Converting year and month ("yyyy-mm" format) to a date?

Question

I have a dataset that looks like this:

Month    count 2009-01  12 2009-02  310 2009-03  2379 2009-04  234 2009-05  14 2009-08  1 2009-09  34 2009-10  2386 

I want to plot the data (months as x values and counts as y values). Since there are gaps in the data, I want to convert the Information for the Month into a date. I tried:

as.Date("2009-03", "%Y-%m") 

But it did not work. Whats wrong? It seems that as.Date() requires also a day and is not able to set a standard value for the day? Which function solves my problem?

Answer 1

Since dates correspond to a numeric value and a starting date, you indeed need the day. If you really need your data to be in Date format, you can just fix the day to the first of each month manually by pasting it to the date:

month <- "2009-03" as.Date(paste(month,"-01",sep="")) 

Answer 2

Try this. (Here we use text=Lines to keep the example self contained but in reality we would replace it with the file name.)

Lines <- "2009-01  12 2009-02  310 2009-03  2379 2009-04  234 2009-05  14 2009-08  1 2009-09  34 2009-10  2386"  library(zoo) z <- read.zoo(text = Lines, FUN = as.yearmon) plot(z) 

The X axis is not so pretty with this data but if you have more data in reality it might be ok or you can use the code for a fancy X axis shown in the examples section of ?plot.zoo .

The zoo series, z, that is created above has a "yearmon" time index and looks like this:

> z Jan 2009 Feb 2009 Mar 2009 Apr 2009 May 2009 Aug 2009 Sep 2009 Oct 2009        12      310     2379      234       14        1       34     2386  

"yearmon" can be used alone as well:

> as.yearmon("2000-03") [1] "Mar 2000" 

Note:

1. "yearmon" class objects sort in calendar order.

2. This will plot the monthly points at equally spaced intervals which is likely what is wanted; however, if it were desired to plot the points at unequally spaced intervals spaced in proportion to the number of days in each month then convert the index of z to "Date" class: time(z) <- as.Date(time(z)) .