6 Matching Annotations
  1. Jan 2019
    1. flights, dep_time %% 2400 <= 600)

      这个膜运算选择了一个最大值

    2. There is one remaining issue. Midnight is represented by 2400, which would correspond to 1440 minutes since midnight, but it should correspond to 0. After converting all the times to minutes after midnight, x %% 1440 will convert 1440 to zero while keeping all the other times the same. Now we will put it all together. The following code creates a new data frame flights_times with columns dep_time_mins and sched_dep_time_mins. These columns convert dep_time and sched_dep_time, respectively, to minutes since midnight. flights_times <- mutate(flights, dep_time_mins = (dep_time %/% 100 * 60 + dep_time %% 100) %% 1440, sched_dep_time_mins = (sched_dep_time %/% 100 * 60 + sched_dep_time %% 100) %% 1440

      这个计算变量用的小技巧非常好,要深入体会一下

    3. arrange(flights, distance / air_time * 60)

      arrange也可以接受一个表达式生成的新变量然后根据这个新变量排序

    4. c(600, 1200, 2400) %% 2400

      这个是膜运算,类似于取余数,但是又不太一样,详细见:https://baike.baidu.com/item/%E5%8F%96%E6%A8%A1%E8%BF%90%E7%AE%97/10739384?fr=aladdin

    5. filter(flights, between(month, 7, 9))

      这个不错,可以在对连续型变量转换成分类变量的时 候使用

    6. desc(is.na(dep_time)), dep_time)

      通过两个变量排序,第一个生成一个逻辑变量T,F。因为缺失值是T,所以缺失值就排在了前边,然后再按照第二个变量dep_time排序