Functions Power Apps

Every app on your phone is a chain of functions. Tap a pixel at (x, y) → a function finds the button → a function runs the action → a function updates the screen. Instagram filters are functions: input a pixel color, output a new color. Google Maps is a function: input two addresses, output a route.

This is why f(x) = x² is a function: every x gives exactly one answer. A rule that outputs "either +3 or -3 for input 9" is not a function — it breaks the one-output rule, and software built on it would be unpredictable.