Subsampling-oversampling from a data frame

Dear members,

Consider the following data frame (first 4 rows shown)


  age sex class
  15   m   low
  20   f  high
  15   f   low
  10   m   low

in my original data set i have 1200 rows and a class distribution of
low=0.3 and high=0.7


My question : how can i create a new data frame as the one shown above but
with the 'high' class subsampled so that in the new data frame the class
distribution is low=0.5 and high=0.5?

I tried looking at the sample function and prob option but all examples i
seen do not use an imbalanced class problem as the one shown above


Thank you in advance


Thank you in advance