How to create ID column in R

Question

I have a longitudinal data in a long format. I want to create an ID variable based on the variable-column that identifies each observation of my data. How do I do that in R?

Answer 1

tc='
name year var1 var2
A    1    4    3
A    2    5    1
A    3    4    2
B    1    .    .
B    2    4    3
B    3    5    1'

df <- read.table(text=tc, header=T)

df$ID <- match(df$name, LETTERS)

Although is not clear if name is a column or are the rownames of the data frame. If is not a column then try rownames(df) instead of df$name

Answer 2

If your name column doesn't just contain single letters (or even if it does), you can use:

dat$id <- as.numeric(as.factor(dat$name))

or, more simply:

dat$id <- c(as.factor(dat$name))

where dat is your data.frame.