] !='a'), j = group, value = 'b')
    set(md, i = NULL, j = group, value = factor(md[[group]], levels = c('a', 'b'),
      ordered = TRUE))
}
md
#    group_1 group_2
#1:       a       a
#2:       b       a
#3:       b       b
#4:       b       b

edited Mar 28, 2017 at 12:50

answered Mar 28, 2017 at 10:20

akrun

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