How to create a consecutive group number

Question

I have a data frame (all_data) in which I have a list of sites (1... to n) and their scores e.g.

  site  score      1    10      1    11        1    12      4    10       4    11      4    11      8    9      8    8      8    7 

I want create a column that numbers each level of site in numerical order, like a counter. In the example, the sites (1, 4, and 8) would be have a corresponding counter from 1 to 3 in the 'number' column:

site  score number      1    10    1      1    11    1       1    12    1       4    10    2      4    11    2      4    11    2      8    9     3      8    8     3       8    7     3 

I am sure this must be easily solved, but I have not found a way yet.

Answer 1

Try Data$number <- as.numeric(as.factor(Data$site))

On a sidenote : the difference between the solution of me and @Chase on one hand, and the one of @DWin on the other, is the ordering of the numbers. Both as.factor and factor will automatically sort the levels, whereas that doesn't happen in the solution of @DWin :

Dat <- data.frame(site = rep(c(1,8,4), each = 3), score = runif(9))  Dat$number <- as.numeric(factor(Dat$site)) Dat$sitenum <- match(Dat$site, unique(Dat$site) )  

Gives

> Dat   site     score number sitenum 1    1 0.7377561      1       1 2    1 0.3131139      1       1 3    1 0.7862290      1       1 4    8 0.4480387      3       2 5    8 0.3873210      3       2 6    8 0.8778102      3       2 7    4 0.6916340      2       3 8    4 0.3033787      2       3 9    4 0.6552808      2       3