Prefactoring

13.2. The Language of the Client

Sam came by to discuss the discount mechanism that he was thinking about. He started talking. I was listening with a pen in hand. He said something like this:

"I want to give a 5% discount if a customer rented more than six CDDiscs in the past month and another 5% if he averaged more than five rentals in the last six months," he said, without pausing for a breath.

"Say again," I asked.

He replied at the same speed, as though he thought that if he did not get it out fast enough, he would forget it.

I took out a clean sheet of paper and copied down what I was pretty sure I heard:

Start with discount_percentage = 0.0.

Compute number_of_rentals_in_last_month:

Compute average_number_of_rentals_during_last_six_months:

If (average_number_of_rentals_during_last_six_months > 5) discount_percentage = discount_percentage + 5.0

I asked him if this was what he meant. He looked at it and asked what the > symbol meant.

I told him that it meant, "is greater than."

He said, "Great, that's exactly what I want."

"Is this what you'll want forever?" I asked.

"Well, probably not," he replied.

"I'll tell you what; I'm going to let you maintain this code," I said. "If those numbers change, you just change the numbers and I'll add it to the program."

"So I'm a programmer now?" Sam asked.

"Of course. We're part of a team," I replied.

Suppose Sam were fairly sure that the algorithm was not going to change, but the numbers were going to change. Then a configuration file ...