按数据框列汇总行，然后将这些行转换为列

Question

我下面有数据框

year<-c(2016,2016,2016,2016)
city<-c("NY","NY","NY","NY")
spec<-c("df","df","vb","vb")
num<-c(45,67,89,90)
df<-data.frame(year,city,spec,num)

year city spec num
1 2016   NY   df  45
2 2016   NY   df  67
3 2016   NY   vb  89
4 2016   NY   vb  90

，我想按year和city分组，按spec级别进行汇总，然后将spec汇总转换为数据框列。像这样：

    year city  df  vb
1 2016   NY 112 179

Answer 1

使用dplyr和tidyr：

library(dplyr)
    library(tidyr)

    x <- df %>%
      group_by(year, city, spec) %>%
      summarise(num = sum(num)) %>%
      spread(spec, num)

# Groups:   year, city [1]
   year city     df    vb
  <dbl> <fct> <dbl> <dbl>
1  2016 NY      112   179