使用dplyr和lubridate按月和年組合計數和分組

我有一个資料框，其中每一行代表一个城市中發生的單个事件.資料框顯示城市名稱和發生日期，如下所示：

df <- data.frame(city = c("Seattle", "Seattle", "Seattle", "Seattle", "Seattle", "NYC", "NYC", "NYC", "Chicago",
                         "Chicago", "Chicago", "Chicago", "Chicago"),
                     date_of_event = c("01/13/2011", "01/17/2011", "03/15/2011", "05/21/2011", "05/23/2011",
                                      "01/20/2011", "01/22/2011", "03/23/2011", "01/18/2011", "02/24/2011",
                                       "02/26/2011", "04/30/2011", "06/18/2011"),
                     stringsAsFactors = FALSE)
df$date_of_event <- as.Date(df$date_of_event, "%m/%d/%Y")

以上只是一个示例，我的資料實際上是在具有數千行，许多城市，许多日期等的csv中。我想做的是生成一个新的資料框，该資料框每个城市和每个月都有一行 /年在資料集中表示，並且相應的計數列顯示原始資料框中每个城市每个月發生的次數.第二个資料幀看起来像這樣：

df2 <- data.frame(city = c("Seattle", "Seattle", "Seattle", "Seattle", "Seattle", "Seattle", "NYC", "NYC", "NYC", "NYC",
                           "NYC", "NYC", "Chicago", "Chicago", "Chicago", "Chicago", "Chicago", "Chicago"),
                     month_year = c("01/01/2011", "02/01/2011", "03/01/2011", "04/01/2011", "05/01/2011", "06/01/2011",
                                    "01/01/2011", "02/01/2011", "03/01/2011", "04/01/2011", "05/01/2011", "06/01/2011",
                                    "01/01/2011", "02/01/2011", "03/01/2011", "04/01/2011", "05/01/2011", "06/01/2011"),
                  count = c(2, 0, 1, 0, 2, 0, 2, 0, 1, 0, 0, 0, 1, 2, 0, 1, 0, 1),
                     stringsAsFactors = FALSE)
df2$month_year <- as.Date(df2$month_year, "%m/%d/%Y")

我知道您可以使用dplyr中的count，也可以將日期四舍五入到每个月的第一天，但​​是我尝試並未能正確进行分組和計數以产生我想要的第二个資料幀 .有人可以帮我吗？ 提前非常感谢。