第 7 章 新增變數欄：由現有欄位轉換生成

- mutate: 新增欄位變數（保留原始data frame欄位）
- transmute: 新增欄位變數（「不」保留原始data frame欄位）

計算dep_delay排名

flights %>% mutate(dep_delay排名=dense_rank(dep_delay))

## # A tibble: 336,776 x 20
##     year month   day dep_time sched_dep_time dep_delay
##    <int> <int> <int>    <int>          <int>     <dbl>
##  1  2013     1     1      517            515         2
##  2  2013     1     1      533            529         4
##  3  2013     1     1      542            540         2
##  4  2013     1     1      544            545        -1
##  5  2013     1     1      554            600        -6
##  6  2013     1     1      554            558        -4
##  7  2013     1     1      555            600        -5
##  8  2013     1     1      557            600        -3
##  9  2013     1     1      557            600        -3
## 10  2013     1     1      558            600        -2
## # ... with 336,766 more rows, and 14 more variables:
## #   arr_time <int>, sched_arr_time <int>,
## #   arr_delay <dbl>, carrier <chr>, flight <int>,
## #   tailnum <chr>, origin <chr>, dest <chr>,
## #   air_time <dbl>, distance <dbl>, hour <dbl>,
## #   minute <dbl>, time_hour <dttm>,
## #   dep_delay排名 <int>

flights %>% transmute(dep_delay排名=dense_rank(dep_delay))

## # A tibble: 336,776 x 1
##    dep_delay排名
##            <int>
##  1            34
##  2            36
##  3            34
##  4            31
##  5            26
##  6            28
##  7            27
##  8            29
##  9            29
## 10            30
## # ... with 336,766 more rows