Often I like to see the performance of a trading strategy summarized annually, quarterly or by month. In R, we start off with the summary function:

aggregateReturns = function( xx, leverage=1 )
{
   return( tail( cumprod( 1 + xx*leverage ), 1 ) )
}

Given a series xx, usually a chunk of the original, this function returns the accumulative returns for the period. The leverage is useful to somewhat simulate leveraged ETFs.

The rest is to call this function for various periods:

summarizeDailyReturns = function(
      ss,
      indicator,
      returns="closeToClose",
      period="annually",
      leverage=1 )
{
   stopifnot( length( index( ss ) ) == length( index( indicator ) ) )
   stopifnot( is.xts( ss ) )
   stopifnot( is.xts( indicator ) )

   if( tolower( returns ) == "opentoclose" )
   {
      stopifnot( has.Op( ss ) && has.Cl( ss ) )
      rets = Cl( ss ) / Op( ss ) - 1
   }
   else 
   {
      stopifnot( has.Ad( ss ) || has.Cl( ss ) )
      if( has.Ad( ss ) )
      {    
         rets = Ad( ss ) / lag( Ad( ss ) ) - 1
      }    
      else 
      {    
         rets = Cl( ss ) / lag( Cl( ss ) ) - 1
      }    
   }

   rets[as.character(head( index( ss ), 1 ))] = 0
   rets = as.xts( indicator * coredata( rets ) )

   if( tolower( period ) == "annually" )
   {
      yy = as.numeric( format( index( rets ), "%Y" ) )
      rets = aggregate( rets, yy, aggregateReturns, leverage )
   }
   else if( tolower( period ) == "quarterly" )
   {
      rets = aggregate( rets, as.yearqtr, aggregateReturns, leverage )
   }
   else if( tolower( period ) == "monthly" )
   {
      rets = aggregate( rets, as.yearmon, aggregateReturns, leverage )
   }

   return( round( rets, 4 ) )
}

I will finish the post with an illustration how to use this function (assuming the two functions reside in returns.R from the current directory):

library(quantmod)

source("returns.R")

getSymbols("^GSPC", from="1900-01-01")

# The indicator - buy and hold: 1 every day
ind = xts(rep(1, length(index(GSPC))), order.by=index(GSPC))

summarizeDailyReturns(GSPC, ind, returns="OpenToClose")
summarizeDailyReturns(GSPC, ind, leverage=2, period="quarterly")
summarizeDailyReturns(GSPC, ind, leverage=2, period="daily")

The last call computes the compound growth on a daily basis.

The final version of the function is available from the quntscript project.