Question

I would like to use the data.table package in R to dynamically generate aggregations, but I am running into an error. Below, let my.dt be of type data.table.

sex <- c("M","F","M","F")
age <- c(19, 23, 26, 21)
dependent.variable <- c(1400, 1500, 1250, 1100)
my.dt <- data.table(sex, age, dependent.variable)
grouping.vars <- c("sex", "age")
for (i in 1:2) {
     my.dt[,sum(dependent.variable), by=grouping.vars[i]]
}

If I run this, I get errors:

Error in `[.data.table`(my.dt, , sum(dependent.variable), by = grouping.vars[i] :
  by must evaluate to list

Yet the following works without error:

my.dt[,sum(dependent.variable), by=sex]

I see why the error is occurring, but I do not see how to use a vector with the by parameter.

Answer 1

I did two changes to your original code:

sex <- c("M","F","M","F")
age <- c(19, 23, 26, 21) 

age<-as.factor(age)

dependent.variable <- c(1400, 1500, 1250, 1100)
my.dt <- data.table(sex, age, dependent.variable)

for ( a in 1:2){
print(my.dt[,sum(dependent.variable), by=list(sex,age)[a]]) 
}

Numerical vector age should be forced into factors. As to by parameter, do not use quote for column names but group them into list(…). At least this is what the author has suggested.