R help - Feb 2012 - Combining month and year into a single variable

Hello all,

## I am trying to convert some year and month data into a single
variable that has a date format so I can plot a proper x axis.
## I've made a few tries at this and search around but I haven't found
anything. I am looking for something of the format "%Y-%m"

## A data.frame
df <- data.frame(x=rnorm(36, 1, 10), month=rep(1:12, each = 3),
year=c(2000,2001,2002))

## One option. I'm not totally sure why this doesn't work
df$Date <- as.Date(paste(df$year, df$month,sep="-"),
"%Y-%m")

## This adds the monthly total to the day rather than the MOnday and this option
## is messy anyway as I am adding a day to the variable
or = format(ISOdate(df$year-1, 12, 31), "%Y-%m-%d")
df$Date2 = as.Date(or) + df$month

## Just a plot to illustrate this.
plot(x~Date2, data=df)

## Any thoughts on how I can combine the year and the month into a
form that is useful for plotting?

Thanks in advance!

Sam

On Wed, Feb 1, 2012 at 5:26 PM, Sam Albers <tonightsthenight at gmail.com>
wrote:
> Hello all,
>
> ## I am trying to convert some year and month data into a single
> variable that has a date format so I can plot a proper x axis.
> ## I've made a few tries at this and search around but I haven't
found
> anything. I am looking for something of the format "%Y-%m"
>
> ## A data.frame
> df <- data.frame(x=rnorm(36, 1, 10), month=rep(1:12, each = 3),
> year=c(2000,2001,2002))
>
> ## One option. I'm not totally sure why this doesn't work
> df$Date <- as.Date(paste(df$year, df$month,sep="-"),
"%Y-%m")
>
> ## This adds the monthly total to the day rather than the MOnday and this
option
> ## is messy anyway as I am adding a day to the variable
> or = format(ISOdate(df$year-1, 12, 31), "%Y-%m-%d")
> df$Date2 = as.Date(or) + df$month
>
> ## Just a plot to illustrate this.
> plot(x~Date2, data=df)
>
> ## Any thoughts on how I can combine the year and the month into a
> form that is useful for plotting?
>
Try "yearmon" class in zoo:

library(zoo)

df$tt <- as.yearmon(paste(df$year, df$month, sep = "-"))
head(df$tt) # look at it
plot(x ~ tt, df) # plot it

or you could create a zoo object:

z <- zoo(df$x, df$tt)
head(z) # look at it
plot(z) # plot it


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