i have a question regarding data.table package in R. Now i have a data like this
a=data.table(person=c(1:3),grade=c(4:6),age=(30:32))
a
person grade age
1: 1 4 30
2: 2 5 31
3: 3 6 32
Now i want to generate a new data which merge the amount of grade and age into one column, and we will have criteria column next to it, like this
person criteria amount
1: 1 grade 4
2: 2 grade 5
3: 3 grade 6
4: 1 age 30
5: 2 age 31
6: 3 age 32
Do you know how could i achieve that in data.table. Thanks so much
