另一个列的计数

Question

假设我有以下数据框，其中用户ID和位置为两列。用户ID可以包含多个位置。我有兴趣根据用户ID找到每个可能位置序列的可能性。

所以如果我的数据看起来像这样：

places = data.frame(user_id=c(1,1,2,3,3,3,4,4,5,5,5,5),
                  location=c("home","school","work","home","school","work",
                             "lunch","airport","gym","breakfast","work","home"))
places

我想找到以下内容：

freq = data.frame(location_path=c("home - school", "work", "home - school - work", 
                                  "lunch - airport", "gym - breakfast - work - home"),
                  count=c(1,1,1,1,1))
freq

这第二个数据框告诉我“家”＆＃39;和学校＆＃39;配对按此顺序发生两次。此外，家庭，学校和工作配对也只发生过一次。

当然可能存在配对多次发生的情况。在下列情况下，家庭，学校和工作配对的计数为2。

   places = data.frame(user_id=c(1,1,2,3,3,3,4,4,5,5,5,5,6,6,6),
                      location=c("home","school","work","home","school","work",
                                 "lunch","airport","gym","breakfast","work","home",
                                 "home","school","work"))
places