When trying to explain the interaction variable explanation, I saw this link.
In this link, I saw a paragraph

GPA is the student’s Grade Point Average (higher values indicate better grades). The average gpa is about 2.81. The range of gpa theoretically goes from 0 to 4 but in actuality the lowest gpa in the sample is 1.45. MALE is coded 1 if the student is male, 0 if Female. MALEGPA = MALE * GPA
Array

I do not understandwhy they do not do
Code:
reg drink gpa male
but
Code:
reg drink gpa i.male