#1
fourdim = array(1:10,c(2,3,2,2))

#2
fourdim[2,2,2,2]= NA

#3
mean(fourdim, na.rm=TRUE)

#4
threed = array(c(155, 261, 132000,423.4, 321, 137000,105, 240, 118000,157.64, 260, 139000),
                c(3,2,2),
                list(c("Stock Price", "Revenue", "Employees"),
                c("2018","2019"),
                c("Apple","Microsoft")))

#5
diff_price = (threed["Stock Price","2019","Apple"] - threed["Stock Price","2018",'Apple'])
diff_price
#6
(diff_price)/((threed["Stock Price","2019","Apple"] + threed["Stock Price","2018",'Apple'])/2)

#7
Apple = threed[,,"Apple"]

#8
stock_price = threed[c("Stock Price"),,]
stock_price

#9
dim(stock_price)

#10
class(stock_price) == class(threed)

#11
price_vector = stock_price[,"Microsoft"]
price_vector