总结这个数据框架的更好方法是什么？

我们可以用 melt/dcast

library(reshape2)
dcast(melt(df1, id.var='message.id'), value~variable, 
                 value.var='message.id', length)

或者使用包装器 recast

recast(df1, id.var='message.id', value~variable, length)
#    value sender recipient
#1     A      3         0
#2     B      3         2
#3     C      0         2
#4     D      0         1
#5     Q      0         1

如果我们需要使用 dplyr/tidyr

library(dplyr)
library(tidyr)
gather(df1, messages, address, 2:3) %>%
          group_by(messages, address) %>%
          summarise(n=n()) %>% 
          spread(messages, n, fill=0)
#     address sender recipient
#     (chr)  (dbl)     (dbl)
#1       A      3         0
#2       B      3         2
#3       C      0         2
#4       D      0         1
#5       Q      0         1