creating new column in an R dataframe based on values in multiple columns -

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i have following data frame including restaurants (id#), owner , purchase date. restaurant changes ownership, analysis purposes want create column keeps name of latest owner, determined "purchase date". how can create new column? in example restaurant 23 changes hand on 1/1/2013, want include new owner's name (bob) current owner rows restaurant shown below. if restaurant not change hands, keep same name "current owner" column "owner" column.

    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")

you can use this:

do.call(rbind, by(df, df$restaurant, function(d)         within(d, currentowner2 <- as.character(             owner[which.max(as.date(purchasedate, format="%d/%m/%y"))]))))

thanks @thomas excavating question :-)


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