running a command on a subset of data (similar to if condition in stata)



``` hist(coffee.exercise$met[coffee.exercise$coffee.consumption=="A"])
hist(coffee.exercise$met[coffee.exercise$coffee.consumption=="B"])
hist(coffee.exercise$met[coffee.exercise$coffee.consumption=="C"])
hist(coffee.exercise$met[coffee.exercise$coffee.consumption=="D"])
````

Introduction to the Analysis of Survival Data in the Presence of Competing Risks

 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4741409/