How to sort by year and month in R (using as.yearmon from zoo package)

Question

I'm trying to sort a vector with year and month information. I'm getting year and month from a Date type variable. Here's an example of what I'm trying to accomplish:

example <- as.Date(c("2010-04-22", "2010-04-26", "2010-04-26",
  "2010-06-04", "2010-06-07", "2010-06-18", "2010-06-21",
  "2010-07-14", "2010-07-16", "2010-07-30", "2010-08-20"))

mes_ano <- as.character(as.yearmon(as.Date(example)))

mes_ano <- factor(mes_ano, levels = c(unique(mes_ano),
 "mai 2010", "set 2010", "out 2010", "nov 2010"))

Now I would like to sort mes_ano by year and month, but I don't know how.

Actually, my real goal is to sort the levels, since I'll make a plot with ggplot2 and I need the levels to be sorted. But I thought that it would make our life easier if I just asked about sorting a vector. Then I can figure out how to sort levels.

Because when I change mes_ano to factor, mes_ano isn't sorted in the right way. I could manually set the order of levels for each factor, but in my actual dataset the number of levels is too big, so it would be really burdensome. — Manoel Galdino, Sep 25 '12 at 21:28
I need my Dates to be on month-year format only, because I'll do some aggregation (using plyr package) by month and year. — Manoel Galdino, Sep 26 '12 at 17:07

Sven Hohenstein · Accepted Answer · 2012-09-26T08:34:26.140

I suppose you want to use the Potuguese month names. Let's go:

Your data:

example <- as.Date(c("2010-04-22", "2010-04-26", "2010-04-26",
                     "2010-06-04", "2010-06-07", "2010-06-18", "2010-06-21",
                     "2010-07-14", "2010-07-16", "2010-07-30", "2010-08-20"))

We need a vector with these names:

port_mon <- c("jan", "fev", "mar", "abr", "mai", "jun",
              "jul", "ago", "set", "out", "nov", "dez")

Now, sort the data:

ex_sorted <- sort(example)

Choose the correct month names in Portuguese:

month_names <- port_mon[as.numeric(format(ex_sorted, "%m"))]

Combine months and years:

mes_ano <- paste(month_names, format(ex_sorted, "%Y"))

Create factor:

mes_ano_fac <- factor(as.yearmon(ex_sorted), labels = unique(mes_ano))

Plot (with meaningless data on the y-axis for illustration purposes):

library(ggplot)

qplot(x = mes_ano_fac, y = seq(length(example)))

enter image description here

Thank you. I'll try it and if it works (as it seems to do), I'll chose it as my accepted answer. Thanks again. — Manoel Galdino, Sep 26 '12 at 17:06

How to sort by year and month in R (using as.yearmon from zoo package)

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