如@DWin所述，table()不是用于求和，而是用于记录计数.

As @DWin states, table() is not for summation, but for record counts.

我给出了使用plyr，data.table和aggregate

all_data <- expand.grid(country = paste('Country', LETTERS[1:3]), 
  date = seq(as.Date('2012/01/01'), as.Date('2012/12/31'), by = 1) )

all_data[['ed_visits']] <- rpois(nrow(all_data), lambda = 5)



# using plyr

library(plyr)

by_date_plyr <- ddply(all_data, .(date), summarize, visits = sum(ed_visits))


# using data.table
library(data.table)
all_DT <- data.table(all_data)
by_date_dt <- all_DT[, list(visits = sum(ed_visits)), by = 'date' ]

# using aggregate
by_date_base <- aggregate(ed_visits ~ date, data = all_data, sum)