# problem-4.16

problem-4.16 This can be done as follows:

    > before.72 = year < 72 & year > 0
    > tapply(runs.scored[before.72],league[before.72],mean)
       AL    NL
    653.9 673.9
    > tapply(runs.scored[!before.72],league[!before.72],mean)
       AL    NL
    718.7 675.6

Alternatively, tapply() can use two factors to split up the results in one command.

    > tapply(runs.scored,list(factor(before.72),league),mean)
             AL    NL
    FALSE 718.7 675.6
    TRUE  653.9 673.9

The tapply() syntax above uses a list of factors to split up the value of runs.scored.