[R] Calculate monthly means

[Daniel Kaschek]

On Tue, 2011-03-15 at 07:24 -0700, Carl Nim wrote: 
> I am trying to calculate monthly means by year of phosphates and nitrates from a multi year data set. Can anybody help me out with the most effective way to do this?
> 
> My data looks like this:
> 
> Collection_Date Test.Name Value
> 2000-01-24 17:00:00 Phosphate 0.108
> 2000-01-24 17:00:00 Nitrate 0.037
> 2001-11-12 08:45:00 Phosphate 0.45
> ...
> 
> 
> Thanks and sorry for the blatantly "newbie" question.

Let's say you have a data.frame, mydata, with the above data. Then you
could write a function

mymean <- function(year, month, substance) {
  mysub <- subset(mydata,
format(as.PPSIXlt.date(Collection_Date), "%Y") == year,
format(as.POSIXlt.date(Collection_Date), "%b") == month,
Test.Name == substance)
  return(mean(mysub$value))
}

Then you need to apply this function to every combination of year, month
and substance in your data.frame. You can do this by

M <- expand.grid(2000:2010, month.abb, c("Phosphate", "Nitrate"))
meanValues <- apply(M, 1, mymean(myRowEntry[1], myRowEntry[2],
myRowEntry[3])

In the end you can put the result together with M, i.e.

M <- cbind(M, meanValues)




> 
> 
> 
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-- 
Daniel Kaschek
Physikalisches Institut, Freiburg
Hermann-Herder-Str. 3
79104 Freiburg

Office:  Westbau, 02020
Phone:   +49-761-203-8531
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