基於多列中的值在R數據框中創建新列

Question

我有以下數據框，其中包括餐館（id＃），所有者和購買日期。有時餐廳會更改所有權，但爲了分析目的，我想創建另一個只保留最新所有者名稱的列，這由「購買日期」決定。我如何創建這個新列？在這個例子中，餐館23在2013年1月1日改變了手，所以我想只包括新所有者的名字（Bob）作爲該餐廳的所有行的當前所有者，如下所示。如果餐廳不換手，請爲​​「當前所有者」列保留與「所有者」列相同的名稱。

Restaurant <- c(23,23,23,23,23,23,23,15,15,15,15,17,17,17,17) 

    owner <- c("Chuck","Chuck","Chuck","Chuck","Bob","Bob","Bob", 
    "Hazel","Hazel","Hazel","Hazel","Pete","Pete","Pete","Pete") 

    purchaseDate <- c("3/4/2011","3/4/2011","3/4/2011","3/4/2011", 
    "1/1/2013","1/1/2013","1/1/2013","4/11/2010","4/11/2010","4/11/2010", 
    "4/11/2010","9/2/2012","9/2/2012","9/2/2012","9/2/2012") 

    df <- data.frame(Restaurant = Restaurant, owner=owner, purchaseDate=purchaseDate) 

    df$currentOwner <- c("Bob","Bob","Bob","Bob","Bob","Bob","Bob","Hazel","Hazel","Hazel","Hazel","Pete","Pete","Pete","Pete") 

Answer 1

您可以使用此：

do.call(rbind, by(df, df$Restaurant, function(d) 
     within(d, currentOwner2 <- as.character(
      owner[which.max(as.Date(purchaseDate, format="%d/%m/%Y"))])))) 

感謝@Thomas爲挖掘這個問題:-)