Convert months mmm to numeric

Question

I have been given a csv with a column called month as a char variable with the first three letters of the month. E.g.:

"Jan", "Feb","Mar",..."Dec" 

Is there any way to convert this to a numeric representation of the month, 1 to 12, or even a type that is in a date format?

Answer 1

Use match and the predefined vector month.abb:

tst <- c("Jan","Mar","Dec") match(tst,month.abb) [1]  1  3 12 

Answer 2

You can use the built-in vector month.abb to check against when converting to a number, eg :

mm <- c("Jan","Dec","jan","Mar","Apr")  sapply(mm,function(x) grep(paste("(?i)",x,sep=""),month.abb)) Jan Dec jan Mar Apr    1  12   1   3   4  

The grep construct takes care of differences in capitalization. If that's not needed,

match(mm,month.abb)  

works just as fine.

If you also have a day and a year column, you can use any of the conversion functions, using the appropriate codes (see also ?strftime)

eg

mm <- c("Jan","Dec","jan","Mar","Apr") year <- c(1998,1998,1999,1999,1999) day <- c(4,10,3,16,25)  dates <- paste(year,mm,day,sep="-")  strptime(dates,format="%Y-%b-%d") [1] "1998-01-04" "1998-12-10" "1999-01-03" "1999-03-16" "1999-04-25"