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.