a simplified version of aaply() from the R plyr package, much faster for simple cases

df.by.rows

df.by.rows <- function(df, fn, agg.type=numeric, .progress=NULL) 
{
  # A simplified and more-efficient version of aaply for data frames. Iterates over rows of the df, applies the
  # fn to each single-row slice, and aggregates the **scalar** results.
  #
  # Args:
  #    df - data frame to split
  #    fn - function to apply to each row
  #    agg.type - a constructor, one of numeric, character
  #    .progress - set to "text" to get a progress bar
  #
  # Returns: a vector of type agg.type of length nrow(df)
  ######################

  stopifnot(is.data.frame(df))
  stopifnot(is.function(fn))
  #stopifnot(agg.type %in% c(numeric, character))
  
  df.rows <- nrow(df)
  new.col <- agg.type(df.rows)
  
  if (.progress == "text")
    progbar <- txtProgressBar(min=1, max=df.rows, initial=1, style=3)
    
  for (i in seq_len(df.rows)) {
    new.col[[i]] <- fn(df[i, ])
    setTxtProgressBar(progbar, i)
  }
  close(progbar)
  
  new.col
}