# problem-4.10

problem-4.10 The data set is stored in a matrix, so we can reverse the years using indexing.

    > barplot(t(dvdsales[7:1,]), beside=TRUE)

Using apply() on the rows, we get the yearly amounts:

    > apply(dvdsales, 1, sum)
        2004     2003     2002     2001     2000     1999     1998
          NA 21994389 17089823 12706584  8498545  4019389  1089261
        1997
          NA
    > apply(dvdsales, 1, function(x) sum(x,na.rm=TRUE)) # avoid NA
        2004     2003     2002     2001     2000     1999     1998
     5957387 21994389 17089823 12706584  8498545  4019389  1089261
        1997
      315136

Applying sum() to the columns gives the monthly sales. We do so only for years that have complete data.

    > apply(dvdsales[2:6,], 2, sum)
        JAN     FEB     MAR     APR     MAY     JUN     JUL     AUG
    2407354 2545896 4761189 3677919 3539725 5634658 3521254 3812934
        SEP     OCT     NOV     DEC
    8362963 7551936 9323618 9169284

November is the big winner. (Christmas shopping?)