> head(stock)
                  symbol Open               High         Low          Close              volume      Adjusted   
2007-01-03 "AAPL" "12.327143"  "12.368571"  "11.7"       "11.971429" "309579900" "8.104136"
2007-01-03 "AMZN" "38.68"      "39.060001"  "38.049999"  "38.700001" "12405100"  "38.700001"
2007-01-03 "GOOG" "231.494354" "236.789917" "229.065155" "232.28421" "15513200"  "232.28421"
2007-01-03 "MSFT" "29.91"      "30.25"      "29.4"       "29.860001" "76935100"  "22.858253"
2007-01-04 "AAPL" "12.007143"  "12.278571"  "11.974286"  "12.237143" "211815100" "8.284014"
2007-01-04 "AMZN" "38.59"      "39.139999"  "38.259998"  "38.900002" "6318400"   "38.900002"

这是用quantmod包整理出来的数据框，老师给的作业是1. Using the filter function to select stock quotes only for the last two years (tip: filter(date >= today() — years(2))
请教诸位大牛一下问题如何解决：
[size=18.6667px]> filter(stock,date >= today() - years(2))

Errerror in UseMethod("filter_") :

no no applicable method for 'filter_' applied to an object of class "c('xts', 'zoo')

q