R: ddply function applied to certain months obtained from date field

Can functions be applied to only certain month/year combinations of a Date field in ddply? I want to figure the mean (among other functions) by specifying month/year.

monthlySummary <- ddply(tempData, .(Date, SiteID, SubstrateID), summarize, monthlyMean=mean(Temp_C))


Not sure what you mean by "only certain month/year combinations" so perhaps subset is what you want, but I thought you might be asking for summarization by month. So assuming that Date is a field of class Date:

monthlySummary <- ddply(tempData, .(format(Date, "%m" ),
                        summarize, monthlyMean=mean(Temp_C))

If it's not a Date class variable, maybe you should make it one:

tempData$Date2 <- as.Date(tempData$Date, "%d/%m/%Y") # or your format

And if you wanted it by site and substrate as well as month then:

monthlySummary <- ddply(tempData,
                  .( format(Date, "%m" ), SiteID, SubstrateID), 
                   summarize, monthlyMean=mean(Temp_C))

Other date-aggregation options besides format.POSIXt do include the functions in package:lubridate and the 'yearmon' class supported in package:zoo. The exaple offered above would lump together any event occurring in January of any year. If you wanted the year-month distinction to be maintained you would only need to include that in the format-string: format(Date, "%m-%Y").

Need Your Help

C++ Issues with Struct array in class definition

c++ arrays class struct enums

I'm currently having a lot of issues in trying to create a struct in a class header file. In the public accessors &amp; mutators its saying that the variables are 'undeclared identifiers', as well ...

About UNIX Resources Network

Original, collect and organize Developers related documents, information and materials, contains jQuery, Html, CSS, MySQL, .NET, ASP.NET, SQL, objective-c, iPhone, Ruby on Rails, C, SQL Server, Ruby, Arrays, Regex, ASP.NET MVC, WPF, XML, Ajax, DataBase, and so on.