Modify the harmonic_mean() function so that the return value has the class harmonic. Now write an S3 print() method for this class that displays the message The harmonic mean is Y where Y is the value computed by the function harmonic_mean(), e.g.:

 

> harmonic_mean(x = c(1, 2, 4))
The harmonic mean is 1.714286 
>

Hint: You will need the function cat() in the S3 method.

 

(from: Cotton, 2013, Learning R, O’Reilly)