Part of making code reusable is removing the data we want to perform some logic on to leave only the logic. Then we can use our logic on any data. Here's what we mean:
Imagine you're writing a weather app. Your thermometer outside reports the temperature in Celsius and your goal is to report the temperature in Fahrenheit.
You write a program that takes a temperature like
15 degrees Celsius, then multiplies and divides it until you get the temperature in Fahrenheit.
Once you've done this though, you see the temperature now reads
16 degrees Celsius. In order to find Fahrenheit again, you'd need to write a whole new program to convert
16 degrees Celsius to Fahrenheit.
This would happen because the Celsius-to-Fahrenheit logic is operating directly on the Celsius data.